libcurl can in some circumstances reuse the wrong connection when asked to do an Negotiate-authenticated HTTP or HTTPS request.
libcurl features a pool of recent connections so that subsequent requests can reuse an existing connection to avoid overhead.
When reusing a connection a range of criterion must first be met. Due to a logical error in the code, a request that was issued by an application could wrongfully reuse an existing connection to the same server that was authenticated using different credentials. One underlying reason being that Negotiate sometimes authenticates *connections* and not *requests*, contrary to how HTTP is designed to work.
An application that allows Negotiate authentication to a server (that responds
wanting Negotiate) with user1:password1 and then does another operation to
the same server also using Negotiate but with user2:password2 (while the
previous connection is still alive) - the second request wrongly reused the
same connection and since it then sees that the Negotiate negotiation is
already made, it just sends the request over that connection thinking it uses
the user2 credentials when it is in fact still using the connection
authenticated for user1...
The set of authentication methods to use is set with CURLOPT_HTTPAUTH.
Applications can disable libcurl's reuse of connections and thus mitigate this
problem, by using one of the following libcurl options to alter how
connections are or are not reused: CURLOPT_FRESH_CONNECT,
CURLOPT_MAXCONNECTS and CURLMOPT_MAX_HOST_CONNECTIONS (if using the
curl_multi API).