In Eclipse Vert.x versions [4.0.0,4.5.35] or [5.0.0,5.2.2], when one "CachingWebClient" is shared across users — the documented pattern of a per-user "WebClientSession" over a shared caching client — a response that is specific to one user but is cacheable and not marked "private"/"no-store" is stored in the client-level shared cache tier and served to other users requesting the same URL. A later user's session, or an anonymous caller through the same client, can receive an earlier user's response. Depending on the response this discloses authenticated content or a session cookie, the latter enabling session fixation or hijacking.