bad reuse of HTTP Negotiate connection ======================================
Project curl Security Advisory, March 11th 2026 [Permalink](https://curl.se/docs/CVE-2026-1965.html) 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](CVE-2014-0015.md), 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. - Affected versions: from curl 7.10.6 to and including 8.18.0 - Not affected versions: curl < 7.10.6 and >= 8.19.0 - Introduced-in: https://github.com/curl/curl/commit/e56ae1426c 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. - Fixed-in: https://github.com/curl/curl/commit/34fa034d9a390c4bd6 - Fixed-in: https://github.com/curl/curl/commit/f1a39f221d57354990 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 ------- - Reported-by: Zhicheng Chen - Patched-by: Daniel Stenberg Thanks a lot! -- / daniel.haxx.se || https://rock-solid.curl.dev -- Unsubscribe: https://lists.haxx.se/mailman/listinfo/curl-users Etiquette: https://curl.se/mail/etiquette.html
