CVE-2026-91768: IPv6 ACL bypass in FastCGI listen.allowed_clients due to partial address comparison (memcmp 12 bytes)
Partial address comparison can bypass listen.allowed_clients.
CVE published 2026-09-25 · Record updated 2026-09-28 · Snapshot checked 24 October 2026
Why this matters
Partial address comparison can bypass listen.allowed_clients.
CVSS 3.1: 6.5 (MEDIUM). Source: CVE assigning authority.
A severity score describes the vulnerability, not the probability that your own website has been compromised. Exposure depends on the installed version, configuration and reachable code path.
When to check your system
Affected PHP branches and the relevant FPM or OpenSSL code paths; match the vendor package to the advisory.
How to address it
Included in the September security releases. Use the applicable October security release or a later vendor-supported fix.
Record the installed package and vendor build, check the advisory’s prerequisites, back up, apply the vendor fix and test the affected site functions. Distribution packages may include backported fixes without changing the upstream version number.
Sources
This page summarises a selected advisory. It is not evidence that a particular website is vulnerable. Where an advisory covers several CVEs, check the linked record for this CVE’s exact affected range.