CVE-2026-21003 – JWT "none" Algorithm Bypass via kid Header Omission

Program Code (Node.js)

// jwt_verify_server.js - Vulnerable JWT verification
const jwt = require('jsonwebtoken');
const express = require('express');
const app = express();
app.use(express.json());

const publicKey = `-----BEGIN PUBLIC KEY-----
MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAu1SU1LfVLPHCYZM5rPY4...
-----END PUBLIC KEY-----`;

app.post('/verify', (req, res) => {
    const token = req.body.token;
    // Vulnerability: if no algorithm specified, defaults to HS256? Actually, jsonwebtoken can be tricked.
    // We simulate a custom verifier that accepts "none" if kid is missing.
    const decoded = jwt.decode(token, { complete: true });
    if (!decoded.header.alg || decoded.header.alg === 'none') {
        // Accept token without signature
        res.json({ status: 'authenticated', user: decoded.payload.sub });
    } else {
        jwt.verify(token, publicKey, { algorithms: ['RS256'] }, (err, payload) => {
            if (err) res.status(401).send('Invalid');
            else res.json({ status: 'authenticated', user: payload.sub });
        });
    }
});
app.listen(3000);

CVE-2026-21003 – JWT "none" Algorithm Bypass via kid Omission

Overview

A custom JWT verifier does not properly enforce that a signing algorithm is present. If the token’s header omits the alg parameter or explicitly sets it to none, the server accepts the token without verifying a signature, allowing privilege escalation.

Vulnerability Details

Type: Authentication Bypass
Impact: Impersonation of any user.
Root Cause: The code path for missing algorithm defaults to skipping signature verification.

Exploit Demonstration

Start the server:

npm install express jsonwebtoken
node jwt_verify_server.js

Run the exploit:

pip install pyjwt requests
python exploit_jwt_none.py

Response shows authenticated as admin.

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CVE-2026-21003-JWT-none-Algorithm-Bypass-via-kid-Header-Omission