CVE-2026-44523: Note Mark JWT Secret is Too Weak for Security

#### 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):

```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)
* Optionally log warnings or fail startup if the secret is insecure

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### Resources

* RFC 7518 Section 3.2 (JSON Web Algorithms - HMAC key size requirements)
* CWE-326: Inadequate Encryption Strength
* CWE-345: Insufficient Verification of Data Authenticity

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CWE-326 Inadequate Encryption Strength

CWE-345