Attack Chain

1. The attacker authenticates to the phpBB application.
2. The attacker crafts a malicious ZIP archive containing serialized PHP objects designed for remote code execution. This archive is designed to be processed by the phar:// stream wrapper.
3. The attacker uploads the crafted ZIP archive through the plupload functionality, potentially disguised as a legitimate attachment type.
4. The phpBB application processes the uploaded file. The application uses the phar:// stream wrapper to extract the contents of the uploaded ZIP file.
5. The application deserializes the malicious PHP objects, triggered by the imagick parameter in attachment settings.
6. Deserialization of the crafted PHP objects leads to arbitrary code execution on the server.
7. The attacker gains control of the web server, potentially escalating privileges.

Impact

Successful exploitation of CVE-2019-25685 allows an attacker to execute arbitrary code on the phpBB server. The attacker could gain complete control of the web server, potentially leading to data theft, website defacement, or denial of service. The impact is significant due to the potential for full system compromise. The number of victims is dependent on the number of phpBB installations exposed and targeted.

Recommendation

• Inspect web server logs for POST requests to attachment upload endpoints containing ZIP archives and the “phar://” wrapper in request parameters to detect potential exploit attempts. (Log Source: webserver, Rule: phpbb_phar_upload)
• Monitor phpBB file upload directories for the creation of unexpected files, particularly PHP scripts or other executable files. (Log Source: file_event, Rule: phpbb_suspicious_file_creation)
• Apply available patches or updates for phpBB to address CVE-2019-25685 as soon as possible.