---
title: "CVE-2026-107722: fast-jwt: Incomplete patch of CVE-2026-34950: Non-whitespace key-prefix re-enables RSA→HS256 algorithm confusion"
url: https://erroneo.us/security/cve/CVE-2026-107722/
description: "CVE-2026-107722 (fast-jwt (npm)): fast-jwt provides fast JSON Web Token (JWT) implementation. From 6.2.0 until 6.3.0, fast-jwt can misclassify RSA…"
type: vulnerability-record
section: "Security Digest"
date_published: 2026-10-08
date_modified: 2026-10-13
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2026-107722"
kev: false
cvss: 9.8
epss: 0.00272
license: All rights reserved. Quote with attribution and a link.
---

# CVE-2026-107722: fast-jwt: Incomplete patch of CVE-2026-34950: Non-whitespace key-prefix re-enables RSA→HS256 algorithm confusion

**Signal:** Critical and remotely reachable. No known exploitation yet, but it is network-reachable without credentials. (Computed from CISA KEV, EPSS and CVSS; not an editorial verdict.)

## Should you act?

- Not in CISA KEV , but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
- EPSS 0.3% , percentile 18: FIRST’s model estimates a 0.3% chance of exploitation activity in the next 30 days, higher than 18% of all scored CVEs (low). EPSS is a probability, not a verdict.
- Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
- Only applies if your dependency tree includes fast-jwt (npm) >= 6.2.0, <= 6.2.4, fixed in 6.3.0 . Check your lockfile, including transitive dependencies.

## What it is

fast-jwt provides fast JSON Web Token (JWT) implementation. From 6.2.0 until 6.3.0, fast-jwt can misclassify RSA public-key text as an HMAC secret when the key has non-whitespace content before its PEM header. In src/crypto.js, performDetectPublicKeyAlgorithms trims whitespace but publicKeyPemMatcher remains start-anchored, so comments, control characters, zero-width characters, or wrapper text can prevent PEM detection and reach the HMAC fallback. An attacker who knows the public key bytes can sign arbitrary HS256 claims with that public material when HS256 is inferred or allowed, resulting in authentication or authorization bypass. An asymmetric-only algorithm allowlist prevents the attack. This issue is fixed in version 6.3.0.

## Affected packages

- npm `fast-jwt` >= 6.2.0, <= 6.2.4 → fixed: 6.3.0

## Affected products (CVE record)

- nearform fast-jwt: >= 6.2.0, < 6.3.0

## Severity

- GitHub_M CVSS 3.1: 9.8 `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`
- GitHub CVSS 3.1: 9.8 `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`

## Exploitation

- CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
- CISA SSVC assessment: exploitation poc , automatable yes , technical impact total .
- FIRST EPSS (model of 2026-10-21): 0.27% probability of exploitation activity in the next 30 days; percentile 18.

## Sources

- [GitHub Advisory GHSA-ww5h-9m49-7xx4](https://github.com/advisories/GHSA-ww5h-9m49-7xx4)
- [CVE.org record CVE-2026-107722](https://www.cve.org/CVERecord?id=CVE-2026-107722)
- [NVD entry CVE-2026-107722](https://nvd.nist.gov/vuln/detail/CVE-2026-107722)
- [github.com](https://github.com/nearform/fast-jwt/security/advisories/GHSA-ww5h-9m49-7xx4)
- [github.com](https://github.com/nearform/fast-jwt/pull/632)
- [github.com](https://github.com/nearform/fast-jwt/commit/d96bbc6c5336055a6dbfa318fdbab01197264cfa)
- [github.com](https://github.com/nearform/fast-jwt/releases/tag/v6.3.0)

---
Source page: https://erroneo.us/security/cve/CVE-2026-107722/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt