PHP-FPM FastCGI IPv6 Client Access Check /96 Prefix Bypass
CVE-2026-91768 Published on September 25, 2026

IPv6 ACL bypass in FastCGI listen.allowed_clients due to partial address comparison (memcmp 12 bytes)
The IPv6 branch of the FastCGI client access check compares only the first 12 bytes of a 16-byte IPv6 address, so listen.allowed_clients matches on a /96 prefix instead of the exact address. An attacker who can source an address sharing the first 96 bits with an allowed one passes the check and reaches the FastCGI endpoint.

Vendor Advisory NVD

Vulnerability Analysis

Attack Vector:
ADJACENT_NETWORK
Attack Complexity:
LOW
Privileges Required:
NONE
User Interaction:
NONE
Scope:
UNCHANGED
Confidentiality Impact:
HIGH
Integrity Impact:
NONE
Availability Impact:
NONE

Weakness Type

Incomplete Comparison with Missing Factors

The software performs a comparison between entities that must consider multiple factors or characteristics of each entity, but the comparison does not include one or more of these factors. An incomplete comparison can lead to resultant weaknesses, e.g., by operating on the wrong object or making a security decision without considering a required factor.


Products Associated with CVE-2026-91768

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Affected Versions

PHP Group PHP: