Attack Chain

1. An attacker authenticates to the e107 CMS application with an account that has theme installation permissions.
2. The attacker crafts a malicious theme package containing a PHP web shell (e.g., payload.php).
3. The attacker uploads the malicious theme package via the theme.php endpoint.
4. The e107 CMS installs the uploaded theme, placing the web shell (e.g., payload.php) within the e107_themes directory.
5. The attacker sends an HTTP request to the deployed web shell (e107_themes/payload.php).
6. The web shell executes arbitrary system commands specified in the HTTP request parameters (e.g., payload.php?cmd=whoami).
7. The server executes the command, and the web shell returns the output to the attacker.
8. The attacker uses the web shell to perform further actions such as escalating privileges, installing malware, or exfiltrating data.

Impact

Successful exploitation of CVE-2021-47937 leads to remote code execution, allowing attackers to gain complete control over the e107 CMS server. An attacker could potentially deface websites, steal sensitive data, install malware, or use the compromised server as a foothold for further attacks within the network. The CVSS v3.1 score of 8.8 highlights the high severity of this vulnerability.

Recommendation

• Apply any available patches or upgrades provided by e107 to address CVE-2021-47937.
• Restrict theme installation permissions to only highly trusted administrators to limit the attack surface.
• Deploy the Sigma rule Detect Suspicious e107 Theme Upload - CVE-2021-47937 to identify attempts to upload malicious theme files.
• Monitor web server logs for access to unusual PHP files within the e107_themes directory to detect web shell activity (e.g., payload.php).
• Implement strict file upload validation to prevent the upload of potentially malicious files.