joserfc: HS256/HS384/HS512 verify accepts empty/nil HMAC key (cross-language sibling of CVE-2026-45363)

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

8.7 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N · score source

EPSS

• CVE-2026-49852: 0.17% estimated exploitation activity in the next 30 days; percentile 6.51 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary joserfc.jwt.decode accepts attacker-forged HMAC-signed tokens when the caller-supplied verification key is the empty string or None. HMACAlgorithm.sign and HMACAlgorithm.verify in src/joserfc/rfc7518/jwsalgs.py:62-70 feed whatever OctKey.getopkey(...) produced into hmac.new(...), and OctKey.importkey only emits a SecurityWarning when the raw key is shorter than 14 bytes without rejecting zero-length input. Any application whose JWT secret is sourced from an unset environment variable, an unset Redis / DB row, a key finder fallback that returns "", or a Hash.new("")-style default verifies attacker tokens forged with HMAC(key=b"", signinginput) because the attacker trivially reproduces the same digest with no secret knowledge. This is a cross-language sibling of jwt/ruby-jwt GHSA-c32j-vqhx-rx3x / CVE-2026-45363 (HS256/HS384/HS512 verify accepted an empty/nil HMAC key, filed 2026-05-13). ruby-jwt v3.2.0 added an ensurevalidkey! precondition that rejects empty keys at both sign…

Published by the source

2 Jul 2026, 19:12 UTC

Last modified by the source

10 Sep 2026, 03:50 UTC

Source last checked

26 Oct 2026, 13:46 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-gg9x-qcx2-xmrh

2. Revision 2 · amended · 25 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-gg9x-qcx2-xmrh

Preview the exact text before you copy it. Commands are read-only suggestions; review them before running anything.

# Patch Next advisory brief: CVE-2026-49852

Advisory: joserfc: HS256/HS384/HS512 verify accepts empty/nil HMAC key (cross-language sibling of CVE-2026-45363)
IDs: CVE-2026-49852, GHSA-gg9x-qcx2-xmrh
Record: https://patchnext.com/advisories/cve-2026-49852 (revision 3, source checked 26 Oct 2026, 13:46 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: joserfc 1.6.8. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI joserfc: affected <= 1.6.7; fixed in 1.6.8

## Suggested inspection (read-only)
- `python -m pip show joserfc`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on joserfc.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-gg9x-qcx2-xmrh (OSV record GHSA-gg9x-qcx2-xmrh lists affected ranges: PyPI joserfc >=0 <1.6.8 (fixed))
- Verified: https://github.com/advisories/GHSA-gg9x-qcx2-xmrh (GitHub-reviewed advisory GHSA-gg9x-qcx2-xmrh lists affected ranges: PyPI joserfc >=0 <=1.6.7)

Commands are suggestions: review each one before running it.