A company runs a web application on Amazon EC2 instances behind an Application Load Balancer...

AWS WAF is the correct service because AWS documents that it protects web applications at Layer 7 and defends against common web exploits such as SQL injection and cross-site scripting. Associating WAF with the ALB allows malicious requests to be filtered before they reach the EC2 instances, which minimizes application impact and avoids pushing inspection logic onto the hosts. CloudTrail, GuardDuty, and network ACLs are not the best tools for active application-layer request filtering against SQLi and XSS. Shield Advanced can add further protection for broader attack scenarios, but the key control in this answer is WAF on the ALB with rules that block malicious traffic patterns.

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