Objective

Generate a JSON Web Token (JWT) with the following specifications:

• Payload: {"userid": 0}
• Algorithm: HS256
• Secret Key:
  • Derived from the variable APP_SECRET.
  • APP_SECRET is generated using the SHA-256 hash function.
  • The input to SHA-256 is the application's start time.
  • Use the function hashlib.sha256(time), where time is the timestamp when the application started.
  • Create the JWT token and use it as cookie "session".
  • Then go to /api/files to see which file is present in that directory.

Overview

The application uses a variable named APP_SECRET to generate a secret key with the following process:

1. Generate Secret Key:

   APP_SECRET = hashlib.sha256(str(time_started).encode()).hexdigest()

   Here, time_started is the time when the application started.

2. Check for Session Cookie:

   The application looks for a session cookie. If it exists, it attempts to decode it.

3. Decode JWT Token:

   The session cookie should be a JWT token with:

   • A payload containing {"userid": 0}.
   • Encoded using HS256 algorithm with APP_SECRET as the secret key.
4. Validation:

   If the token does not pass these checks, the application returns an error including the time since the app started. This indicates a business logic flaw that permits token creation.

Overview

I developed a Python script to exploit a vulnerability in the application's JWT handling. The steps are as follows:

1. Generate Random JWT Token:

   Create a JWT token with the payload {"userid": 0} and a random secret, encoded with HS256.

2. Send Initial Request:

   Send the token to the /api/files endpoint and receive an error response containing the application's runtime in seconds.

3. Create Correct Secret:

   Use the runtime information to reconstruct the correct APP_SECRET:

     import hashlib
     time_started = current_time - seconds_since_start
     APP_SECRET = hashlib.sha256(str(time_started).encode()).hexdigest()
                       

4. Generate Valid JWT Token:

   Create a new JWT token with the payload {"userid": 0}, encoded with HS256 using the correct APP_SECRET.

5. Send Valid Request:

   Send the new token to the /api/files endpoint successfully.

6. Access Restricted File:

   Send a request to /api/file?filename=/dirFromEtcPasswd/flag.txt to access the restricted file.