1. A cookie is set using the secure keyword for https://target
2. curl is redirected to or otherwise made to speak with http://target (same
   hostname, but using clear text HTTP) using the same cookie set
3. The same cookie name is set - but with just a slash as path (path='/').
   Since this site is not secure, the cookie should just be ignored.
4. A bug in the path comparison logic makes curl read outside a heap buffer
   boundary

The bug either causes a crash or it potentially makes the comparison come to
the wrong conclusion and lets the clear-text site override the contents of the
secure cookie, contrary to expectations and depending on the memory contents
immediately following the single-byte allocation that holds the path.

The presumed and correct behavior would be to plainly ignore the second set of
the cookie since it was already set as secure on a secure host so overriding
it on an insecure host should not be okay.

References

• https://nvd.nist.gov/vuln/detail/CVE-2025-9086
• https://hackerone.com/reports/3294999
• https://curl.se/docs/CVE-2025-9086.html
• https://curl.se/docs/CVE-2025-9086.json
• http://www.openwall.com/lists/oss-security/2025/09/10/1
• https://lists.debian.org/debian-lts-announce/2026/01/msg00002.html