CVE-2020-5300 in Hydra
Summary
by MITRE
In Hydra (an OAuth2 Server and OpenID Certifiedâ„¢ OpenID Connect Provider written in Go), before version 1.4.0+oryOS.17, when using client authentication method 'private_key_jwt' [1], OpenId specification says the following about assertion `jti`: "A unique identifier for the token, which can be used to prevent reuse of the token. These tokens MUST only be used once, unless conditions for reuse were negotiated between the parties". Hydra does not check the uniqueness of this `jti` value. Exploiting this vulnerability is somewhat difficult because: - TLS protects against MITM which makes it difficult to intercept valid tokens for replay attacks - The expiry time of the JWT gives only a short window of opportunity where it could be replayed This has been patched in version v1.4.0+oryOS.17
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Analysis
by VulDB Data Team • 05/09/2025
The vulnerability identified as CVE-2020-5300 affects Hydra, an OAuth2 server and OpenID Connect provider implemented in Go, specifically addressing a critical flaw in the private_key_jwt client authentication method. This issue exists in versions prior to 1.4.0+oryOS.17 and represents a significant security weakness that undermines the integrity of the authentication process. The flaw resides in the server's failure to validate the uniqueness of the JSON Web Token (JWT) identifier field known as 'jti' within the OpenID Connect assertions.
The technical implementation defect stems from Hydra's non-compliance with the OpenID Connect specification requirements for the 'jti' parameter. According to the OpenID specification, the jti field must contain a unique identifier that prevents token reuse, with explicit requirements that these tokens "MUST only be used once, unless conditions for reuse were negotiated between the parties." This requirement is fundamental to preventing replay attacks and maintaining the security guarantees of the authentication protocol. Hydra's failure to enforce this uniqueness constraint creates a potential vector for attackers to reuse valid JWT tokens within their limited time windows.
The operational impact of this vulnerability extends beyond simple replay attacks, as it creates a window of opportunity for malicious actors to exploit the system's trust model. While the attack complexity is moderately high due to TLS protection mechanisms that prevent man-in-the-middle attacks, the reduced time window for token reuse still presents a meaningful risk. The combination of the short expiry time and the lack of jti uniqueness validation creates a scenario where an attacker who successfully intercepts a valid token could potentially reuse it during its validity period, thereby bypassing the intended authentication mechanisms.
Security practitioners should recognize this vulnerability as a variant of CWE-310, which encompasses cryptographic issues related to the lack of proper validation of unique identifiers in cryptographic protocols. The vulnerability also aligns with ATT&CK techniques related to credential access and privilege escalation through token manipulation. Organizations using Hydra versions prior to 1.4.0+oryOS.17 must implement immediate mitigations including upgrading to the patched version, which resolves the jti uniqueness validation issue, and monitoring for potential unauthorized token usage patterns. Additional defensive measures could include implementing more robust token tracking mechanisms and strengthening the overall authentication infrastructure to reduce the attack surface for such vulnerabilities.