fast-jwt provides fast JSON Web Token (JWT) implementation. In 6.2.4, fast-jwt can classify raw serialized public JWK or JWKS JSON as an HMAC secret because src/crypto.js performDetectPublicKeyAlgorithms treats non-PEM strings as symmetric key material. If HS256 is explicitly allowed or inferred, an attacker who knows the exact serialized public-key bytes can use those bytes as an HMAC key and create a token containing arbitrary claims that createVerifier accepts. Serialization ordering or whitespace differences can prevent exploitation, and applications using supported PEM keys with an asymmetric-only algorithm allowlist are not affected. This issue is fixed in version 6.3.0.
fast-jwt before version 6.3.0 improperly validates the type of JWT payloads, allowing JSON arrays to bypass claim checks such as expiry, issuer, audience, and replay protections. This occurs because the payload type check accepts arrays without rejecting them, causing configured claim validations to be skipped silently. The vulnerability is fixed in version 6.3.0.
fast-jwt versions from 6.2.0 up to but not including 6.3.0 contain a vulnerability where RSA public keys with non-whitespace content before the PEM header are misclassified as HMAC secrets. This misclassification allows an attacker who knows the public key bytes to forge HS256 signatures, potentially bypassing authentication or authorization. The issue is resolved in version 6.3.0.
fast-jwt provides fast JSON Web Token (JWT) implementation. Prior to 6.3.1, fast-jwt createVerifier accepts an unsigned JWT when key is an empty string or null and algorithms is a non-empty allowlist. Falsy synchronous keys bypass prepareKeyOrSecret, allowedAlgorithms remains active, hasKey is false, and the empty signature avoids the verifySignature gate. An attacker can therefore submit a token containing arbitrary claims without possessing a signing key, resulting in authentication or authorization bypass. Claim validators still run, and non-empty keys, an empty key without algorithms, and the async key resolver path do not have this behavior. This issue is fixed in version 6.3.1.
fast-jwt provides fast JSON Web Token (JWT) implementation. Prior to 6.3.4, the fast-jwt createVerifier cache can continue accepting a previously valid, signed JWT after its exp time when caching is enabled and the token has exp but no iat. In src/verifier.js, cacheSet derives the exp cache deadline only when iat is present, so the cache falls back to cacheTTL, and a later cache hit returns the saved payload before verifyToken rechecks expiration. An attacker who can replay the same cached bearer token can extend access until the cache entry expires, but cannot forge a token through this issue. This issue is fixed in version 6.3.4.
fast-jwt provides fast JSON Web Token (JWT) implementation. Prior to 6.2.4, a critical authentication-bypass vulnerability in fast-jwt's async key-resolver flow allows any unauthenticated attacker to forge arbitrary JWTs that are accepted as authentic. When the application's key resolver returns an empty string (''), for example via the common keys[decoded.header.kid] || '' JWKS-style fallback, fast-jwt converts it to a zero-length Buffer, hands it to crypto.createSecretKey, derives allowedAlgorithms = ['HS256','HS384','HS512'] from it, and then verifies the token's signature against an empty-key HMAC. The attacker simply computes HMAC-SHA256(key='', input='${header}.${payload}'), which Node accepts without complaint — and the verifier returns the attacker-chosen payload (sub, admin, scopes, etc.) as authentic. This vulnerability is fixed in 6.2.4.
fast-jwt provides fast JSON Web Token (JWT) implementation. From 5.0.0 to 6.2.0, a denial-of-service condition exists in fast-jwt when the allowedAud verification option is configured using a regular expression. Because the aud claim is attacker-controlled and the library evaluates it against the supplied RegExp, a crafted JWT can trigger catastrophic backtracking in the JavaScript regex engine, resulting in significant CPU consumption during verification. This vulnerability is fixed in 6.2.1.
fast-jwt provides fast JSON Web Token (JWT) implementation. Prior to 6.2.1, 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) and /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. This vulnerability is fixed in 6.2.1.
fast-jwt versions 6.1.0 and earlier do not validate the 'crit' (Critical) Header Parameter in JSON Web Tokens as required by RFC 7515 §4.1.11. This means that if a token contains a 'crit' array listing extensions that fast-jwt does not recognize, the library incorrectly accepts the token instead of rejecting it. This behavior violates the mandatory validation requirement defined in the RFC and can lead to insufficient verification of data authenticity.
fast-jwt provides fast JSON Web Token (JWT) implementation. From 0.0.1 to before 6.2.0, 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 and users being mis-identified as other users based on the wrong token. Version 6.2.0 contains a patch.
fast-jwt provides fast JSON Web Token (JWT) implementation. In 6.1.0 and earlier, the publicKeyPemMatcher regex in fast-jwt/src/crypto.js uses a ^ anchor that is defeated by any leading whitespace in the key string, re-enabling the exact same JWT algorithm confusion attack that CVE-2023-48223 patched.