fast-jwt: Stateful RegExp (/g or /y) causes non-deterministic allowed-claim validation (logical DoS)
Impact
Using certain modifiers on RegExp objects in the allowedAud, allowedIss, allowedSub, allowedJti, or allowedNonce options in verify functions can cause certain unintended behaviours. This is because some modifiers are stateful and will cause failures in every second verification attempt regardless of the validity of the token provided.
Such modifiers are:
- /g : Global matching
- /y : Sticky matching
This does NOT allow invalid tokens to be accepted, only for valid tokens to be improperly rejected in some configurations. Instead it causes 50% of valid authentication requests to fail in an alternating pattern, leading to:
- Intermittent user authentication failures
- Potential retry storms in applications
- Operational monitoring alerts
Affected Configurations
This vulnerability ONLY affects applications that:
- Use RegExp objects (not strings) in the allowedAud, allowedIss, allowedSub, allowedJti, or allowedNonce options
- Use stateful RegExp modifiers such a /g or /y
Example: allowedAud: /abc/g ← IMPACTED
Example: allowedAud: "/abc/" ← SAFE
Not Affected
- Applications using string patterns for audience validation (most common)
- Applications using RegExp patterns without stateful modifiers
Assessment Guide
To determine if you're affected:
Check if allowedAud, allowedIss, allowedSub, allowedJti, or allowedNonce options use RegExp objects (/pattern/ or new RegExp())
If yes, review the pattern for stateful modifiers like /g, /y
If no RegExp usage or no stateful modifiers, you are NOT affected
Mitigation Options
While a fix will be coming in the next version of the package you can take steps to mitigate the issue immediately by removing any such modifiers (/g, /y) from the regex.
Summary
fast-jwt accepts RegExp for allowedAud, allowedIss, allowedSub, allowedJti, and allowedNonce.
If the provided regular expression uses the g (global) or y (sticky) flag, verification becomes non-deterministic: the same valid token alternates between acceptance and rejection across successive calls.
This occurs because RegExp.prototype.test() is stateful when g/y is set (it mutates lastIndex), and fast-jwt reuses the same RegExp object without resetting lastIndex.
Affected component
src/verifier.js
ensureStringClaimMatcher() returns the RegExp object directly.
validateClaimValues() performs repeated a.test(v) calls without resetting lastIndex.
Impact
Logical denial-of-service / authentication flapping.
A valid signed JWT can be intermittently rejected.
Causes unpredictable authentication outcomes across repeated verification calls.
Can trigger retry storms and cascading failures in API gateways and authentication middleware.
Affects any deployment that configures allowed* using RegExp and includes g or y flags.
Root cause
validateClaimValues() uses: allowed.some(a => a.test(v))
When a is a RegExp with g or y, a.test() mutates a.lastIndex.
Subsequent calls against the same input can return different results.
Proof of concept
Environment
- fast-jwt: 6.1.0 (repo HEAD)
- Node.js: v24.13.1
PoC
const { createSigner, createVerifier } = require('fast-jwt')
const sign = createSigner({ key: 'secret' })
const token = sign({ aud: 'admin', iss: 'issuer' })
function run(name, opts) {
const verify = createVerifier({ key: 'secret', ...opts })
console.log('\n==', name)
for (let i = 0; i < 8; i++) {
try { verify(token); console.log(i, 'PASS') }
catch (e) { console.log(i, 'FAIL', e.code || e.message) }
}
}
run('allowedAud global regex', { allowedAud: /^admin$/g })
run('allowedIss global regex', { allowedIss: /^issuer$/g })
run('control (non-global regex)', { allowedAud: /^admin$/ })
Observed behavior
- allowedAud with
/g alternates PASS/FAIL across calls - allowedIss with
/g alternates PASS/FAIL across calls - control regex (no g/y) is deterministic and always PASS
Expected behavior
Validation must be deterministic.
The same token under the same verifier configuration must always yield the same decision.
Suggested fix (minimal and safe)
Wrap RegExp matchers inside ensureStringClaimMatcher() to reset lastIndex before calling test():
if (r instanceof RegExp) {
return { test: v => { r.lastIndex = 0; return r.test(v) } }
}
This preserves semantics for non-global regexes, makes g/y deterministic, and avoids changes in the rest of the verifier logic.
Security classification
Logical DoS / authentication reliability failure.
This can be weaponized to produce production outages via retry storms and auth instability.
Why this is not “misuse”
- The library explicitly accepts RegExp for allowed* claim validation.
- The behavior difference is caused by internal state mutation of RegExp.test().
- The same token, same verifier config, same runtime yields different outcomes.
- Security decisions must be deterministic; non-determinism at the verification layer is a co
