nearform fast-jwt up to 6.2.3 JSON Web Token improper authentication

Summaryinfo

A vulnerability identified as critical has been detected in nearform fast-jwt up to 6.2.3. The affected element is an unknown function of the component JSON Web Token Handler. The manipulation leads to improper authentication. This vulnerability is listed as CVE-2026-44351. The attack may be initiated remotely. There is no available exploit. You should upgrade the affected component.

Detailsinfo

A vulnerability was found in nearform fast-jwt up to 6.2.3. It has been rated as critical. This issue affects some unknown functionality of the component JSON Web Token Handler. The manipulation with an unknown input leads to a improper authentication vulnerability. Using CWE to declare the problem leads to CWE-287. When an actor claims to have a given identity, the product does not prove or insufficiently proves that the claim is correct. Impacted is confidentiality, integrity, and availability. The summary by CVE is:

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.

It is possible to read the advisory at github.com. The identification of this vulnerability is CVE-2026-44351 since 05/05/2026. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. The technical details are unknown and an exploit is not publicly available.

Upgrading to version 6.2.4 eliminates this vulnerability.

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Productinfo

Vendor

• nearform

Name

• fast-jwt

Version

• 6.2.0
• 6.2.1
• 6.2.2
• 6.2.3

Website

• Product: https://github.com/nearform/fast-jwt/

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