Note Mark has a JWT Secret Weakness that allows Full Account Takeover via Token Forgery

Summary

No minimum length or entropy is enforced on the JWT_SECRET configuration value. The application accepts any base64-decodable secret regardless of size, including secrets as short as 1 byte.

HS256 secrets below 32 bytes are brute-forceable offline, allowing attackers to recover the signing key and forge valid JWTs for arbitrary users.

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Impact

An attacker who captures a single valid JWT (e.g, from cookies, logs, or network traffic) can:

> Crack the signing secret offline using brute-force or wordlist attacks > Forge valid JWTs for any user ID (including administrators) > Authenticate without knowing any credentials

This results in full account takeover across the entire application with no server-side detection or rate limiting possible.

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Details

In backend/config/utils.go, the Base64Decoded.UnmarshalText function decodes the JWT secret but does not validate its length or entropy.

In backend/core/auth.go, JWT tokens are signed using HS256 without enforcing minimum key size requirements.

According to RFC 7518 Section 3.2, HS256 keys must be at least 256 bits (32 bytes). Libraries such as PyJWT explicitly warn against shorter keys, but note-mark performs no such validation.

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PoC

1- Deploy note-mark with a weak secret:

JWT_SECRET = base64("testsecret123456789012345")

2- Register an account and capture the Auth-Session-Token cookie

3- Crack the secret offline (example using Python):

import jwt, base64
jwt.decode(TOKEN, base64.b64decode(SECRET), algorithms=["HS256"])

4- Forge a new token for any user UUID with extended expiry

5- Send the forged token in requests → server returns 200 Ok and authenticates as that user

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Reproduction Steps

1- Deploy the application with a JWT secret shorter than 32 bytes (after base64 decoding) 2- Authenticate and capture a valid JWT 3- Perform offline brute-force or dictionary attack against the token signature 4- Recover the secret 5- Generate a forged JWT for another user 6- Use the forged token to access protected endpoints

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Fix Recommendation

• Enforce a minimum of 32 bytes (256 bits) for JWT secrets after base64 decoding
• Reject weak secrets during configuration parsing (e.g., in Base64Decoded.UnmarshalText or config validation)