FastCGI Security Flaw in PHP Affects IPv6 Address Validation

CVE-2026-91768

What is CVE-2026-91768?

CVE-2026-91768 is a security vulnerability affecting the FastCGI implementation within PHP, specifically relating to the handling of IPv6 address validation. FastCGI is a protocol designed to interface interactive web applications with web servers, enhancing performance by allowing persistent connections. This vulnerability arises from an inadequate check in which the FastCGI client only evaluates the first 12 bytes of a 16-byte IPv6 address. Consequently, it permits matches based on a /96 prefix instead of requiring an exact address match. This flaw could be exploited by attackers who can manipulate their source address to resemble an allowed address, potentially allowing unauthorized access to sensitive FastCGI endpoints.

Organizations utilizing PHP in conjunction with FastCGI could face significant operational and security risks if the vulnerability is not addressed. Attackers targeting this weakness could gain access to server resources that would otherwise be restricted, leading to a compromise of web applications and data confidentiality.

Potential impact of CVE-2026-91768

1. Unauthorized Access: The most immediate risk is the potential for attackers to gain unauthorized access to secured FastCGI endpoints by exploiting the flawed IPv6 address validation. This could lead to unauthorized actions being performed on behalf of legitimate users.

2. Data Breaches: By exploiting this vulnerability, threat actors could gain access to sensitive data processed by web applications. This could involve access to user credentials, private data, or critical operational information, leading to significant breaches of confidentiality and trust.

3. Service Disruption: With unauthorized access, attackers could manipulate or take control of web applications, leading to potential disruptions in service. This could exacerbate the impact on organizations by affecting availability and reliability, resulting in loss of revenue and reputational damage.

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