CVE-2026-32597 in pyjwt
Summary
by MITRE • 03/13/2026
PyJWT is a JSON Web Token implementation in Python. Prior to 2.12.0, PyJWT does not validate the crit (Critical) Header Parameter defined in RFC 7515 §4.1.11. When a JWS token contains a crit array listing extensions that PyJWT does not understand, the library accepts the token instead of rejecting it. This violates the MUST requirement in the RFC. This vulnerability is fixed in 2.12.0.
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Analysis
by VulDB Data Team • 03/20/2026
The vulnerability identified as CVE-2026-32597 affects PyJWT, a widely-used Python implementation for handling JSON Web Tokens that plays a critical role in authentication and authorization systems across numerous applications and services. This flaw resides in the library's handling of the crit (Critical) Header Parameter as defined in RFC 7515 section 4.1.11, which establishes mandatory validation requirements for critical extensions within JWS tokens. The vulnerability represents a significant security gap that undermines the integrity of token validation processes and exposes systems to potential exploitation through crafted malicious tokens.
The technical flaw manifests when PyJWT encounters a JWS token containing a crit array that references extensions the library does not recognize or support. Instead of enforcing the RFC-mandated rejection of such tokens, the vulnerable versions of PyJWT accept them without proper validation, effectively bypassing security controls that should prevent tokens with unknown critical extensions from being processed. This behavior directly violates the MUST requirement specified in RFC 7515, which explicitly states that implementations must reject JWS tokens containing critical header parameters they do not understand or support. The flaw creates a dangerous scenario where attackers can craft tokens with unrecognized critical extensions that will be accepted by vulnerable systems, potentially leading to bypass of security mechanisms that depend on proper crit parameter validation.
The operational impact of this vulnerability extends beyond simple token acceptance issues, as it fundamentally compromises the security model of systems relying on PyJWT for token validation. Attackers could exploit this weakness by crafting tokens with malicious critical extensions that appear legitimate to vulnerable systems, potentially leading to privilege escalation, unauthorized access, or other security breaches depending on how the tokens are processed within the target environment. This vulnerability aligns with CWE-1107, which addresses the issue of "Improper Validation of Critical Extensions in JWT Tokens," and represents a clear violation of the principle of least privilege and security by design. The flaw affects systems that depend on PyJWT for validating JWS tokens, particularly those implementing authentication flows where token integrity is paramount, making it a critical concern for organizations maintaining security-sensitive applications.
Mitigation strategies should prioritize immediate upgrading to PyJWT version 2.12.0 or later, which properly implements crit parameter validation as required by RFC 7515. Organizations should conduct comprehensive testing of their token validation workflows to ensure that existing tokens with legitimate crit parameters continue to function correctly after the upgrade. Security teams should also implement monitoring for suspicious token patterns and consider implementing additional validation layers beyond the library's built-in checks. The vulnerability demonstrates the importance of strict RFC compliance in security-critical libraries and highlights the necessity of thorough validation of all header parameters in JWT implementations. This fix addresses the ATT&CK technique T1550.001, which involves using valid accounts to gain access, by ensuring that tokens with invalid critical parameters are properly rejected rather than accepted, thus preventing potential credential theft or unauthorized access through token manipulation.