CVE-2026-62995

Deferred Deferred - Pending Action

JWT Padding Malleability in joserfc Library

Publication date: 2026-07-29

Last updated on: 2026-07-30

Assigner: GitHub, Inc.

Description

joserfc is a Python library that provides an implementation of several JSON Object Signing and Encryption (JOSE) standards. in versions 1.7.1 and prior, joserfc accepts JWTs with trailing padding (==) which are not conforming to the JOSE specifications. This leads to malleability of the JWTs when consumed by joserfc. Depending on this application this might or not be an issue. This could lead to bypass of token revocation or anti-replay protection when implemented as a deny list of tokens or a deny list of token hashes. Note that ECDSA JWS are always malleable because of the malleability of ECDSA signatures (first test case in the code bellow). This makes a scheme which assumes that JWTs are not malleable brittle. However for other signatures (or MAC) schemes it might make sense to assume non malleability of the token. This issue has been fixed in version 1.7.2.

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Meta Information

Published

2026-07-29

Last Modified

2026-07-30

Generated

2026-08-19

AI Q&A

2026-07-30

EPSS Evaluated

2026-08-18

NVD

CVE-2026-62995

Affected Vendors & Products

Vendor	Product	Version / Range
joserfc	joserfc	to 1.7.2 (exc)
joserfc	joserfc	1.7.2

Helpful Resources

Exploitability

CWE

KEV

CWE ID	Description
CWE-345	The product does not sufficiently verify the origin or authenticity of data, in a way that causes it to accept invalid data.

Attack-Flow Graph

Executive Summary

joserfc is a Python library for JOSE standards. Versions 1.7.1 and prior accept JWTs with trailing padding (==) which violates JOSE specifications. This allows JWT malleability, meaning tokens can be altered without detection. ECDSA signatures are always malleable due to ECDSA properties. The issue was fixed in version 1.7.2.

Detection Guidance

This vulnerability is specific to the joserfc library and relates to JWT handling. To detect it, inspect applications using joserfc versions 1.7.1 or earlier for JWT processing. Check for trailing padding (==) in JWT tokens. No direct network commands detect this, but you can audit code or logs for joserfc usage and malformed JWTs.

Impact Analysis

This could allow bypass of token revocation or anti-replay protections if implemented as a deny list. Attackers might manipulate tokens to bypass security controls. Impact depends on how the library is used in applications. ECDSA-based tokens are always at risk due to inherent signature malleability.

Compliance Impact

This vulnerability could impact compliance with standards like GDPR and HIPAA if the affected application relies on non-malleable JWTs for token revocation or anti-replay protection. Bypassing these protections might lead to unauthorized access or data breaches, violating confidentiality and integrity requirements under these regulations.

Mitigation Strategies

Upgrade joserfc to version 1.7.2 or later to address the JWT padding issue. Review applications using joserfc for JWT validation logic, especially those relying on token revocation or anti-replay protections. Consider disabling JWT padding acceptance if custom handling is required.

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