High [CVE-2026-15076] Information disclosure via improper cookie domain validation

In versions up to and including 4.5.29 (4.x branch) and 5.1.4 (5.x branch), the WebClientSession component of Eclipse Vert.x Web Client does not validate that the Domain attribute of a Set-Cookie response header matches the originating server's domain, in violation of RFC 6265 section 5.3.

An attacker who controls any server that the victim application contacts can inject a cookie scoped to an arbitrary third-party domain; because the session store performs no cross-domain ownership check, it stores and later transmits that cookie to the targeted domain.

When the victim application subsequently sends a request to the targeted domain using the same WebClientSession, it presents the attacker-injected cookie, causing the receiving service to process the request under the attacker's account.

Sensitive data included in the victim application's requests, such as payment amounts, card details, or other API payloads, may then be accessible to the attacker through their own account on that service.

The vulnerability arises from insufficient validation of the `Domain` attribute in `Set-Cookie` response headers, allowing a remote attacker to inject cookies scoped to arbitrary third-party domains.

This could lead to sensitive data exposure and unauthorized access if a victim application subsequently sends requests to the targeted domain, presenting the attacker-controlled cookie.