The JWT Algorithm "none" Attack: The Vulnerability in 1 Line of Code

JWT authentication is everywhere. It's also one of the most misconfigured security mechanisms.

One line of code can compromise everything.

The Vulnerable Code

// ❌ This looks fine...
const decoded = jwt.verify(token, secret, {
  algorithms: ['HS256', 'none'], // 💀 The vulnerability
});

The Attack

// 1. Attacker takes a valid JWT:
eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.
eyJzdWIiOiIxMjM0NTYiLCJyb2xlIjoidXNlciJ9.
signature_here

// 2. Modifies the header to use "none" algorithm:
eyJhbGciOiJub25lIiwidHlwIjoiSldUIn0.
eyJzdWIiOiIxMjM0NTYiLCJyb2xlIjoiYWRtaW4ifQ.
// No signature needed!

// 3. Server accepts it because "none" is in algorithms list
// Attacker is now admin

Real CVEs

CVE	Library	Impact
CVE-2015-2951	jwt-simple	Algorithm confusion
CVE-2016-10555	jose2go	None algorithm bypass
CVE-2018-0114	node-jose	Key confusion

The Fix

// ✅ Explicitly whitelist algorithms
const decoded = jwt.verify(token, secret, {
  algorithms: ['HS256'], // Only what you use!
});

All JWT Vulnerabilities

1. Algorithm None

// ❌ Dangerous
jwt.verify(token, secret, { algorithms: ['none'] });

// ✅ Safe
jwt.verify(token, secret, { algorithms: ['HS256'] });

2. Algorithm Confusion

// ❌ Dangerous: RS256 token verified with symmetric secret
jwt.verify(token, publicKey);

// ✅ Safe: Explicit algorithm
jwt.verify(token, publicKey, { algorithms: ['RS256'] });

3. Weak Secret

// ❌ Dangerous: Brute-forceable
jwt.sign(payload, 'password123');

// ✅ Safe: Strong secret
jwt.sign(payload, process.env.JWT_SECRET); // 256+ bits

4. Missing Expiration

// ❌ Dangerous: Token valid forever
jwt.sign({ userId: 123 }, secret);

// ✅ Safe: Short expiration
jwt.sign({ userId: 123 }, secret, { expiresIn: '1h' });

5. Sensitive Payload

// ❌ Dangerous: Password in token (tokens can be decoded!)
jwt.sign({ userId: 123, password: 'secret' }, key);

// ✅ Safe: Only IDs
jwt.sign({ userId: 123 }, key);

ESLint Coverage

// eslint.config.js
import jwtPlugin from 'eslint-plugin-jwt';

export default [jwtPlugin.configs.recommended];

13 JWT Rules

Rule	CWE	What it catches
no-algorithm-none	CWE-347	Algorithm "none" allowed
no-algorithm-confusion	CWE-327	RS/HS confusion attacks
no-weak-secret	CWE-326	Brute-forceable secrets
no-hardcoded-secret	CWE-798	Secrets in code
no-sensitive-payload	CWE-312	PII in tokens
require-expiration	CWE-613	Missing exp claim
require-algorithm-whitelist	CWE-327	No explicit algorithms
require-issuer-validation	CWE-345	Missing iss check
require-audience-validation	CWE-345	Missing aud check
no-decode-without-verify	CWE-347	jwt.decode() misuse
require-issued-at	CWE-613	Missing iat claim
require-max-age	CWE-613	No maxAge in verify
no-timestamp-manipulation	CWE-345	Clock skew exploits

Error Messages

src/auth.ts
  15:3  error  🔒 CWE-347 CVSS:9.8 | JWT algorithm 'none' is allowed
               Risk: Attackers can forge tokens without a signature
               Fix: Remove 'none' from algorithms: ['HS256']

Quick Install

📦 npm install eslint-plugin-jwt

import jwtPlugin from 'eslint-plugin-jwt';
export default [jwtPlugin.configs.recommended];

13 rules. Full JWT security. Zero false positives.

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📦 npm: eslint-plugin-jwt
📖 Rule: no-algorithm-none

⭐ Star on GitHub

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🚀 Check your JWT config now. Is "none" in your algorithms?

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