Trying to inspect the JWT with https://jwt.io, I discovered that the HS256 algorithm is used, which is vulnerable to brute-forcing the secret key with which the jwt is signed:
Jwt.io
As soon as we find this type of algorithm, the first thing to try is using jwt-cracker with rockyou.txt to attempt cracking the secret key, so let’s proceed with the exploitation.
🔬 Vulnerability Analysis
Potential Vulnerabilities
JWT Secret Brute-Forcing
🎯 Solution Path
Exploitation Steps
Initial setup
As mentioned in the analysis phase, I will try to crack the secret key of the previously found JWT:
As we can see, the cracking was successful and the key ilovepico was found. We can now use this key to make any modifications we want to the JWT and then sign it with it.
Exploitation
Since we need to access as the admin user, we must modify the previously extracted JWT payload to {"user":"admin"} and sign the JWT with the key we previously found (ilovepico). We can do this in different ways, for example, by writing a Python script using the jwt library, or we can do it directly via jwt-tools from the CLI:
Now, all that’s left is to replace the old JWT value in the cookie using ChromeDevTools, for example, and refresh the page (F5) to “save the changes”. This way, I obtained the flag.
Flag capture
Manual Flag
🛠️ Exploitation Process
Approach
The automatic exploit takes a wordlist.txt argument and performs cracking by trying all the keys in the wordlist. As soon as it finds the valid key, it forges the token with user=admin and makes a request with the forged token, extracting the flag from the response using a regex.
I learned that the key of the HS256 algorithm in JWT can be brute-forced.
Time Optimization
When facing a challenge with a JWT using the HS256 algorithm, we can try to brute-force the key right away, perhaps using a wordlist like rockyou.txt with jwt-cracker. If this method doesn’t work, we can try using the none algorithm as done in the JAuth challenge.
