Attack Chain

1. An unauthenticated attacker identifies a WordPress website using Contact Form 7 plugin version 2.6.7 or earlier.
2. The attacker crafts a malicious HTTP POST request targeting the WordPress REST API endpoint.
3. The POST request includes a large integer value for the iteration count parameter, which is passed directly to the hide_hidden_mail_fields_regex_callback() method.
4. The hide_hidden_mail_fields_regex_callback() method, lacking input validation, reads the attacker-controlled integer.
5. The method initiates an unbounded loop, performing preg_replace() operations based on the attacker-supplied iteration count.
6. Each preg_replace() operation consumes server memory.
7. The excessive number of iterations rapidly exhausts available server memory.
8. The PHP process crashes due to memory exhaustion, resulting in a denial-of-service condition for the website.

Impact

Successful exploitation of this vulnerability leads to a denial-of-service condition. Attackers can crash the PHP process on vulnerable WordPress websites by exhausting server memory. This can result in website downtime, impacting user experience and potentially leading to data loss or corruption. While the exact number of affected websites is unknown, the widespread use of Contact Form 7 makes this vulnerability a significant threat.

Recommendation

• Upgrade the Contact Form 7 WordPress plugin to a version greater than 2.6.7 to patch CVE-2026-25863.
• Deploy the Sigma rule Detect Contact Form 7 Uncontrolled Resource Consumption Attempt to your SIEM to detect malicious POST requests targeting the WordPress REST API.
• Monitor web server logs for abnormally large POST request sizes to the WordPress REST API endpoint, as this may indicate an attempted exploitation of CVE-2026-25863.