Prior to version 3.14.1, aio-libs AIOHTTP mistakenly strips the host‑only flag from cookies when the CookieJar is saved and later restored, causing the cookies to be treated as domain cookies. This conversion can expose session identifiers or other sensitive data to a wider set of subdomains, potentially enabling attackers to read or hijack user session information. The bug is triggered only when CookieJar.save() and CookieJar.load() are used, and it is fixed in AIOHTTP 3.14.1.

Any deployment of aio-libs AIOHTTP older than 3.14.1 that uses persistent cookie storage via CookieJar.save() or load() is impacted. The vulnerability does not depend on the operating system or specific Python environment, so all projects that import aiohttp and rely on that functionality are at risk.

The CVSS base score of 1.3 and the absence of an EPSS score or KEV listing indicate a very low overall risk. Exploitation requires local access to the persisted CookieJar or manipulation of the persistence process, limiting the attack surface to trusted users or compromised systems. While the flaw can lead to data leakage, the impact is constrained to cookie visibility and does not provide remote code execution or denial of service.