Improper Validation of Specified Type of Input Affecting fast-jwt package, versions <6.3.0
Threat Intelligence
Snyk has a proof-of-concept or detailed explanation of how to exploit this vulnerability.
The probability is the direct output of the EPSS model, and conveys an overall sense of the threat of exploitation in the wild. The percentile measures the EPSS probability relative to all known EPSS scores. Note: This data is updated daily, relying on the latest available EPSS model version. Check out the EPSS documentation for more details.
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Test your applications- Snyk IDSNYK-JS-FASTJWT-20825052
- published11 Oct 2026
- disclosed8 Oct 2026
- creditRitvik Arya
Introduced: 8 Oct 2026
NewCVE-2026-107723 (opens in a new tab)How to fix?
Upgrade fast-jwt to version 6.3.0 or higher.
Overview
fast-jwt is a Fast JSON Web Token implementation
Affected versions of this package are vulnerable to Improper Validation of Specified Type of Input via the decode function in src/decoder.js, where a JWT with a JSON array as its payload passes the object type check because typeof [] === 'object' in JavaScript. As a result, every configured claim validator (exp, nbf, iss, aud, sub, jti, nonce) is silently skipped for array-shaped payloads, because the claim in payload guard always returns false for arrays. An authenticated attacker can forge a token with an array payload to bypass all claim validation and gain unauthorized access.