undici: Cache poisoning and cross-user data disclosure via cross-origin redirects (CVE-2026-109829)

The undici HTTP client library contains a vulnerability (CVE-2026-109829) in its cache interceptor when used in conjunction with the redirect interceptor. Specifically, when an originless Agent is configured with the interceptor composition compose(redirect(), cache()) and a shared cache, the cache key is generated based on the original request URL before the redirect occurs.

This allows the response from a cross-origin redirect to be stored under the key of the initial trusted URL. An attacker controlling the redirect target can poison the cache with malicious content, or cause private, user-specific data to be stored and served to unauthenticated callers if the redirect path contains sensitive information. The vulnerability affects versions 7.x (< 7.30.1) and 8.x (< 8.12.2).

Impact

Successful exploitation allows for persistent cache poisoning of trusted URLs or the disclosure of sensitive, private data across users within the same shared cache instance.

Recommendation

• Upgrade undici to version 7.30.1 or 8.12.2 immediately.
• If upgrading is not immediately possible, update the interceptor composition order to compose(cache(), redirect()).
• Ensure the use of per-origin dispatchers (Client or Pool) or disable shared caching for requests that are susceptible to redirects.