CVE-2026-1965 bad reuse of HTTP Negotiate connection
libcurl may reuse authenticated connections with wrong credentials, allowing attackers to access data as another user.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-1965?
CVE-2026-1965 is a medium-severity authentication bypass vulnerability in Haxx Curl, published on 11 Mar 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official description
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).
Weakness: CWE-305
Which versions are affected by CVE-2026-1965?
It affects 1 product.
Products include Haxx Curl.
How do I fix CVE-2026-1965?
Apply the vendor's patch.
Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.
Is CVE-2026-1965 being exploited?
There is no known exploitation of CVE-2026-1965 in the wild and no public exploit code.
Other vulnerabilities in Haxx Curl
Sources
Also known as ALPINE-CVE-2026-1965, AZL-79676, BELL-CVE-2026-1965, CURL-CVE-2026-1965, DEBIAN-CVE-2026-1965, ECHO-3c02-5957-bbd3 and 6 more.