Improper JWT Audience Validation occurs when an application accepts a JSON Web Token (JWT) without verifying the aud (audience) claim. The aud claim specifies which service or application the token is intended for. If this validation step is skipped, a token issued for one service (e.g., Service A) can be replayed against another service (e.g., Service B). This allows adversaries to bypass authentication boundaries, impersonate users across services, or escalate privileges by reusing tokens outside their intended scope.

Remediation

• Always validate the aud claim during JWT verification, ensuring the token was issued for the current service or API.
• Reject tokens with missing or unexpected aud values.
• Apply strict validation of other critical claims (iss, exp, nbf) in addition to aud.
• Use distinct audience values for each service and avoid reusing the same JWT across multiple applications.
• Prefer well-vetted JWT libraries that enforce claim validation by default rather than rolling custom verification logic.
• Regularly audit authentication flows to confirm that audience checks are consistently applied.