WebCore/platform/network/soup/SocketStreamHandleImplSoup.cpp in the libsoup network backend of WebKit, as used in WebKitGTK+ prior to version 2.20.0 or without libsoup 2.62.0, unexpectedly failed to use system proxy settings for WebSocket connections. As a result, users could be deanonymized by crafted web sites via a WebSocket connection.