Note Mark is an open-source note-taking application. Prior to 0.19.4, 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. This vulnerability is fixed in 0.19.4.

Affected: Go: github.com/enchant97/note-mark/backend

#### 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):

```…