async-http-client: Cookie header not stripped on cross-origin redirect

## Summary async-http-client leaks `Cookie` headers to cross-origin redirect targets. When following a redirect across a security boundary (different origin, or HTTPS→HTTP downgrade), the `propagatedHeaders()` method in `Redirect30xInterceptor.java` strips `Authorization` and `Proxy-Authorization` headers but does not strip `Cookie`, so session cookies and other sensitive cookie values are forwarded to the redirect target — which may be attacker-controlled. ## Details The vulnerability is in `client/src/main/java/org/asynchttpclient/netty/handler/intercept/Redirect30xInterceptor.java`. The caller computes `stripAuth` on each redirect: ```java boolean sameBase = request.getUri().isSameBase(newUri); boolean stripAuth = !sameBase || schemeDowngrade || stripAuthorizationOnRedirect; // ... requestBuilder.setHeaders(propagatedHeaders(request, realm, keepBody, stripAuth)); ``` `stripAuth` is `true` whenever the redirect crosses an origin, downgrades the scheme, or the caller opted in via `AsyncHttpClientConfig#isStripAuthorizationOnRedirect()`. In the vulnerable version, `propagatedHeaders()` only removes `Authorization` and `Proxy-Authorization` in that branch — `Cookie` is left untouched: ```java private static HttpHeaders propagatedHeaders(Request request, Realm realm, boolean keepBody, boolean stripAuthorization) { HttpHeaders headers = request.getHeaders() .remove(HOST) .remove(CONTENT_LENGTH); if (!keepBody) { headers.remove(CONTENT_TYPE); } if (stripAuthorization || (realm != null && (realm.getScheme() == AuthScheme.NTLM || realm.getScheme() == AuthScheme.SCRAM_SHA_256))) { headers.remove(AUTHORIZATION) .remove(PROXY_AUTHORIZATION); // BUG: COOKIE is not removed here, so cookies leak across the security boundary. } return headers; } ``` The companion test class `RedirectCredentialSecurityTest` covers `Authorization` / `Proxy-Authorization` stripping on cross-origin redirects and scheme downgrades, but has no coverage for `Cookie`, which is why the regression went unnoticed. ## Proof of concept ```java import org.asynchttpclient.*; AsyncHttpClient client = asyncHttpClient(); // trusted-api.com responds 302 -> https://evil.com Request request = new RequestBuilder("GET") .setUrl("https://trusted-api.com/endpoint") .setHeader("Cookie", "session=abc123; csrf=xyz789; api_key=secret") .setHeader("Authorization", "Bearer token123") .build(); client.executeRequest(request).get(); // Request seen by evil.com after the redirect: // Authorization: <stripped> // Cookie: session=abc123; csrf=xyz789; api_key=secret <-- leaked ``` ## Impact - **Session hijacking** — leaked session cookies allow impersonation. - **CSRF token theft** — CSRF tokens carried in cookies are disclosed. - **API key theft** — API keys stored in cookies are disclosed. - **Privacy** — tracking identifiers leak to third-party origins. Realistic attack paths: - Open-redirect in a trusted API endpoint. - Compromised CDN or API gateway injecting redirects. - MITM on a plaintext hop in the redirect chain. ## Fix Add `COOKIE` to the headers removed alongside `AUTHORIZATION` / `PROXY_AUTHORIZATION` on the security-boundary branch: ```java if (stripAuthorization) { headers.remove(AUTHORIZATION) .remove(PROXY_AUTHORIZATION) .remove(COOKIE); } else if (realm != null && (realm.getScheme() == AuthScheme.NTLM || realm.getScheme() == AuthScheme.SCRAM_SHA_256)) { headers.remove(AUTHORIZATION) .remove(PROXY_AUTHORIZATION); } ``` Note that the URI-scoped `CookieStore` will re-add any cookies that legitimately match the new target after `propagatedHeaders` returns, so legitimate cross-origin sessions tracked by the client are not broken. Fixed in **3.0.10** and **2.15.0** by commit [`3b0e3e9e`](https://github.com/AsyncHttpClient/async-http-client/commit/3b0e3e9e).