JWT token is found to be using weak encryption key i.e. HMAC

  1. Home

What is the Vulnerability?

JSON Web Tokens (JWT) are commonly used for securely transmitting information between parties as a JSON object. However, when JWTs are signed using HMAC (Hash-based Message Authentication Code) with a weak or hardcoded secret key, they become vulnerable to brute-force or dictionary attacks.

An attacker can exploit this by:

  • Guessing or cracking the weak signing key.
  • Forging tokens to impersonate other users or elevate privileges.
HMAC, when used with weak keys or poor key management, fails to provide the cryptographic integrity that JWTs are supposed to guarantee.
JWT token is found to be using weak encryption key i.e. HMAC-image

Impact of the Vulnerability

  • Token Forgery: An attacker can generate valid JWTs and gain unauthorized access to protected resources.

  • Privilege Escalation: Attackers can impersonate admin accounts or bypass role-based controls.

  • Data Exposure: Sensitive endpoints may be exposed to unauthorized parties.

  • System Compromise: In severe cases, system-level compromise can occur due to trust placed on the JWT.

This compromises authentication, authorization, and session integrity of the application.

Solution to Fix the Vulnerability

    1. Avoid using HMAC with weak keys

    • If HMAC is used, ensure keys are at least 256 bits long and randomly generated.
    • Avoid hardcoded, guessable, or short secrets (e.g., "admin", "123456").

    2. Use Asymmetric Algorithms

    • Prefer RS256 (RSA) or ES256 (Elliptic Curve) instead of HS256 (HMAC).
    • These provide public/private key separation, which is more secure and scalable.

    3. Enforce Key Rotation

    • Implement key rotation policies using kid (Key ID) header for version tracking.

    4. Validation Checks

    • Always validate the token’s algorithm, expiration, and issuer/audience.
    • Do not allow tokens signed with none algorithm.

    5. Server Version Consideration

    • If using old libraries (e.g., jsonwebtoken < v8 in Node.js), upgrade immediately, as older versions may contain bypasses for weak keys.
    • Avoid PHP JWT libraries that allow none as a signing algorithm without proper checks.