auto client cert connection reuse mixup
=======================================

Project curl Security Advisory, October 14 2026
[Permalink](https://curl.se/docs/CVE-2026-86415.html)

VULNERABILITY
-------------

libcurl's connection cache matcher never considers the automatic client
certficate option, so a cached, already authenticated Schannel connection can
be handed to a transfer that just got redirected to a different origin, even
though the code explicitly switched off the auto client cert option for that
transfer to prevent exactly this.

INFO
----

This flaw only affects curl builds using Schannel, which is a TLS backend only
available on Windows.

The Common Vulnerabilities and Exposures (CVE) project has assigned the name
CVE-2026-86415 to this issue.

CWE-201: Information Exposure Through Sent Data

Severity: Low

AFFECTED VERSIONS
-----------------

- Affected versions: curl 8.21.0 to and including 8.22.0
- Not affected versions: curl < 8.21.0 and >= 8.23.0
- Introduced-in: https://github.com/curl/curl/commit/d69bfad3fa3daf5e72331f68

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

This flaw also affects the curl command line tool.

SOLUTION
------------

- Fixed-in: https://github.com/curl/curl/commit/00271cfcb95b6481a1f91ce0a0dad

RECOMMENDATIONS
--------------

 A - Upgrade curl to version 8.23.0

 B - Apply the patch to your local version

 C - Build curl with a different TLS backend

TIMELINE
--------

This issue was reported to the curl project on September 3, 2026.

curl 8.23.0 was released on October 14, 2026, coordinated with the publication
of this advisory.

CREDITS
-------

- Reported-by: Stanislav Fort (Aisle Research)
- Patched-by: Stefan Eissing

Thanks a lot!