CVE-2026-3300 is a critical insecure deserialization vulnerability in Everest Forms Pro that enables unauthenticated remote code execution (RCE). The flaw originates from the plugin passing unsanitized user-supplied serialized strings to the PHP unserialize() function, facilitating PHP Object Injection. Attackers can leverage gadget chains to trigger sinks like call_user_func() and file_put_contents(), allowing the deployment of web shells and full server compromise. Immediate patching is required to prevent unauthorized system takeover and subsequent lateral movement within the hosting environment.

• Vulnerability Mechanics: PHP Object Injection

  • Root cause is the lack of input sanitization for serialized data within the plugin's core form-processing logic [CVE-2026-3300">1].
  • Triggered when attacker-controlled data is passed directly to the unserialize() function, enabling PHP Object Injection (POI) [2].
  • Execution flow is redirected via WordPress-specific gadget chains to reach dangerous sinks, specifically call_user_func() [4].

• Exploitation Vector: Unauthenticated Access

  • The vulnerability is reachable via unauthenticated HTTP POST requests, requiring no valid user session or credentials [2].
  • Public Proof-of-Concept (PoC) code has been disclosed, increasing the likelihood of automated scanning and mass exploitation by botnets [4].
  • Payloads typically employ file_put_contents() to write malicious PHP shells to the server's web root for persistent access [CVE-2026-3300">1].

• Impact Assessment: Critical System Risk

  • Carries a Critical CVSS rating due to the total loss of confidentiality, integrity, and availability of the affected host [2].
  • The extensive install base of Everest Forms Pro significantly expands the global attack surface for WordPress sites [3].
  • Successful exploitation allows for the exfiltration of sensitive database credentials and lateral movement across the internal network [4].

• Defensive Posture: Remediation and Detection

  • Immediate Patching: Upgrade Everest Forms Pro to the latest patched version provided by the vendor to eliminate vulnerable code paths [3].
  • WAF Integration: Implement signatures to identify and block POST requests containing PHP serialized object patterns (e.g., O:, a:, s:) [4].
  • Log Analysis: Monitor web server access logs for oversized, obfuscated payloads targeting plugin endpoints or signatures of unsafe PHP function calls [CVE-2026-3300">1].