The AsyncHttpClient (AHC) library allows Java applications to easily execute HTTP requests and asynchronously process HTTP responses. In versions from 2.0.0 prior to 2.16.0 and from 3.0.0.Beta1 prior to 3.0.11, ThreadSafeCookieStore stored a cookie under the value of its Domain attribute without verifying that the responding host is allowed to set a cookie for that domain, leading to a cookie tossing / cookie injection issue. A host the client connects to can therefore plant a cookie scoped to an unrelated domain, and the client will then send that cookie on later requests to that domain. Applications that use a single AsyncHttpClient instance - and thus the default, shared CookieStore - to reach both an attacker-influenced host and a trusted host are impacted. This issue has been fixed in versions 2.16.0 and 3.0.11.

Affected: Maven: org.asynchttpclient:async-http-client

### Impact A **cookie tossing / cookie injection** issue (CWE-1275). `ThreadSafeCookieStore` stored a cookie under the value of its `Domain` attribute without verifying that the responding host is allowed to set a cookie for that domain (RFC 6265 §5.3 step 6). A host the client connects to can therefore plant a cookie scoped to an unrelated domain, and the client will then send that cookie on later requests to that domain.

### Who is Impacted Applications that use a single `AsyncHttpClient` instance - and thus the default, shared `CookieStore` - to reach **both** an attacker-influenced host and a trusted host. Typical exposure: crawlers, link-preview / webhook fetchers, SSRF-style "fetch this URL" features, multi-backend aggregators, or following redirects to an attacker-controlled host. The attacker can *write* a cookie the client presents to the victim host (session fixation, overwriting a session id / CSRF-token cookie); they cannot *read* the victim host's cookies. Applications that talk only to a fixed trusted backend, or that disable/scope the cookie store, are not exposed.

### Patches Fixed in 3.0.11 and 2.16.0

### Workarounds - Disable the cookie store (setCookieStore(null)) when cookies are not needed; or - Use a separate AsyncHttpClient (separate cookie store) per trust domain so an attacker-influenced host and a trusted host never share a jar - Supply a custom CookieStore whose add(Uri, Cookie) rejects cookies whose Domain is not domain-matched by the request…

Affected: Ubuntu:Pro:14.04:LTS: async-http-client, Ubuntu:16.04:LTS: async-http-client, Ubuntu:18.04:LTS: async-http-client, Ubuntu:Pro:20.04:LTS: async-http-client, Ubuntu:22.04:LTS: async-http-client, Ubuntu:24.04:LTS: async-http-client, Ubuntu:25.10: async-http-client, Ubuntu:26.04:LTS: async-http-client

The AsyncHttpClient (AHC) library allows Java applications to easily execute HTTP requests and asynchronously process HTTP responses. In versions from 2.0.0 prior to 2.16.0 and from 3.0.0.Beta1 prior to 3.0.11, ThreadSafeCookieStore stored a cookie under the value of its Domain attribute without verifying that the responding host is allowed to set a cookie for that domain, leading to a cookie tossing / cookie injection issue. A host the client connects to can therefore plant a cookie scoped to an unrelated domain, and the client will then send that cookie on later requests to that domain. Applications that use a single AsyncHttpClient instance - and thus the default, shared CookieStore - to reach both an attacker-influenced host and a trusted host are impacted. This issue has been fixed in versions 2.16.0 and 3.0.11.