Medium [CVE-2026-62995] JWT Malleability via Non-Standard Padding

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.

This vulnerability allows the library to accept JSON Web Tokens (JWTs) that include non-standard trailing padding. An attacker could exploit this to create malleable JWTs, potentially bypassing security mechanisms like token revocation or anti-replay protection.

This could lead to unauthorized access or continued use of tokens that should be invalid.

Affected products named by the advisory: Red Hat Hardened Images; Lightspeed Core; Migration Toolkit for Applications 8; OpenShift Lightspeed; and 3 more.