Attack Chain

1. An unauthenticated attacker submits a malicious serialized PHP object through a public Everest Forms form field.
2. The submitted payload bypasses the sanitize_text_field() function due to the function’s failure to remove serialization control characters.
3. The crafted serialized object is stored in the wp_evf_entrymeta database table associated with the form entry.
4. An administrator accesses the WordPress administration panel and navigates to the Everest Forms entries section.
5. The html-admin-page-entries-view.php file is executed to display form entries and their associated metadata.
6. The plugin retrieves the stored serialized object from the wp_evf_entrymeta table.
7. The unserialize() function is called on the retrieved data without the allowed_classes parameter, triggering PHP Object Injection.
8. The injected PHP object is instantiated, leading to arbitrary PHP code execution on the server, potentially granting the attacker complete control over the WordPress site.

Impact

Successful exploitation of this vulnerability (CVE-2026-3296) can lead to complete compromise of the WordPress website. An attacker can gain remote code execution, allowing them to inject malware, deface the site, steal sensitive data (including user credentials and financial information), or use the compromised server as part of a botnet. Given the widespread use of the Everest Forms plugin, a large number of WordPress sites are potentially vulnerable. The CVSS v3.1 base score of 9.8 reflects the critical severity of this vulnerability.

Recommendation

• Immediately update the Everest Forms plugin to the latest version (greater than 3.4.3) to patch CVE-2026-3296.
• Deploy the Sigma rule Detect Suspicious unserialize Call in Everest Forms to identify potential exploitation attempts in web server logs.
• Monitor web server logs for suspicious POST requests to WordPress form submission endpoints containing serialized PHP objects, as detected by the Detect Suspicious Form Submission with Serialized Data Sigma rule.
• Implement a Web Application Firewall (WAF) rule to block requests containing serialized PHP objects in form submission data.