fast-jwt accepts unknown crit header extensions (RFC 7515 violation)

Exploitation

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

CVSS

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

EPSS

• CVE-2026-35042: 0.16% estimated exploitation activity in the next 30 days; percentile 4.97 (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 does not validate the crit (Critical) Header Parameter defined in RFC 7515 §4.1.11. When a JWS token contains a crit array listing extensions that fast-jwt does not understand, the library accepts the token instead of rejecting it. This violates the MUST requirement in the RFC. --- RFC Requirement RFC 7515 §4.1.11: If any of the listed extension Header Parameters are not understood and supported by the recipient, then the JWS is invalid. --- Proof of Concept Expected: Error — unsupported critical extension Actual: Token accepted. Comparison --- Impact - Split-brain verification in mixed-library environments - Security policy bypass when crit carries enforcement semantics - Token binding bypass (RFC 7800 cnf confirmation) - See CVE-2025-59420 for full impact analysis --- Suggested Fix In src/verifier.js, add crit validation after header decoding:

Published by the source

3 Apr 2026, 22:01 UTC

Last modified by the source

6 Apr 2026, 23:49 UTC

Source last checked

26 Oct 2026, 12:47 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-hm7r-c7qw-ghp6

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-hm7r-c7qw-ghp6

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# Patch Next advisory brief: CVE-2026-35042

Advisory: fast-jwt accepts unknown `crit` header extensions (RFC 7515 violation)
IDs: CVE-2026-35042, GHSA-hm7r-c7qw-ghp6
Record: https://patchnext.com/advisories/cve-2026-35042 (revision 2, source checked 26 Oct 2026, 12:47 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor response not established. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm fast-jwt: affected <= 6.1.0

## 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
No complete vendor fix is recorded. Follow the primary source's mitigation guidance and check this record again before closing anything.

## 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://osv.dev/vulnerability/GHSA-hm7r-c7qw-ghp6 (OSV record GHSA-hm7r-c7qw-ghp6 lists affected ranges: npm fast-jwt >=0 <=6.1.0)
- Verified: https://github.com/advisories/GHSA-hm7r-c7qw-ghp6 (GitHub-reviewed advisory GHSA-hm7r-c7qw-ghp6 lists affected ranges: npm fast-jwt >=0 <=6.1.0)

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