fast-jwt treats raw public JWK JSON as an HMAC secret, enabling HS256 token forgery

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-2026-107724: 0.28% estimated exploitation activity in the next 30 days; percentile 20.75 (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 fast-jwt 6.2.4 silently classifies raw serialized public JWK JSON as an HMAC secret. If an application supplies public JWK JSON text as the verifier key and HS256 is explicitly allowed or automatically inferred, an attacker who knows the same public JSON text can use it as an HMAC key and create an arbitrary HS256 token that fast-jwt accepts as valid. This can result in authentication or authorization bypass through forged JWT claims. The PoC demonstrates the issue with a serialized RSA public JWK. The same non-PEM classification also applies to raw JWKS JSON text, although the attached standalone PoC focuses on the smallest JWK case. Details The verifier accepts a string or Buffer as key material. In src/crypto.js, performDetectPublicKeyAlgorithms() attempts to infer the permitted algorithm family from the supplied string. Strings matching supported PEM public-key formats are classified as asymmetric keys. Any other non-empty string is assumed to be an HMAC secret: js…

Published by the source

8 Oct 2026, 22:01 UTC

Last modified by the source

8 Oct 2026, 22:15 UTC

Source last checked

26 Oct 2026, 13:07 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-g3jj-5cmm-3hxx

2. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GHSA-g3jj-5cmm-3hxx

3. Revision 1 · created · 23 Oct 2026

   Published from ghsa record GHSA-g3jj-5cmm-3hxx

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# Patch Next advisory brief: CVE-2026-107724

Advisory: fast-jwt treats raw public JWK JSON as an HMAC secret, enabling HS256 token forgery
IDs: CVE-2026-107724, GHSA-g3jj-5cmm-3hxx
Record: https://patchnext.com/advisories/cve-2026-107724 (revision 3, source checked 26 Oct 2026, 13:07 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: fast-jwt 6.3.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm fast-jwt: affected >= 6.2.4, <= 6.2.4; fixed in 6.3.0

## Suggested inspection (read-only)
- `npm ls fast-jwt --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## 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 fast-jwt.
- 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://github.com/advisories/GHSA-g3jj-5cmm-3hxx (GitHub-reviewed advisory GHSA-g3jj-5cmm-3hxx lists affected ranges: npm fast-jwt >=6.2.4 <=6.2.4)
- Verified: https://osv.dev/vulnerability/GHSA-g3jj-5cmm-3hxx (OSV record GHSA-g3jj-5cmm-3hxx lists affected ranges: npm fast-jwt >=6.2.4 <6.3.0 (fixed))

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