JWT token exploitation by Bruteforce weak signing key

Bruteforce weak signing key
Try to brute force a weak signing key. If you get the secret key that is used for creating the signature, then you can modify data and use that key to create a new signature. In this scenario, the server is validating the signature, but the server is using the weak key for creating the signature.
I know some of you might be confused about this. Check this blog to understand the digital signature.

Check the above image, if you know the secret you can create your own signature, and that same secret is used for validating the signature at the server end. This is how it works.

For brute force, you can sue any signature dictionary. https://raw.githubusercontent.com/wallarm/jwt-secrets/master/jwt.secrets.list
Copy the JWT token and paste it into the below command and give the path to the dictionary.

Bruteforcing:

hashcat -a 0 -m 16500 <YOUR-JWT> /path/to/jwt.secrets.list
or
python3 jwt_tool.py <JWT Token> -C -d jwt.secrets.list

Let’s say after using the above command you get the secret key: secret1
Now convert the secret key in base64 (c2VjcmV0MQ==) by using the Burp decoder.
Go to the “JWT Editor Keys” in Burp and then click on “New Symmetric key”. Click on “Generate” and replace the value of “k” with the base64 secret key.
We are just saving our key in base64 in the formula so that it can be used for signing the header and payload in the next steps.

Now go back to the repeater in Burp, and change the value in the payload by using “JSON Web Token”. At the bottom of the tab, click “sign”, then select the key that you generated in the previous section. Make sure that the Don’t modify header option is selected, then click OK. The modified token is now signed with the correct signature.

Send the request and observe the successful response.

JWT:
https://lazyhacker22.blogspot.com/2022/07/jwt-vulnerabilities-list-simple.html
https://lazyhacker22.blogspot.com/2022/06/what-is-jwt-json-web-tokens-simple.html

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