Attack Chain

1. An attacker gains authenticated administrative access to an EGroupware instance.
2. The attacker navigates to the Admin -> Filemanager -> VFS Mounts section within EGroupware and clicks "Install custom templates" to mount the /etemplates directory with administrative write access.
3. The attacker crafts a malicious index.xet XML file that contains a payload, such as <textbox id="$row\touch /tmp/pwned_egw 2>/dev/null`"/>, embedding a shell command (touch /tmp/pwned_egw`) within a widget ID attribute.
4. The attacker uploads this malicious index.xet file to a specific path within the mounted VFS, typically /etemplates/admin/templates/default/index.xet, using the EGroupware file manager.
5. To trigger the vulnerability, the attacker then navigates to an EGroupware admin panel URL, such as https://<target>/egroupware/index.php?menuaction=admin.admin_ui.index, which causes the application to load and process the custom template.
6. During template processing, the Widget::expand_name() method is invoked, and the specially crafted widget ID containing the backtick shell command is passed to a PHP eval() call.
7. Due to the unescaped backticks, the PHP interpreter executes the embedded shell command (touch /tmp/pwned_egw 2>/dev/null) on the underlying Linux operating system.
8. The shell command runs with the privileges of the web server user (e.g., www-data), creating or modifying files on the server and achieving RCE.

Impact

Successful exploitation of CVE-2026-40187 allows an attacker with administrative EGroupware credentials to elevate their privileges to arbitrary OS command execution as the web server user. From this position, an attacker can access sensitive system files (e.g., database credentials), establish persistence mechanisms, pivot to other internal systems, or completely compromise the EGroupware server and its data. The vulnerability directly affects any EGroupware installation that is not using the official Docker deployment with its hardened php.ini settings or has had disable_functions modified. While no specific victim count is provided, all non-hardened EGroupware instances are at risk.

Recommendation

• Prioritize patching EGroupware installations to a version that addresses CVE-2026-40187 immediately.
• Ensure that PHP's disable_functions directive in php.ini is properly configured to block functions like exec, passthru, shell_exec, system, proc_open, and popen on all EGroupware servers, especially non-Docker deployments.
• Deploy the provided Sigma rule to detect suspicious process creation activities indicating potential exploitation of CVE-2026-40187.
• Enable comprehensive process_creation logging on all Linux servers hosting EGroupware to capture command line arguments, parent processes, and user information, which is critical for the Sigma rule.
• Block the execution of the commands identified in the IOCs (e.g., touch /tmp/pwned_egw, id > /tmp/pwned_egw) at endpoint protection layers where possible.