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(CONTENTLENGTH);

if (!keepBody) { headers.remove(CONTENTTYPE); }

if (stripAuthorization || (realm != null && (realm.getScheme() == AuthScheme.NTLM || realm.getScheme() == AuthScheme.SCRAMSHA256))) { headers.remove(AUTHORIZATION) .remove(PROXYAUTHORIZATION); // 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; apikey=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; apikey=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 / PROXYAUTHORIZATION on the security-boundary branch:

java if (stripAuthorization) { headers.remove(AUTHORIZATION) .remove(PROXYAUTHORIZATION) .remove(COOKIE); } else if (realm != null && (realm.getScheme() == AuthScheme.NTLM || realm.getScheme() == AuthScheme.SCRAMSHA256)) { headers.remove(AUTHORIZATION) .remove(PROXYAUTHORIZATION); }

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.