PyJWT: Public-key JWK accepted as HMAC secret enables forged HS256 tokens when mixed families are allowed

Exploitation

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

CVSS

7.4 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N

EPSS

• CVE-2026-48526: 0.43% estimated exploitation activity in the next 30 days; percentile 37.00 (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.

[!NOTE] Exploitation requires a verifier configured with both symmetric and asymmetric algorithms in algorithms=[…] and a raw-JSON JWK as the key= argument, both contrary to documented usage, hence the High attack-complexity rating. Summary When the verifier is decoding JSON Web Tokens, while supporting both asymmetric and HMAC algorithms, the library does not validate use of JSON Web Keys in HMAC algorithm, allowing attacker to use the issuer public key as the secret key for HMAC algorithm. Details In JWT algorithm confusion attack, the verifier is mistakenly use of public key to be used as the shared secret in symmetric algorithms. In pyjwt case, when the verifier is supporting both HMAC with other asymmetric algorithm and mistakenly using the public key of the issuer to verify the token as demonstrated in the following example: jws.decode(token, key=rsajwkjson, algorithms=["HS256","RS256"])) An attacker who specifies in the token header to use HMAC, will cause the verifier to…

Published by the source

15 Jun 2026, 19:28 UTC

Last modified by the source

10 Sep 2026, 03:51 UTC

Source last checked

26 Oct 2026, 13:45 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-xgmm-8j9v-c9wx

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-xgmm-8j9v-c9wx

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# Patch Next advisory brief: CVE-2026-48526

Advisory: PyJWT: Public-key JWK accepted as HMAC secret enables forged HS256 tokens when mixed families are allowed
IDs: CVE-2026-48526, GHSA-xgmm-8j9v-c9wx
Record: https://patchnext.com/advisories/cve-2026-48526 (revision 2, source checked 26 Oct 2026, 13:45 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.4)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: pyjwt 2.13.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI pyjwt: affected < 2.13.0; fixed in 2.13.0

## Suggested inspection (read-only)
- `python -m pip show pyjwt`: 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 pyjwt.
- 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-xgmm-8j9v-c9wx (OSV record GHSA-xgmm-8j9v-c9wx lists affected ranges: PyPI pyjwt >=0 <2.13.0 (fixed))
- Verified: https://github.com/advisories/GHSA-xgmm-8j9v-c9wx (GitHub-reviewed advisory GHSA-xgmm-8j9v-c9wx lists affected ranges: PyPI pyjwt >=0 <2.13.0)

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