guzzlehttp/guzzle versions before 7.15.1 fail to preserve host-only cookie scope, storing the request host in the Domain field instead of marking cookies as host-only. Attackers controlling child hosts can receive host-only cookies intended only for parent hosts, potentially disclosing session identifiers and authorization tokens when the same cookie jar is reused across trust boundaries.

Affected: Packagist: guzzlehttp/guzzle

### Impact

In affected versions, `CookieJar` does not preserve whether a response cookie was set without a `Domain` attribute or with an empty one. A cookie without `Domain` is host-only and must be returned only to the exact host that set it. Under current cookie processing rules, an empty `Domain` value is also host-only. Guzzle instead stores the request host in the cookie's `Domain` field and later applies ordinary domain matching, as though the server had supplied a valid domain. For example, a host-only `sid=secret` cookie set by `example.com` can subsequently be sent to `child.example.com`. `FileCookieJar` and `SessionCookieJar` also persist the request host without recording the host-only state, so reloading a jar preserves the widened scope.

An attacker who controls or can observe a child host can therefore receive cookies that were intended only for its parent host. Depending on the cookie, this can disclose session identifiers, authorization tokens, or other sensitive state. Exploitation requires the application to enable Guzzle's cookie support, reuse the same built-in cookie jar, receive a host-only cookie from a parent host, and later make a matching request to a less-trusted child host. The cookie's other restrictions still apply. Its path must match, a `Secure` cookie is sent only over a secure connection, and an expired cookie is not sent.

Applications that do not use Guzzle's cookie support are not affected. Applications are also not affected by this dis…

Affected: Ubuntu:24.04:LTS: guzzle, Ubuntu:26.04:LTS: guzzle

guzzlehttp/guzzle versions before 7.15.1 fail to preserve host-only cookie scope, storing the request host in the Domain field instead of marking cookies as host-only. Attackers controlling child hosts can receive host-only cookies intended only for parent hosts, potentially disclosing session identifiers and authorization tokens when the same cookie jar is reused across trust boundaries.

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