CVE-2017-18239

Vulnerability

The JsonWebToken.validate method in main/scala/authentikat/jwt/JsonWebToken.scala of authentikat-jwt (com.jason-goodwin/authentikat-jwt) versions 0.4.5 and earlier performs a time-sensitive equality check when comparing the JWT signature [1][2]. Instead of using a constant-time string comparison, the implementation returns early as soon as a mismatched byte is encountered, enabling a timing side-channel attack [3]. This vulnerability affects all versions up to and including 0.4.5.

Exploitation

An attacker who is able to supply a crafted JWT token to a service using this library can repeatedly submit validation requests and measure the response time [1]. By observing slight timing differences, the attacker can incrementally guess each bit or character of the correct HMAC signature or private key [3]. No authentication or special network position is required beyond being able to send tokens to a validating endpoint that returns timing information, either directly or through observable side effects.

Impact

Successful exploitation allows the attacker to recover the secret key used for signing JWT tokens [3]. With the key, the attacker can forge arbitrary valid tokens, leading to complete authentication bypass and unauthorized access to any resource or action protected by JWT verification [2]. This results in a full compromise of confidentiality, integrity, and availability of the affected system.

Mitigation

The project maintainer recommended updating to version 0.4.5, but that version still contains this timing vulnerability [1]. The pull request #36 on GitHub introduces constant-time string comparison to fix the issue, but as of the advisory, no patched release has been published [1][2]. The project is not actively maintained, and the author explicitly recommends migrating to a more mature and well-supported JWT library for production use [2]. No workaround is documented in the references; the only reliable mitigation is to replace authentikat-jwt with an actively maintained alternative.