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EUVD-2026-9980

fkie_cve-2023-48223

fast-jwt provides fast JSON Web Token (JWT) implementation. Prior to version 3.3.2, the fast-jwt library does not properly prevent JWT algorithm confusion for all public key types. 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. 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. Version 3.3.2 contains a patch for this issue. As a workaround, change line 29 of `blob/master/src/crypto.js` to include a regular expression.

GHSA-c2ff-88x2-x9pg — JWT Algorithm Confusion

Affected: npm: fast-jwt

### 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: 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;

co…

Most recent entries from all

EUVD-2026-9980

fkie_cve-2023-48223

GHSA-c2ff-88x2-x9pg — JWT Algorithm Confusion

gsd-2023-48223