Attack Chain

1. An unauthenticated attacker identifies a WordPress site using a vulnerable version (<= 2.0.0) of the ProSolution WP Client plugin.
2. The attacker crafts a malicious HTTP POST request to the file upload endpoint of the plugin, typically located at /wp-content/plugins/wp-client/upload.php or a similar path.
3. The POST request includes a multipart/form-data payload containing an array of files. The first file is a benign file with a valid extension (e.g., .jpg, .png) and MIME type.
4. The subsequent file in the array is a malicious PHP script with a .php extension, disguised or named to evade basic detection.
5. The server-side validation process checks only the first file’s extension and MIME type, passing it.
6. The plugin processes and uploads all files in the array to the web-accessible directory, such as /wp-content/uploads/wp-client/.
7. The attacker then sends an HTTP request to the uploaded malicious PHP script (e.g., /wp-content/uploads/wp-client/malicious.php).
8. The server executes the PHP script, granting the attacker remote code execution on the server, enabling them to perform actions like data exfiltration, defacement, or further exploitation.

Impact

Successful exploitation of this vulnerability allows unauthenticated attackers to execute arbitrary code on the targeted WordPress server. This can lead to complete compromise of the website, including data theft, defacement, or use of the server as a platform for further attacks. Given the widespread use of WordPress and the popularity of the WP Client plugin, a large number of websites are potentially vulnerable. The impact of a successful attack is high, potentially leading to significant financial and reputational damage.

Recommendation

• Upgrade the ProSolution WP Client plugin to a version greater than 2.0.0 to patch CVE-2026-6555.
• Deploy the Sigma rule “Detect CVE-2026-6555 Exploitation Attempt — ProSolution WP Client Arbitrary File Upload” to your SIEM to detect potential exploitation attempts based on HTTP POST requests to the plugin’s upload endpoint.
• Monitor web server logs for HTTP requests to the /wp-content/uploads/wp-client/ directory containing PHP files, which could indicate successful exploitation of CVE-2026-6555.
• Implement strict file extension and MIME type validation on all file upload endpoints, ensuring that all files in an array are validated, not just the first one.