CVE-2026-13697: undici vulnerable to cross-user information disclosure and parse-time crash via degenerate private cache directives

Summary

Two issues in undici's cache interceptor, both fixed by the same patch on lib/util/cache.js:

1. Shared-cache disclosure: Responses with malformed qualified Cache-Control: private directives such as private="" or private="," can be incorrectly stored in the default shared cache, then served to a later caller with the same cache key. 2. Parse-time crash: Mixed unqualified-and-qualified private directives in the same header (such as public, max-age=60, private, private="hdr") cause an uncaught TypeError in the cache-control parser, terminating the request.

Impact

Shared-cache disclosure

Applications using interceptors.cache() in shared mode may cache a user-specific response and serve it to a later caller with the same cache key. This can disclose private response bodies and headers, including Set-Cookie.

Required conditions:

- the cache interceptor is enabled in shared mode, including the default configuration; - an upstream returns a malformed directive such as Cache-Control: public, max-age=300, private=""; - another request later matches the same cache key, without a separating Vary header.

Parse-time crash

Applications using interceptors.cache() against an upstream that returns a Cache-Control header combining unqualified private with qualified private="..." see an uncaught TypeError: output.private.concat is not a function during response handling. The request rejects; depending on the consumer's error handling, the process may exit.

Details

private="" is parsed as { private: [''] }. The shared-cache guard only rejects private === true, so the response can be stored. When served from cache, the previous user's body and headers may be returned to a different user.

For the crash variant, an unqualified private directive sets output.private = true, then a subsequent qualified private="hdr" directive attempts output.private.concat(['hdr']), which throws because boolean has no concat method.

The patch routes the qualified-directive path through a shared helper that normalizes empty-after-trim arrays to true and preserves existing true values, closing both vectors.

Patches

Upgrade to undici 7.29.0 or 8.9.0. Both releases fix the qualified private directive handling that caused the shared-cache storage and the parser crash.

Workarounds

Until patched, avoid shared interceptors.cache() for user-specific responses, use type: 'private', or disable caching for affected origins.

Credit

Disclosure variant reported by @h0rk1p via HackerOne report #3817497.