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Critical, CVSS 9.1amended

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

CVE-2026-35039GHSA-rp9m-7r4c-75qg

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (fast-jwt 6.2.0), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

  • npm fast-jwt

    Affected
    >= 0.0.1, < 6.2.0
    Fixed in
    6.2.0

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS
9.1 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
EPSS
  • CVE-2026-35039: 0.22% estimated exploitation activity in the next 30 days; percentile 12.09 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

NOTE: While the library exposes a mechanism which could introduce the vulnerability, this issue is created by developer-supplied code and not by the library itself. We will add a warning and some education for users around the possible issues however since the defaults work we will not be updating the library beyond that for this advisory. Impact 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 - Users being mis-identified as other users based on the wrong token This could result in: - User impersonation - UserB receives UserA's identity and permissions - Privilege escalation - Low-privilege users inherit admin-level access - Cross-tenant data access - Users gain access to other tenants' resources - Authorization bypass - Security decisions made on…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 3 · amended ·

    Amended from ghsa record GHSA-rp9m-7r4c-75qg

  2. Revision 2 · amended ·

    Amended from osv record GHSA-rp9m-7r4c-75qg

  3. Revision 1 · created ·

    Published from ghsa record GHSA-rp9m-7r4c-75qg

Agent brief

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# Patch Next advisory brief: CVE-2026-35039

Advisory: fast-jwt: Cache Confusion via cacheKeyBuilder Collisions Can Return Claims From a Different Token (Identity/Authorization Mixup)
IDs: CVE-2026-35039, GHSA-rp9m-7r4c-75qg
Record: https://patchnext.com/advisories/cve-2026-35039 (revision 3, source checked 26 Oct 2026, 12:47 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Critical (CVSS 9.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: fast-jwt 6.2.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm fast-jwt: affected >= 0.0.1, < 6.2.0; fixed in 6.2.0

## Suggested inspection (read-only)
- `npm ls fast-jwt --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on fast-jwt.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-rp9m-7r4c-75qg (GitHub-reviewed advisory GHSA-rp9m-7r4c-75qg lists affected ranges: npm fast-jwt >=0.0.1 <6.2.0)
- Verified: https://osv.dev/vulnerability/GHSA-rp9m-7r4c-75qg (OSV record GHSA-rp9m-7r4c-75qg lists affected ranges: npm fast-jwt >=0.0.1 <6.2.0 (fixed))

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