wrong reuse of HTTP Negotiate connection ========================================
Project curl Security Advisory, April 29 2026 [Permalink](https://curl.se/docs/CVE-2026-5545.html) VULNERABILITY ------------- libcurl might in some circumstances reuse the wrong connection when asked to do an authenticated HTTP(S) request after a Negotiate-authenticated one, when both use the same host. 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 criteria must 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. An application that first uses Negotiate authentication to a server with `user1:password1` and then does another operation to the same server asking for any authentication method but for `user2:password2` (while the previous connection is still alive) - the second request gets confused and wrongly reuses the same connection and sends the new request over that connection thinking it uses a mix of user1's and user2's credentials when it is in fact still using the connection authenticated for user1... INFO ---- 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). The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2026-5545 to this issue. This flaw is similar to [CVE-2026-1965](CVE-2026-1965.md). 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 8.19.0 - Not affected versions: curl < 7.10.6 and >= 8.20.0 - 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 8.20.0 makes sure that connections using Negotiate are not wrongly reused. - Fixed-in: https://github.com/curl/curl/commit/33e43985b8f3b9e6669 RECOMMENDATIONS --------------- We suggest you take one of the following actions immediately, in order of preference: A - Upgrade to curl and libcurl 8.20.0 B - Apply the patch and rebuild libcurl C - Avoid using HTTP Negotiate in your application TIMELINE --------- It was reported to the curl project on April 1st 2026. We contacted distros@openwall on April 23. libcurl 8.20.0 was released on April 29th 2026, coordinated with the publication of this advisory. CREDITS ------- - Reported-by: Quac Tran and Ngoc Hieu - Patched-by: Stefan Eissing 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
