---
title: "CVE-2026-85394: python-jose algorithm confusion guard bypassed by DER-encoded public keys"
url: https://erroneo.us/security/cve/CVE-2026-85394/
description: "CVE-2026-85394 (python-jose (PyPI)): python-jose through 3.5.0 fails to properly validate asymmetric keys in HMAC initialization, accepting DER-encoded…"
type: vulnerability-record
section: "Security Digest"
date_published: 2026-09-03
date_modified: 2026-10-05
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2026-85394"
kev: false
cvss: 9.3
epss: 0.00222
license: All rights reserved. Quote with attribution and a link.
---

# CVE-2026-85394: python-jose algorithm confusion guard bypassed by DER-encoded public keys

**Signal:** Critical and remotely reachable. No known exploitation yet, but it is network-reachable without credentials. (Computed from CISA KEV, EPSS and CVSS; not an editorial verdict.)

## Should you act?

- No confirmed exploitation. It is not in CISA’s KEV catalog and CISA’s SSVC assessment records no known exploitation. That can change quickly after a public write-up; re-check if you are affected.
- EPSS 0.2% , percentile 12: FIRST’s model estimates a 0.2% chance of exploitation activity in the next 30 days, higher than 12% of all scored CVEs (low). EPSS is a probability, not a verdict.
- Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
- Only applies if your dependency tree includes python-jose (PyPI) <= 3.5.0, no fixed version listed . Check your lockfile, including transitive dependencies.

## What it is

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

- PyPI `python-jose` <= 3.5.0 → fixed: none listed

## Affected products (CVE record)

- mpdavis python-jose: <= 3.5.0

## Severity

- VulnCheck CVSS 4.0: 9.3 `CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N`
- VulnCheck CVSS 3.1: 9.1 `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N`
- GitHub CVSS 4.0: 9.3 `CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N`

## Exploitation

- CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
- CISA SSVC assessment: exploitation none , automatable no , technical impact total .
- FIRST EPSS (model of 2026-10-21): 0.22% probability of exploitation activity in the next 30 days; percentile 12.

## Sources

- [GitHub Advisory GHSA-3qf3-8w2g-rqmx](https://github.com/advisories/GHSA-3qf3-8w2g-rqmx)
- [CVE.org record CVE-2026-85394](https://www.cve.org/CVERecord?id=CVE-2026-85394)
- [NVD entry CVE-2026-85394](https://nvd.nist.gov/vuln/detail/CVE-2026-85394)
- [GitHub Issue #414](https://github.com/mpdavis/python-jose/issues/414)
- [github.com](https://github.com/mpdavis/python-jose)
- [github.com](https://github.com/mpdavis/python-jose/blob/018b310ddb8b50dcfd09a0c152117835a21dd656/jose/backends/native.py)
- [github.com](https://github.com/mpdavis/python-jose/blob/018b310ddb8b50dcfd09a0c152117835a21dd656/jose/utils.py)
- [GitHub Security Advisory (GHSA-6c5p-j8vq-pqhj)](https://github.com/advisories/GHSA-6c5p-j8vq-pqhj)
- [VulnCheck Advisory: python-jose through 3.5.0 Algorithm Confusion via DER-encoded Public Key as HMAC Secret](https://www.vulncheck.com/advisories/python-jose-through-3.5.0-algorithm-confusion-via-der-encoded-public-key-as-hmac-secret)

---
Source page: https://erroneo.us/security/cve/CVE-2026-85394/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt