CVE-2025-66200 is a vulnerability affecting Apache HTTP Server versions 2.4.7 through 2.4.65, specifically involving the interaction between the mod_userdir module and the suexec feature when the AllowOverride directive is set to FileInfo. The vulnerability arises because users who have permission to use the RequestHeader directive within .htaccess files can manipulate the execution context of CGI scripts. Normally, suexec is designed to run CGI scripts under a specific user ID to enforce security boundaries. However, due to this flaw, an attacker with limited privileges can bypass suexec's intended user ID enforcement, causing CGI scripts to run under an unexpected or unauthorized user ID. This can lead to unauthorized script execution with elevated privileges or under different user contexts, potentially enabling privilege escalation or unauthorized actions on the server. The vulnerability does not expose confidential data directly but impacts integrity and availability by allowing unauthorized script execution. Exploitation requires that the attacker has at least limited privileges to modify .htaccess files with RequestHeader usage but does not require user interaction. The vulnerability is remotely exploitable over the network. The Apache Software Foundation has addressed this issue in version 2.4.66, and users are strongly advised to upgrade. No public exploits have been reported to date, but the presence of this vulnerability in widely deployed Apache versions makes it a concern for many organizations. The underlying CWE is CWE-288 (Authentication Bypass Using an Alternate Path or Channel), indicating that the vulnerability allows bypassing intended authentication or execution controls.

For European organizations, this vulnerability poses a risk primarily to web servers running affected Apache HTTP Server versions with suexec enabled and AllowOverride set to FileInfo, permitting RequestHeader usage in .htaccess files. Exploitation could allow attackers with limited privileges to execute CGI scripts under unauthorized user IDs, potentially leading to privilege escalation or unauthorized actions on the server. This can compromise the integrity of hosted applications and may disrupt availability if malicious scripts are executed. While confidentiality is not directly impacted, the ability to run scripts under different user contexts can facilitate further attacks or lateral movement within the network. Organizations relying on Apache for hosting critical web services, especially those using shared hosting environments or multi-user setups, are at higher risk. The vulnerability could also affect compliance with European data protection regulations if exploited to alter or disrupt services. 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.

1. Upgrade Apache HTTP Server to version 2.4.66 or later immediately to apply the official patch addressing this vulnerability. 2. Review and restrict the use of AllowOverride directives, particularly limiting or disabling FileInfo overrides in .htaccess files unless absolutely necessary. 3. Audit all .htaccess files for the use of the RequestHeader directive and remove or restrict permissions for untrusted users to modify these files. 4. Harden suexec configurations by ensuring that only trusted users can deploy CGI scripts and that suexec wrappers are properly configured to enforce user boundaries. 5. Implement strict access controls on web server directories to prevent unauthorized modifications to .htaccess files. 6. Monitor web server logs for unusual CGI script executions or unexpected user IDs running scripts, which may indicate exploitation attempts. 7. Employ web application firewalls (WAFs) to detect and block suspicious HTTP header manipulations related to RequestHeader usage. 8. Conduct regular security assessments and penetration testing focusing on web server configurations and privilege boundaries to detect similar misconfigurations.