CVE-2026-1965 — bad reuse of HTTP Negotiate connection

Affected: curl

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`, `CURLO…

USN-8084-1 — curl vulnerabilities

Affected: Ubuntu:22.04:LTS: curl, Ubuntu:24.04:LTS: curl, Ubuntu:25.10: curl

Zhicheng Chen discovered that curl could incorrectly reuse the wrong connection for Negotiate-authenticated HTTP or HTTPS requests. This could result in the use of credentials from a different connection, contrary to expectations. (CVE-2026-1965)

It was discovered that curl incorrectly leaked OAuth2 bearer tokens when following a redirect. This could result in tokens being sent to the wrong host, contrary to expectations. (CVE-2026-3783)

Muhamad Arga Reksapati discovered that curl incorrectly reused existing HTTP proxy connections even if the request used different credentials. This could result in the use of incorrect credentials, contrary to expectations. (CVE-2026-3784)

Daniel Wade discovered that curl incorrectly handled certain memory operations when doing a second SMB request to the same host. An attacker could use this issue to cause curl to crash, resulting in a denial of service, or possibly execute arbitrary code. This issue only affected Ubuntu 25.10. (CVE-2026-3805)

Yihang Zhou discovered that curl incorrectly reused .netrc file credentials when following redirects. This could result in the use of credentials for a different host, contrary to expectations. This issue only affected Ubuntu 22.04 LTS and Ubuntu 24.04 LTS. (CVE-2025-0167)

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CVE-2026-1965 — bad reuse of HTTP Negotiate connection

USN-8084-1 — curl vulnerabilities