CVE-2026-1965

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 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).