stale SigV4 headers sent unintended
===================================

Project curl Security Advisory, October 14 2026
[Permalink](https://curl.se/docs/CVE-2026-86409.html)

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

Setting `CURLOPT_AWS_SIGV4` to NULL but does not disable SigV4 authentication,
contrary to expectations and documentation. A subsequent transfer on the same
handle still sends an `Authorization: AWS4-HMAC-SHA256` header to whatever URL
is set next, leaking state information.

INFO
----

An application that reuses an easy handle, retains `CURLOPT_USERPWD`
credentials, relies on the documented NULL reset, and then performs a request
to an attacker-influenced URL will unintentionally send a signed SigV4
`Authorization:` header to that destination. The header exposes the access-key
identifier and a request-bound signature. The secret key is not transmitted.

The Common Vulnerabilities and Exposures (CVE) project has assigned the name
CVE-2026-86409 to this issue.

CWE-201: Information Exposure Through Sent Data

Severity: Low

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

- Affected versions: curl 7.75.0 to and including 8.21.0
- Not affected versions: curl < 7.75.0 and >= 8.22.0
- Introduced-in: https://github.com/curl/curl/commit/08e8455dddc5e48e58a12ade3

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

This flaw is not accessible using the curl command line tool.

SOLUTION
--------

- Fixed-in: https://github.com/curl/curl/commit/c3c70440e8d9938c0bb732595751e8

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

 A - Upgrade curl to version 8.22.0

 B - Apply the patch to your local version

 C - Make sure no attempts to NULL this option is done

TIMELINE
--------

This issue was reported to the curl project on August 27, 2026.

curl 8.22.0 was released on September 2, 2026. This advisory was published on
October 14, 2026.

CREDITS
-------

- Reported-by: Stanislav Fort (Aisle Research)
- Patched-by: Viktor Szakats

Thanks a lot!