---
title: "CVE-2026-35039: fast-jwt: Cache Confusion via cacheKeyBuilder Collisions Can Return Claims From a Different Token (Identity/Authorization Mixup)"
url: https://erroneo.us/security/cve/CVE-2026-35039/
description: "CVE-2026-35039 (fast-jwt (npm)): fast-jwt provides fast JSON Web Token (JWT) implementation. From 0.0.1 to before 6.2.0, setting up a custom cacheKeyBuilder…"
type: vulnerability-record
section: "Security Digest"
date_published: 2026-04-06
date_modified: 2026-05-13
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2026-35039"
kev: false
cvss: 9.1
epss: 0.00219
license: All rights reserved. Quote with attribution and a link.
---

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

**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?

- No confirmed exploitation. It is not in CISA’s KEV catalog and CISA’s SSVC assessment records no known exploitation. That can change quickly after a public write-up; re-check if you are affected.
- EPSS 0.2% , percentile 12: FIRST’s model estimates a 0.2% chance of exploitation activity in the next 30 days, higher than 12% 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) >= 0.0.1, < 6.2.0, fixed in 6.2.0 . Check your lockfile, including transitive dependencies.

## What it is

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.

## Affected packages

- npm `fast-jwt` >= 0.0.1, < 6.2.0 → fixed: 6.2.0

## Affected products (CVE record)

- nearform fast-jwt: >= 0.0.1, < 6.2.0

## 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 none , automatable yes , technical impact total .
- FIRST EPSS (model of 2026-10-21): 0.22% probability of exploitation activity in the next 30 days; percentile 12.

## Sources

- [GitHub Advisory GHSA-rp9m-7r4c-75qg](https://github.com/advisories/GHSA-rp9m-7r4c-75qg)
- [CVE.org record CVE-2026-35039](https://www.cve.org/CVERecord?id=CVE-2026-35039)
- [NVD entry CVE-2026-35039](https://nvd.nist.gov/vuln/detail/CVE-2026-35039)
- [github.com](https://github.com/nearform/fast-jwt/security/advisories/GHSA-rp9m-7r4c-75qg)
- [github.com](https://github.com/nearform/fast-jwt/commit/de121056c6415b58770c60640881eaec67ac4ceb)

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Source page: https://erroneo.us/security/cve/CVE-2026-35039/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt