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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.
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1 valid exploit after Mallory filtered fakes, detection scripts, and README-only repos.
This is a standalone Node.js proof-of-concept and deliberately vulnerable web demonstration for CVE-2026-44351, an authentication-bypass flaw in fast-jwt versions through 6.2.3. The vulnerable condition is implemented in `server.js`, `demo.js`, and `fixed/demo.js`: an asynchronous JWT key resolver uses `keys[decoded.header.kid] || ''`. For an attacker-chosen unknown `kid`, fast-jwt 6.2.3 treats the empty value as a zero-length HMAC key. An attacker can therefore calculate a valid HS256, HS384, or HS512 signature without possessing the legitimate signing key and submit arbitrary JWT claims. `forge.js` is the primary exploit utility. It creates a forged token using an empty HMAC secret by default and supports configurable algorithm, key ID, subject, admin flag, role, expiration, output file, and JSON output. It also decodes JWT header/payload segments without signature verification. The default forged claims request administrator access and its generated output references the included `/admin` endpoint. `server.js` implements the vulnerable target on TCP port 3000. It serves static content from `public/`, exposes `/health`, issues legitimate tokens through `/login`, verifies bearer tokens using the vulnerable async resolver, and authorizes `/admin` solely on `payload.admin === true`. The public frontend (`public/app.js`) includes a browser-side pure-JavaScript HS256/HMAC implementation for demonstrating local empty-secret token creation; it stores a token in localStorage and uses it for requests to the target API. The application also contains intentionally hard-coded demonstration credentials. The repository contains a comparative patched demonstration under `fixed/`, pinned to fast-jwt 6.2.4, where the same empty-key condition is expected to fail with an invalid-key error. Docker resources build the vulnerable and fixed demos, with the vulnerable server published on host port 3000. Documentation in `README.md` and `docs/CVE-2026-44351.md` explains the vulnerable key-resolution path, conditions, impact, and remediation. No external command-and-control, data-exfiltration endpoint, or third-party network target is embedded; all exploit traffic is directed manually to a chosen target, represented by the local demonstration service.
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