Vulnerability · Published Sep 3, 2026 · Assigned by VulnCheck

CVE-2026-85394python-jose through 3.5.0 Algorithm Confusion via DER-encoded Public Key as HMAC Secret

Critical 9.1

In plain English

A critical-severity security flaw in python-jose from mpdavis. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 13% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)9.1Critical

Exploit odds (30 days)0.2%top 87.1% of all CVEs

Exploited in the wildNo reportsCISA SSVC: none

Attack toolingNoneno public tooling known

Official description

python-jose through 3.5.0 fails to properly validate asymmetric keys in HMAC initialization, accepting DER-encoded public keys that lack PEM armor or SSH prefixes. Attackers holding the service's public key can forge HS256 tokens that pass verification when algorithms are not explicitly restricted. This is an incomplete fix for CVE-2024-33663.

Affected packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
PyPI	python-jose	no fix listed	GHSA-3qf3-8w2g-rqmx
PyPI	python-jose	3.4.0	GHSA-6c5p-j8vq-pqhj
PyPI	python-jose	3.4.0	PYSEC-2024-232
PyPI	python-jose	no fix listed	PYSEC-2026-4298
Ubuntu:22.04:LTS	python-jose	no fix listed	UBUNTU-CVE-2026-85394
Ubuntu:24.04:LTS	python-jose	no fix listed	UBUNTU-CVE-2026-85394

Affected products (as reported)

Vendor	Product	Versions
mpdavis	python-jose	0 – through 3.5.0

Weakness: CWE-347 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

References

GitHub Issue #414 issue-tracking
https://github.com/mpdavis/python-jose product
https://github.com/mpdavis/python-jose/blob/018b310ddb8b50dcfd09a0c152117835a21dd656/jose/backends/native.py technical-description
https://github.com/mpdavis/python-jose/blob/018b310ddb8b50dcfd09a0c152117835a21dd656/jose/utils.py technical-description
GitHub Security Advisory (GHSA-6c5p-j8vq-pqhj) related
VulnCheck Advisory: python-jose through 3.5.0 Algorithm Confusion via DER-encoded Public Key as HMAC Secret third-party-advisory