Description

get-jwks: poisoned JWKS cache allows post-fetch issuer validation bypass

Summary

A vulnerability in get-jwks can lead to cache poisoning in the JWKS key-fetching mechanism.

Details

When the iss (issuer) claim is validated only after keys are retrieved from the cache, it is possible for cached keys from an unexpected issuer to be reused, resulting in a bypass of issuer validation. This design flaw enables a potential attack where a malicious actor crafts a pair of JWTs, the first one ensuring that a chosen public key is fetched and stored in the shared JWKS cache, and the second one leveraging that cached key to pass signature validation for a targeted iss value.

The vulnerability will work only if the iss validation is done after the use of get-jwks for keys retrieval, which usually is the common case.

PoC

Server code:

const express = require('express')
const buildJwks = require('get-jwks')
const { createVerifier } = require('fast-jwt')

const jwks = buildJwks({ providerDiscovery: true });
const keyFetcher = async (jwt) =>
    jwks.getPublicKey({
        kid: jwt.header.kid,...
Show more

Exploit server that generates the JWT pair and send the public RSA key to the victim server:

const { generateKeyPairSync } = require('crypto');
const express = require('express');
const pem2jwk = require('pem2jwk');
const jwt = require('jsonwebtoken');

const app = express();
const port = 3001;
const host = `http://localhost:${port}`;...
Show more

The first JWT token will create a cache entry with the chosen public key and have the following format:

RS256:testkid:http://localhost:3001/?:https://example.com

The second JWT has a valid iss, but will create the exact same cache key as the one before, leading to signature validation with the chosen public key, bypassing any future iss validations:

RS256:testkid:http://localhost:3001/?:https://example.com

Impact

Applications relying on get-jwks for key retrieval, even with iss validation post-fetching, allows attackers to sign arbitrary payloads which will be accepted by the verifiers used.

Solution

Escape each component used in the cache key, so delimiter collisions are impossible.

https://github.com/nearform/get-jwks/blob/57801368adf391a32040854863d81748d8ff97ed/src/get-jwks.js#L76

Mitigation

Aliases

References