VolgaCTF 2021

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The given url lead us to this authentication page:


Once registered, we get an useless page where we are only allowed to β€œsay hi”. We noticed a particular cookie and googling the challenge name we found out that it was a JSON Web Token aka JWT. So using jwt.io we obtained the following decryption of the token.


A known vulnerability of JWT concerns the optional field jku which refers to the location (as url) where the private key adopted in the decryption can be found. The attacker can modify the token providing a maliciuos jku value, and it’s also very easy as we can do it directly with jwt.io. As far as we can impose the key location, we can also trick the server with a forged key, which will be used to encrypt the token. So we created our web server (using php and ngrok), where we exposed a simple file with our key:

    "kty": "oct",
    "kid": "HS256",
    "alg": "HS256"

Where k stands for the key value in base64 and kid is the identifier (the same that appears in the original token). The malicious token looks like this


As you can notice we also changed the sub field from born2scan to admin. After we set the new token in the cookies we can finally say hi to the server.


We liked the response.

🏁 VolgaCTF{jW5_jku_5u85T1TUt10n}