This second action improperly handles file uploads by failing to validate the file type or verify user permissions, permitting the upload of arbitrary PHP files into a publicly accessible directory. The files are stored in wp-content/plugins/payment-gateway-pix-for-woocommerce/Includes/files/certs_c6/, where they can be executed directly by the web server. Successful exploitation grants the attacker remote code execution with the privileges of the web server user, potentially leading to full site takeover, data exfiltration, and establishment of persistent backdoors.

Attack Chain

1. The attacker performs an unauthenticated POST request to wp-admin/admin-ajax.php with the action lkn_pix_for_woocommerce_generate_nonce and action_name=lkn_pix_for_woocommerce_c6_settings_nonce.
2. The server returns a valid security nonce in the JSON response.
3. The attacker crafts a multipart POST request to wp-admin/admin-ajax.php using the action lkn_pix_for_woocommerce_c6_save_settings.
4. The request includes the previously acquired nonce and an arbitrary file (e.g., shell.php) in the certificate_crt_path parameter.
5. The plugin saves the malicious file to wp-content/plugins/payment-gateway-pix-for-woocommerce/Includes/files/certs_c6/.
6. The attacker navigates their browser to the direct URL of the uploaded file to trigger execution.
7. The server executes the PHP payload, providing the attacker with remote command execution capabilities.

Impact

Successful exploitation results in full Remote Code Execution (RCE) on the WordPress server. Attackers can leverage this access to steal sensitive configuration data such as wp-config.php, compromise database credentials, exfiltrate customer information, modify system files to plant backdoors, or gain administrative access to the WordPress site. The vulnerability affects all users running Pix for WooCommerce versions 1.5.0 and earlier.

Recommendation

• Immediately update Pix for WooCommerce to a version later than 1.5.0 that includes the patch for CVE-2026-3891.
• If an update is not immediately available, configure the web server to deny PHP execution within the certs_c6 directory using .htaccess or server configuration blocks.
• Implement a WAF rule to block requests to admin-ajax.php containing the lkn_pix_for_woocommerce_c6_save_settings action if initiated by unauthorized sources.
• Audit the certs_c6 directory for any unexpected PHP files and remove them immediately.
• Deploy the provided Sigma rule to detect suspicious AJAX requests and potential webshell uploads.