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

Mitigations

Mitigations include:

• Ensure uniqueness of keys produced in cacheKeyBuilder
• Remove custom cacheKeyBuilder method
• Disable caching

fast-jwt allows enabling a verification cache through the cache option. The cache key is derived from the token via cacheKeyBuilder.

When a custom cacheKeyBuilder produces collisions between different tokens, the verifier may return the cached payload of a previous token instead of validating and returning the payload of the current token.

This results in cross-token payload reuse and identity confusion.

Two distinct valid JWTs can be verified successfully but mapped to the same cached entry, causing the verifier to return claims belonging to a different token.

This affects authentication and authorization decisions when applications trust the returned payload.

Affected component

src/verifier.js

Relevant logic:

cache enabled via createCache

cache population via cacheSet

lookup based on cacheKeyBuilder(token)

cached payload returned without re-verification

Impact

Identity / authorization confusion via cache collision.

If two tokens generate the same cache key:

token A is verified → payload stored in cache

token B is verified → cache hit occurs

verifier returns payload from token A instead of B

Observed effect:

subject mismatch

claim mismatch

authorization decision performed on wrong identity

Potential real-world consequences:

user impersonation (logical)

privilege confusion

incorrect RBAC evaluation

gateway / middleware auth inconsistencies

This is especially dangerous when:

cache is enabled (recommended for performance)

custom cacheKeyBuilder is used

identity claims (sub / aud / iss) drive authorization

Root cause

The verifier assumes the cache key uniquely identifies the token and its claims.

However:

cacheKeyBuilder is user-controlled

collisions are not detected

cache entries store decoded payload

cached payload is returned without binding validation

This creates a trust boundary break between:

token → cache key → cached payload

Proof of concept

Environment:

fast-jwt: 6.1.0

Node.js: v24.13.1

PoC:

const { createSigner, createVerifier } = require('fast-jwt')

const sign = createSigner({ key: 'secret' })

// Two distinct tokens const t1 = sign({ sub: 'userA', aud: 'admin' }) const t2 = sign({ sub: 'userB', aud: 'admin' })

// Deliberately unsafe cache key builder (collision) const verify = createVerifier({ key: 'secret', cache: true, cacheKeyBuilder: () => 'static-key' })

console.log('verify t1') const p1 = verify(t1) console.log('t1 PASS sub=', p1.sub)

console.log('verify t2') const p2 = verify(t2) console.log('t2 PASS sub=', p2.sub)

console.log('verify t2 again') const p3 = verify(t2) console.log('t2-again PASS sub=', p3.sub)

console.log('verify t1 again') const p4 = verify(t1) console.log('t1-again PASS sub=', p4.sub)

Observed output:

verify t1 t1 PASS sub= userA

verify t2 t2 PASS sub= userA

verify t2 again t2-again PASS sub= userA

verify t1 again t1-again PASS sub= userA

The verifier returns payload from userA when verifying userB.

Expected behavior

Cache must not allow returning claims from a different token.

Verification must remain bound to the actual token being validated.

Even if cache collisions occur, the verifier should:

revalidate signature

re-decode payload

or invalidate cache entry

Why this is not “just misuse”

This is not merely a user mistake.

Reasons:

fast-jwt explicitly exposes cacheKeyBuilder as an extension point.

The documentation suggests performance tuning via custom key builders.

No safeguards exist against collisions.

No verification binding is performed between:

cached payload

original token

The verifier trusts cache output as authoritative identity.

This creates a security-sensitive invariant:

"cache key uniqueness"

which is neither enforced nor validated.

Security-critical libraries must assume extension hooks can be misused and implement defensive checks, especially when identity decisions are derived from cached values.

Security classification

logical authorization flaw

cache confusion vulnerability

identity boundary break

Closest CWE:

CWE-440 — Expected Behavior Violation

Suggested fix (minimal and safe)

Bind cache entries to token integrity.

Option A — safest:

Store token hash along with payload and verify match before returning cache.

Conceptual patch:

const tokenHash = hashToken(token)

cache.set(key, { tokenHash, payload })

...

const entry = cache.get(key)

if (entry && entry.tokenHash === hashToken(token)) { return entry.payload }

Option B — simpler:

Disable cache usage when custom cacheKeyBuilder is provided.

Option C — defensive:

Always re-validate signature when cache hit occurs.

Notes

Default cacheKeyBuilder is safe (hash-based).

Issue appears when custom builders are used — a documented and supported feature.

Impact increases in:

API gateways

auth middleware

RBAC layers relying on payload.sub / payload.aud

This vulnerability is independent from:

RegExp statefulness issue

ReDoS claim validation issue

It is a separate flaw in cache design and trust model.

PoC did on my computer: 'use strict'

const fs = require('node:fs') const path = require('node:path') const { createSigner, createVerifier } = require('./src')

function nowSec() { return Math.floor(Date.now() / 1000) }

const sign = createSigner({ key: 'secret' }) const t1 = sign({ sub: 'userA', aud: 'admin', iat: nowSec() }) const t2 = sign({ sub: 'userB', aud: 'admin', iat: nowSec() })

function badKeyBuilder() { return 'aud=admin' }

const verify = createVerifier({ key: 'secret', cache: true, cacheTTL: 60000, cacheKeyBuilder: badKeyBuilder })

function run(tok) { try { const out = verify(tok) return { ok: true, sub: out.sub, aud: out.aud } } catch (e) { return { ok: false, code: e.code || String(e), message: e.message } } }

const results = [] results.push({ step: 'verify(t1)', token: 't1', result: run(t1) }) results.push({ step: 'verify(t2)', token: 't2', result: run(t2) }) results.push({ step: 'verify(t2) again', token: 't2', result: run(t2) }) results.push({ step: 'verify(t1) again', token: 't1', result: run(t1) })

const evidence = { title: 'fast-jwt cache confusion when cacheKeyBuilder collisions occur', environment: { node: process.version, fastJwt: require('./package.json').version }, config: { cache: true, cacheTTL: 60000, cacheKeyBuilder: "returns constant key 'aud=admin' (realistic collision pattern)" }, tokens: { t1: { claims: { sub: 'userA', aud: 'admin' }, jwt: t1 }, t2: { claims: { sub: 'userB', aud: 'admin' }, jwt: t2 } }, observed: results }

const outPath = path.join(process.cwd(), 'evidence-cache-keybuilder-confusion.json') fs.writeFileSync(outPath, JSON.stringify(evidence, null, 2)) console.log('Wrote evidence to:', outPath)

for (const r of results) { console.log(r.step, '=>', r.result.ok ? PASS sub=${r.result.sub} : FAIL ${r.result.code}) }

Output: PS C:\Users\Franciny Rojas\Desktop\crypto-research\fast-jwt> node poc_cache_keybuilder_confusion_evidence.js Wrote evidence to: C:\Users\Franciny Rojas\Desktop\crypto-research\fast-jwt\evidence-cache-keybuilder-confusion.json verify(t1) => PASS sub=userA verify(t2) => PASS sub=userA verify(t2) again => PASS sub=userA verify(t1) again => PASS sub=userA PS C:\Users\Franciny Rojas\Desktop\crypto-research\fast-jwt>