---
title: "CVE-2026-34950: fast-jwt: Incomplete fix for CVE-2023-48223: JWT Algorithm Confusion via Whitespace-Prefixed RSA Public Key"
url: https://erroneo.us/security/cve/CVE-2026-34950/
description: "CVE-2026-34950 (fast-jwt (npm)): fast-jwt provides fast JSON Web Token (JWT) implementation. In 6.1.0 and earlier, the publicKeyPemMatcher regex in…"
type: vulnerability-record
section: "Security Digest"
date_published: 2026-04-06
date_modified: 2026-04-07
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2026-34950"
kev: false
cvss: 9.1
epss: 0.00274
license: All rights reserved. Quote with attribution and a link.
---

# CVE-2026-34950: fast-jwt: Incomplete fix for CVE-2023-48223: JWT Algorithm Confusion via Whitespace-Prefixed RSA Public Key

**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 19: FIRST’s model estimates a 0.3% chance of exploitation activity in the next 30 days, higher than 19% 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) <= 6.1.0, fixed in 6.2.0 . Check your lockfile, including transitive dependencies.

## What it is

fast-jwt provides fast JSON Web Token (JWT) implementation. In 6.1.0 and earlier, the publicKeyPemMatcher regex in fast-jwt/src/crypto.js uses a ^ anchor that is defeated by any leading whitespace in the key string, re-enabling the exact same JWT algorithm confusion attack that CVE-2023-48223 patched.

## Affected packages

- npm `fast-jwt` <= 6.1.0 → fixed: 6.2.0

## Affected products (CVE record)

- nearform fast-jwt: <= 6.1.0

## Severity

- GitHub_M CVSS 3.1: 9.1 `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N`
- GitHub CVSS 3.1: 9.1 `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N`

## 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 19.

## Sources

- [GitHub Advisory GHSA-mvf2-f6gm-w987](https://github.com/advisories/GHSA-mvf2-f6gm-w987)
- [CVE.org record CVE-2026-34950](https://www.cve.org/CVERecord?id=CVE-2026-34950)
- [NVD entry CVE-2026-34950](https://nvd.nist.gov/vuln/detail/CVE-2026-34950)
- [github.com](https://github.com/nearform/fast-jwt/security/advisories/GHSA-mvf2-f6gm-w987)
- [github.com](https://github.com/advisories/GHSA-c2ff-88x2-x9pg)

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Source page: https://erroneo.us/security/cve/CVE-2026-34950/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt