Vulnerability · Published Jul 29, 2026 · Assigned by GitHub_M
CVE-2026-62995joserfc accepts JWT with padding, leading to JWT malleability
Low 2.3
In plain English
A low-severity insufficient verification of data authenticity in joserfc from authlib. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.1% (higher than 4% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official 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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:13 | joserfc | no fix listed | DEBIAN-CVE-2026-62995 |
| Debian:14 | joserfc | 1.7.2-1 | DEBIAN-CVE-2026-62995 |
| Ubuntu:26.04:LTS | joserfc | no fix listed | UBUNTU-CVE-2026-62995 |
Affected products (as reported)
Weakness: CWE-345 · Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N
References
- https://github.com/authlib/joserfc/security/advisories/GHSA-5jhw-7jv7-qcqq x_refsource_CONFIRM