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curl / Docs / curl CVEs / wrong reuse of HTTP Negotiate connection
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CVE-2026-5545

wrong reuse of HTTP Negotiate connection

Project curl Security Advisory, April 29 2026 Permalink

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.

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 8.20.0 makes sure that connections using Negotiate are not wrongly reused.

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

Thanks a lot!