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.

Versions of Async HTTP Client prior to 1.13.2 are vulnerable to a form of targeted request manipulation called CRLF injection. This vulnerability was the result of insufficient validation of HTTP header field values before sending them to the network. Users are vulnerable if they pass untrusted data into HTTP header field values without prior sanitisation. Common use-cases here might be to place usernames from a database into HTTP header fields. This vulnerability allows attackers to inject new HTTP header fields, or entirely new requests, into the data stream. This can cause requests to be understood very differently by the remote server than was intended. In general, this is unlikely to result in data disclosure, but it can result in a number of logical errors and other misbehaviours.

Async Http Client (aka async-http-client) before 2.0.35 can be tricked into connecting to a host different from the one extracted by java.net.URI if a '?' character occurs in a fragment identifier. Similar bugs were previously identified in cURL (CVE-2016-8624) and Oracle Java 8 java.net.URL.