Advisory Published


First published: Mon Nov 20 2023(Updated: )

### Summary 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: ```javascript 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: }; 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 -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: `` ``` python3 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 ``. Copy the resulting JWT token and use with the following tool: `` ``` python /opt/jwt_tool/ --exploit k -pk public_key token ``` You will now get a resulting JWT token that is validly signed. ### Impact 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. ### Solution Change ```javascript const publicKeyPemMatcher = '-----BEGIN PUBLIC KEY-----' ``` to be regex: ```javascript const publicKeyPemMatcher = /^-----BEGIN( RSA)? PUBLIC KEY-----/ ```

Affected SoftwareAffected VersionHow to fix

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Frequently Asked Questions

  • What is the vulnerability ID for this issue?

    The vulnerability ID for this issue is GHSA-c2ff-88x2-x9pg.

  • What is the severity of GHSA-c2ff-88x2-x9pg?

    The severity of GHSA-c2ff-88x2-x9pg is medium with a severity value of 5.9.

  • What is the affected software for GHSA-c2ff-88x2-x9pg?

    The affected software for GHSA-c2ff-88x2-x9pg is the fast-jwt library with a version up to 3.3.2.

  • How does GHSA-c2ff-88x2-x9pg impact security?

    GHSA-c2ff-88x2-x9pg allows an attacker to exploit JWT algorithm confusion for certain types of public keys.

  • How can I fix GHSA-c2ff-88x2-x9pg?

    To fix GHSA-c2ff-88x2-x9pg, update the fast-jwt library to version 3.3.2 or higher.


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