CVE-2026-107721: fast-jwt clockTolerance: Infinity silently bypasses both exp and nbf validation (and persists in the verifier cache)

Summary

createVerifier({ clockTolerance: Infinity }) silently bypasses both exp (expiry) AND nbf (not-before) validation. Any expired or not-yet-active token is accepted as valid. The same primitive also corrupts the verifier's internal cache so cached entries inherit infinite validity — they remain valid past a later developer-removed Infinity config until LRU eviction.

Vulnerable code

src/verifier.js:531-533 — option validation only rejects negative values, not Infinity:

js if (clockTolerance && (typeof clockTolerance !== 'number' || clockTolerance < 0)) { throw new TokenError(TokenError.codes.invalidOption, 'The clockTolerance option must be a positive number.') }

Infinity passes (it's a number, not less than 0, truthy).

src/verifier.js:583-602 — clockTolerance flows into the date-claim validators:

js if (!ignoreNotBefore) { validators.push({ ..., modifier: -clockTolerance }) // → -Infinity } if (!ignoreExpiration) { validators.push({ ..., modifier: +clockTolerance }) // → Infinity }

src/verifier.js:198-205 — applies modifier additively, producing always-pass comparisons:

js function validateClaimDateValue(value, modifier, now, greater, errorCode, errorVerb) { const adjusted = value 1000 + (modifier || 0) // → ±Infinity const valid = greater ? now >= adjusted : now <= adjusted // → always true ... }

Empirical PoC

js const { createSigner, createVerifier } = require('fast-jwt') const secret = 'test-secret-with-enough-length-to-pass'

const sign = createSigner({ key: secret, algorithm: 'HS256' }) const expiredToken = sign({ sub: 'alice', iat: Math.floor(Date.now()/1000) - 3600, exp: Math.floor(Date.now()/1000) - 1800, // expired 30 min ago })

const v1 = createVerifier({ key: secret }) try { v1(expiredToken) } catch (e) { console.log('baseline rejects:', e.message) } // → "The token has expired at ..."

const v2 = createVerifier({ key: secret, clockTolerance: Infinity }) console.log('bypass:', v2(expiredToken)) // → { sub: 'alice', iat: ..., exp: ... } ← expired token accepted as valid

// Not-yet-active token (nbf in 1 day) — same bypass const futureToken = sign({ sub: 'bob', iat: Math.floor(Date.now()/1000), nbf: Math.floor(Date.now()/1000) + 86400, }) console.log('bypass nbf:', v2(futureToken)) // → { sub: 'bob', ... } ← not-yet-active token accepted as valid

Verified against fast-jwt@HEAD on 2026-06-03 (commit pulled today).

Cache side-effect

The verifier's LRU cache uses clockTolerance to compute the cache entry's expiry window:

src/verifier.js:121-134: js cacheValue[1] = ... payload.nbf 1000 - clockTolerance : 0 // → -Infinity cacheValue[2] = payload.exp 1000 + clockTolerance // → Infinity const maxTTL = clockTimestamp + clockTolerance + cacheTTL // → Infinity

With clockTolerance: Infinity, cache entries are stored with [min=-Infinity, max=Infinity]. The cache-hit check (min === 0 || now < min || now <= max) always passes for cached tokens.

Consequence: a developer who briefly sets clockTolerance: Infinity (e.g., during debug) and then removes it will find that any verifications performed during the debug window remain cached as valid until LRU eviction (default 1000 entries).

Asymmetric hardening — the smoking-gun shape

src/signer.js:98, 104 CORRECTLY uses Number.isFinite() to reject Infinity for expiresIn and notBefore:

js expiresIn != null && Number.isFinite(expiresIn) ? Math.floor((iat + expiresIn) / 1000) : ...

The verifier's clockTolerance validation doesn't apply the same guard. The same < 0 check pattern is also applied to clockTimestamp (line 527-529) and cacheTTL (line 535-537) — both also accept Infinity.

This is the kind of asymmetry that often indicates a missed hardening pass: the sign-side was hardened against Infinity but the verify-side wasn't.

Threat model

The bug requires the developer to (mis)configure clockTolerance: Infinity. Realistic ways this happens:

1. Developer using Infinity as a sentinel for "disable expiry": common JS idiom; many libraries accept Infinity as "no limit." fast-jwt's signer treats Infinity as invalid (Number.isFinite false) but the verifier silently accepts it. 2. JSON / env-var misconfig: config file or env var sets clockTolerance to "Infinity" (string); Number("Infinity") === Infinity. Surprising via JSON.parse + Number cast or even JSON.parse('{"clockTolerance": null}') if the codec accepts null → Infinity. 3. Test config bleed: integration tests use Infinity to make tokens never expire during long-running tests; the config bleeds into production.

For a JWT library, silently disabling token expiry on a "looks like a non-negative number" input is a security boundary failure.

Suggested fix

Single-line addition: use Number.isFinite() consistent with signer.js:

js if (clockTolerance && (typeof clockTolerance !== 'number' || !Number.isFinite(clockTolerance) || clockTolerance < 0)) { throw new TokenError(TokenError.codes.invalidOption, 'The clockTolerance option must be a finite, non-negative number.') }

Same fix for clockTimestamp (line 527-529) and cacheTTL (line 535-537) for consistency.

Optional defense-in-depth: cap clockTolerance to a reasonable upper bound (e.g., 5 minutes = 300000 ms) with a process.emitWarning above that. Most legitimate use cases need <60s tolerance.

Affected versions

All versions since clockTolerance was first introduced in PR #193 (v1.5.1). Current main HEAD on 2026-06-03 is affected.

Reporter

Andrew Ridings (independent security researcher). Happy to coordinate disclosure timing and follow up with any clarifications. Email: ridingsa@gmail.com