CVE-2025-8871 is a deserialization vulnerability classified under CWE-502 affecting the Everest Forms Pro WordPress plugin up to version 1.9.7. The vulnerability stems from unsafe deserialization of untrusted data within the mime_content_type() function, which processes uploaded files. Specifically, when a form includes a non-required signature field alongside an image upload field, an unauthenticated attacker can craft malicious serialized PHP objects to inject into the deserialization process. This PHP Object Injection (POI) can lead to various malicious outcomes if a Property Oriented Programming (POP) chain is available in the environment, typically provided by other installed plugins or themes. Without a POP chain, the vulnerability cannot be exploited to achieve code execution or file manipulation. The vulnerability only affects sites running PHP versions below 8.0, as PHP 8 introduced changes that mitigate this deserialization flaw. The attack vector is remote with no authentication or user interaction required, but the attack complexity is high due to the need for a specific form configuration and the presence of a POP chain elsewhere in the WordPress installation. The CVSS 3.1 score of 5.6 reflects these factors, indicating a moderate risk. No public exploits or proof-of-concept codes have been reported yet, but the potential impact includes arbitrary file deletion, sensitive data disclosure, and remote code execution depending on the POP chain leveraged.

For European organizations using WordPress sites with the Everest Forms Pro plugin (version 1.9.7 or earlier) on PHP versions below 8.0, this vulnerability poses a moderate risk. If exploited, attackers could compromise website integrity by deleting files, exposing sensitive data, or executing arbitrary code, potentially leading to website defacement, data breaches, or further network compromise. The requirement for a specific form configuration and additional vulnerable plugins or themes to enable a POP chain limits the likelihood of exploitation but does not eliminate it. Organizations in sectors with high web presence such as e-commerce, government, healthcare, and finance could face reputational damage, regulatory penalties under GDPR for data breaches, and operational disruptions. The vulnerability's unauthenticated remote attack vector increases exposure, especially for publicly accessible websites. However, the absence of known exploits in the wild and the high attack complexity somewhat reduce immediate risk. Still, the presence of this vulnerability in a widely used WordPress plugin necessitates proactive mitigation to prevent potential exploitation.

European organizations should immediately assess their WordPress environments for the presence of Everest Forms Pro plugin versions up to 1.9.7, especially if running PHP versions below 8.0. Since no official patch links are provided yet, organizations should consider the following specific mitigations: (1) Upgrade PHP to version 8.0 or higher, which mitigates the vulnerability by changing deserialization behavior; (2) Remove or disable any non-required signature fields and image upload fields in forms to reduce attack surface; (3) Audit installed plugins and themes for known POP chains or unsafe deserialization patterns and remove or update them; (4) Implement Web Application Firewall (WAF) rules to detect and block suspicious serialized payloads targeting form submissions; (5) Restrict file upload types and validate uploaded content rigorously to prevent malicious object injection; (6) Monitor logs for anomalous form submissions or deserialization errors; (7) Follow vendor advisories for patches or updates to Everest Forms Pro and apply them promptly once available. These targeted steps go beyond generic advice by focusing on the specific conditions required for exploitation and the environment dependencies.