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

> An attacker can bypass JWT signature verification by using whitespace-prefixed keys, allowing unauthorized access or token forgery.

- **Severity:** Critical 9.1 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 6 Apr 2026
- **Updated:** 6 Apr 2026
- **Also known as:** GHSA-mvf2-f6gm-w987

## What is CVE-2026-34950?

CVE-2026-34950 is a critical vulnerability in nearform fast-jwt, published on 6 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

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.

Weakness: CWE-327 (Use of a Broken or Risky Cryptographic Algorithm)

## Which versions are affected by CVE-2026-34950?

2 open source packages ship an affected version, including fast-jwt. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `fast-jwt` | npm | All versions | 6.2.0 |
| `fast-jwt` | npm | All versions | 6.2.0 |

Products: Nearform Fast-Jwt.

## How do I fix CVE-2026-34950?

Upgrade fast-jwt to 6.2.0 or later.

## Is CVE-2026-34950 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-34950 exists.

## Other vulnerabilities in nearform fast-jwt

- [CVE-2026-35039](https://insights.sbomify.com/CVE-2026-35039/): fast-jwt: Cache Confusion via cacheKeyBuilder Collisions Can Return Claims From a Different Token (Identity/Authorization Mixup) (critical 9.1)
- [CVE-2026-35042](https://insights.sbomify.com/CVE-2026-35042/): fast-jwt accepts unknown `crit` header extensions (RFC 7515 §4.1.11 MUST violation) (high 7.5)
- [CVE-2023-48223](https://insights.sbomify.com/CVE-2023-48223/): fast-jwt JWT Algorithm Confusion (medium 5.9)
- [CVE-2026-35040](https://insights.sbomify.com/CVE-2026-35040/): fast-jwt: Stateful RegExp (/g or /y) causes non-deterministic allowed-claim validation (logical DoS) (medium 5.3)
- [CVE-2026-35041](https://insights.sbomify.com/CVE-2026-35041/): ReDoS in fast-jwt when using RegExp in allowed* leading to CPU exhaustion during token verification (medium 4.2)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-34950 and fix it if it is affected.

What it is: CVE-2026-34950 is a critical vulnerability in nearform fast-jwt, published on 6 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-34950 exists.

Affected packages and fixed versions:
- fast-jwt (npm): affected all versions; fixed in 6.2.0
- fast-jwt (npm): affected all versions; fixed in 6.2.0

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-34950/ (Markdown: https://insights.sbomify.com/CVE-2026-34950/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-34950
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-34950
- OSV: https://osv.dev/vulnerability/CVE-2026-34950

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-34950/
Summaries are generated and may contain mistakes; the official description above is the record.