CVE-2025-66200 is a security vulnerability affecting Apache HTTP Server versions from 2.4.7 through 2.4.65. The flaw arises from an interaction between the mod_userdir module, the suexec wrapper, and the AllowOverride FileInfo directive in .htaccess files. Normally, suexec enforces execution of CGI scripts under the correct user ID to maintain isolation between users on shared hosting systems. However, if an attacker has permission to use the RequestHeader directive within an .htaccess file (enabled by AllowOverride FileInfo), they can manipulate HTTP request headers in a way that causes CGI scripts to run under an unexpected or incorrect user ID. This bypass undermines the security model of suexec, potentially allowing privilege escalation or unauthorized access to files and resources belonging to other users. The vulnerability does not require authentication beyond the ability to write .htaccess files with specific directives, which may be granted in some shared hosting or multi-user environments. The Apache Software Foundation addressed this issue in version 2.4.66 by correcting the interaction to prevent the bypass. No public exploits have been reported yet, but the vulnerability is considered significant due to the potential impact on confidentiality and integrity in multi-user web hosting contexts.

For European organizations, especially those operating shared web hosting services or multi-tenant environments using Apache HTTP Server, this vulnerability can lead to unauthorized privilege escalation and cross-user data access. Attackers able to write .htaccess files with RequestHeader permissions could execute CGI scripts as other users, potentially accessing sensitive data or modifying resources beyond their privileges. This threatens confidentiality and integrity of hosted applications and data. The impact is particularly critical for hosting providers, government agencies, and enterprises relying on Apache for web services, as it could facilitate lateral movement or data breaches. Additionally, disruption of service or defacement could occur if attackers leverage this to execute malicious scripts. The absence of known exploits reduces immediate risk but does not eliminate the threat, especially given the widespread use of Apache HTTP Server across Europe.

European organizations should immediately audit their Apache HTTP Server deployments to identify versions between 2.4.7 and 2.4.65. The primary mitigation is to upgrade all affected servers to Apache HTTP Server version 2.4.66 or later, which contains the fix. Where immediate upgrade is not feasible, administrators should restrict or disable the AllowOverride FileInfo directive in .htaccess files, particularly removing permissions for the RequestHeader directive to prevent exploitation. Additionally, review and tighten file system permissions and suexec configurations to ensure strict user isolation. Monitoring for unusual CGI execution patterns or unexpected user IDs can help detect exploitation attempts. Hosting providers should communicate with customers about the risk and enforce stricter controls on .htaccess usage. Implementing web application firewalls (WAFs) with rules to detect anomalous header manipulations may provide temporary protection. Regular vulnerability scanning and patch management processes should be reinforced to prevent similar issues.