get-jwks contains fetch utils for JWKS keys. In versions prior to 11.0.2, a vulnerability in get-jwks can lead to cache poisoning in the JWKS key-fetching mechanism. 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. This issue has been patched in version 11.0.2.

Affected: npm: get-jwks

### 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:

```js 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, alg: jwt.header.alg, domain: jwt.payload.iss });

const jwtVerifier = createVerifier({ key: keyFetcher, allowedIss: 'https://example.com', });

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

app.use(express.json());

async function verifyToken(req, res, next) { const headerAuth = req.headers.authorization.split(' ') let token = ''; if (headerAuth.length > 1) { token = headerAuth[1]; }

const payload = awa…