Piotnet Forms WordPress Plugin Arbitrary File Upload Vulnerability (CVE-2026-4883)

The Piotnet Forms plugin for WordPress is vulnerable to arbitrary file upload due to insufficient file type validation in the ‘piotnetforms_ajax_form_builder’ function. This vulnerability affects all versions up to and including 2.1.40. The plugin employs an inadequate extension blacklist, blocking only extensions like .php, .phpt, .php5, .php7, and .exe, but failing to prevent uploads of potentially dangerous extensions like .phar or .phtml. An unauthenticated attacker can exploit this vulnerability to upload arbitrary files to the affected WordPress site’s server, which can lead to remote code execution. The vulnerability is only exploitable if a file upload field is present in a form.

Attack Chain

1. Unauthenticated attacker accesses a WordPress page containing a Piotnet Form with a file upload field.
2. Attacker crafts a malicious file (e.g., a .phar or .phtml file) containing malicious code.
3. Attacker submits the form, uploading the malicious file through the ‘piotnetforms_ajax_form_builder’ function.
4. The plugin’s insufficient file type validation allows the file to be uploaded to the server.
5. The attacker determines the upload path of the malicious file.
6. Attacker accesses the uploaded malicious file via a web browser request.
7. The web server executes the malicious code contained in the uploaded file (e.g., .phar or .phtml).
8. Attacker achieves remote code execution on the WordPress server.

Impact

Successful exploitation of this vulnerability allows unauthenticated attackers to upload arbitrary files, potentially leading to remote code execution on the affected WordPress server. This can result in complete compromise of the website, including data theft, defacement, or further malicious activities. The CVSS v3.1 base score for this vulnerability is 9.8, indicating a critical severity.

Recommendation

• Upgrade the Piotnet Forms plugin to a version beyond 2.1.40 to patch CVE-2026-4883.
• Implement a web server rule to block execution of PHP code from the /wp-content/uploads/piotnetforms/ directory to prevent uploaded files from being executed.
• Deploy the Sigma rule detecting uploads of files with dangerous extensions to the /wp-content/uploads/piotnetforms/ directory to identify potential exploitation attempts.