# CVE-2026-35039: fast-jwt: Cache Confusion via cacheKeyBuilder Collisions Can Return Claims From a Different Token (Identity/Authorization Mixup)

> Improper cache key generation in fast-jwt allows token verification to return claims from different tokens, enabling user impersonation and privilege escalation.

- **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:** 13 May 2026
- **Also known as:** GHSA-rp9m-7r4c-75qg

## What is CVE-2026-35039?

CVE-2026-35039 is a critical authentication bypass 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. From 0.0.1 to before 6.2.0, setting up a custom cacheKeyBuilder method which does not properly create unique keys for different tokens can lead to cache collisions. This could cause tokens to be mis-identified during the verification process leading to valid tokens returning claims from different valid tokens and users being mis-identified as other users based on the wrong token. Version 6.2.0 contains a patch.

Weakness: CWE-706 (Use of Incorrectly-Resolved Name or Reference), CWE-345 (Insufficient Verification of Data Authenticity), CWE-1289 (Improper Validation of Unsafe Equivalence in Input)

## Which versions are affected by CVE-2026-35039?

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


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `fast-jwt` | npm | From 0.0.1 | 6.2.0 |
| `fast-jwt` | npm | From 0.0.1 | 6.2.0 |

Products: Nearform Fast-Jwt.

## How do I fix CVE-2026-35039?

Upgrade fast-jwt to 6.2.0 or later.

## Is CVE-2026-35039 being exploited?

There is no known exploitation of CVE-2026-35039 in the wild and no public exploit code.

## 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-2026-35042](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-2026-35039 and fix it if it is affected.

What it is: CVE-2026-35039 is a critical authentication bypass 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: There is no known exploitation of CVE-2026-35039 in the wild and no public exploit code.

Affected packages and fixed versions:
- fast-jwt (npm): affected from 0.0.1; fixed in 6.2.0
- fast-jwt (npm): affected from 0.0.1; 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-2026-35039/ (Markdown: https://insights.sbomify.com/CVE-2026-35039/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-35039
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-35039
- OSV: https://osv.dev/vulnerability/CVE-2026-35039

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-35039/
Summaries are generated and may contain mistakes; the official description above is the record.