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

> Attackers can bypass JWT security checks by using unknown critical header extensions, potentially leading to unauthorized access.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/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:** No
- **Published:** 6 Apr 2026
- **Updated:** 7 Apr 2026
- **Also known as:** GHSA-hm7r-c7qw-ghp6

## What is CVE-2026-35042?

CVE-2026-35042 is a high-severity 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, 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.

Weakness: CWE-636 (Not Failing Securely), CWE-345 (Insufficient Verification of Data Authenticity)

## Which versions are affected by CVE-2026-35042?

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 to 6.1.0 | None |
| `fast-jwt` | npm | All versions to 6.1.0 | None |

Products: Nearform Fast-Jwt.

## How do I fix CVE-2026-35042?

No fix has been published for CVE-2026-35042 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-35042 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-35042 exists.

## Other vulnerabilities in nearform fast-jwt

- [CVE-2026-34950](https://insights.sbomify.com/CVE-2026-34950/): fast-jwt has an incomplete fix for CVE-2023-48223: JWT Algorithm Confusion via Whitespace-Prefixed RSA Public Key (critical 9.1)
- [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-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-2026-35042 and fix it if it is affected.

What it is: CVE-2026-35042 is a high-severity 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-2026-35042 exists.

Affected packages and fixed versions:
- fast-jwt (npm): affected all versions up to 6.1.0; no fix listed
- fast-jwt (npm): affected all versions up to 6.1.0; no fix listed

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-2026-35042/ (Markdown: https://insights.sbomify.com/CVE-2026-35042/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-35042
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-35042
- OSV: https://osv.dev/vulnerability/CVE-2026-35042

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-35042/
Summaries are generated and may contain mistakes; the official description above is the record.