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CVE-2026-1965

bad reuse of HTTP Negotiate connection

Project curl Security Advisory, March 11th 2026 Permalink

VULNERABILITY

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).

INFO

The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2026-1965 to this issue.

This flaw is similar to CVE-2014-0015, but that flaw was for NTLM.

CWE-305: Authentication Bypass by Primary Weakness

Severity: Medium

AFFECTED VERSIONS

This flaw has existed since curl started to support Negotiate.

libcurl is used by many applications, but not always advertised as such!

This bug is not considered a C mistake. It is not likely to have been avoided had we not been using C.

This flaw also affects the curl command line tool.

SOLUTION

curl 8.19.0 makes sure that connections that may use Negotiate cannot reuse another Negotiate-using connection unless the credentials match.

RECOMMENDATIONS

We suggest you take one of the following actions immediately, in order of preference:

A - Upgrade to curl and libcurl 8.19.0

B - Apply the patch and rebuild libcurl

C - Avoid using HTTP Negotiate in your application

TIMELINE

It was reported to the curl project on February 4th 2026. We contacted distros@openwall on March 8.

libcurl 8.19.0 was released on March 11th 2026, coordinated with the publication of this advisory.

CREDITS

Thanks a lot!