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

> Attackers can forge JWT tokens by exploiting algorithm confusion in python-jose when asymmetric keys are used as HMAC secrets without explicit algorithm restrictions.

- **Severity:** Critical 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`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 3 Sep 2026
- **Updated:** 14 Sep 2026
- **Also known as:** CLSA-2026-1791441237, ROOT-APP-PYPI-CVE-2026-85394, UBUNTU-CVE-2026-85394

## What is CVE-2026-85394?

CVE-2026-85394 is a critical vulnerability in mpdavis python-jose, published on 3 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### 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.

Weakness: CWE-347 (Improper Verification of Cryptographic Signature)

## Which versions are affected by CVE-2026-85394?


## How do I fix CVE-2026-85394?

Upgrade to a fixed version.

## Is CVE-2026-85394 being exploited?

There is no known exploitation of CVE-2026-85394 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-85394 and fix it if it is affected.

What it is: CVE-2026-85394 is a critical vulnerability in mpdavis python-jose, published on 3 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-85394 in the wild and no public exploit code.

Affected product: mpdavis python-jose. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-85394/ (Markdown: https://insights.sbomify.com/CVE-2026-85394/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-85394
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-85394
- OSV: https://osv.dev/vulnerability/CVE-2026-85394

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-85394/
Summaries are generated and may contain mistakes; the official description above is the record.