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.
mdeslaur | It looks like this requires a new API introduced in libsoup 2.62.0 to be fixed. We are not going to backport the new API. Marking releases older than bionic as ignored.