PT-2026-30016 · Npm · Fast-Jwt

Published

2026-04-03

·

Updated

2026-04-03

·

CVE-2026-35039

CVSS v3.1

9.1

Critical

AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

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 wrong user identity

Affected Configurations

This vulnerability ONLY affects applications that BOTH:
  1. Enable caching using the cache option
  2. Use custom cacheKeyBuilder functions that can produce collisions
VULNERABLE examples:
// Collision-prone: same audience = same cache key
cacheKeyBuilder: (token) => {
 const { aud } = parseToken(token)
 return `aud=${aud}`
}

// Collision-prone: grouping by user type
cacheKeyBuilder: (token) => {
 const { aud } = parseToken(token)
 return aud.includes('admin') ? 'admin-users' : 'regular-users'
}

// Collision-prone: tenant + service grouping
cacheKeyBuilder: (token) => {
 const { iss, aud } = parseToken(token)
 return `${iss}-${aud}`
}
SAFE examples:
// Default hash-based (recommended)
createVerifier({ cache: true }) // Uses secure default

// Include unique user identifier
cacheKeyBuilder: (token) => {
 const { sub, aud, iat } = parseToken(token)
 return `${sub}-${aud}-${iat}`
}

// No caching (always safe)
createVerifier({ cache: false })

Not Affected

  • Applications using default caching
  • Applications with caching disabled

Assessment Guide

To determine if a consumer application is affected:
  1. Check if caching is enabled: Look for cache: true or cache: in verifier configuration
  2. Check for custom cache key builders: Look for cacheKeyBuilder function in configuration
  3. Analyze collision potential: Review if the application's cacheKeyBuilder can produce identical keys for different users/tokens
  4. If no custom cacheKeyBuilder: The project is NOT affected (default is safe)

Mitigations

While fast-jwt will look to include a fix for this in the next version, immediate mitigations include:
  • Ensure uniqueness of keys produced in cacheKeyBuilder
  • Remove custom cacheKeyBuilder method
  • Disable caching

Fix

Insufficient Verification of Data Authenticity

Weakness Enumeration

CWE-1289CWE-345CWE-706

Related Identifiers

CVE-2026-35039
GHSA-RP9M-7R4C-75QG

Affected Products

Fast-Jwt