This question focuses on API Security, Webhooks abuse, and Webshell mitigation, all of which are addressed in the CEH v13 Web Application Hacking and Security Operations modules. The most appropriate corrective action is Option D, as it directly addresses the root causes and persistence mechanisms of the compromise.

According to CEH v13, attackers frequently exploit improper input validation in APIs and insecure webhook implementations to inject malicious payloads or gain remote command execution. Webhooks, when not properly validated, can accept malicious requests that trigger backend processes, making them a common vector for API abuse.

Input validation on all API endpoints is critical to prevent injection attacks, including command injection and SQL injection. CEH v13 explicitly emphasizes validating request parameters, headers, and payload structures to prevent malicious manipulation.

Reviewing webhook payloads ensures that only trusted sources can trigger automated actions, preventing attackers from abusing webhook functionality. This includes validating signatures, enforcing authentication, and restricting allowed payload formats.

Finally, regular scanning for webshells is essential because webshells provide persistent backdoor access. CEH v13 identifies webshell detection as a key defensive measure during incident response and post-exploitation containment.

Other options are incomplete:

A WAF alone cannot detect all webshells.

IP blocking is ineffective against spoofed or cloud-based attacks.

MFA and server hardening are valuable but do not directly address webhook abuse or existing webshells.

Therefore, Option D provides the most comprehensive, CEH v13–aligned mitigation strategy.