Which type of security tool would be least effective at preventing social engineering attacks but is crucial for defending against web-based attacks?

The Web Application Firewall (WAF) is specifically designed to monitor, filter, and protect web applications from various types of attacks, particularly those that exploit vulnerabilities in the application layer. These can include SQL injection, cross-site scripting (XSS), and other web-based threats. While it plays an essential role in defending against such attacks by inspecting HTTP traffic and blocking malicious requests, it is not effective in preventing social engineering attacks.

Social engineering relies on manipulating people into divulging confidential information through deception, often through means like phishing emails or phone calls, rather than exploiting technical vulnerabilities. Therefore, a WAF does not address the psychological and human factors involved in social engineering. In contrast, tools like Email Security are specifically designed to combat threats that arise from social engineering tactics, such as phishing attempts.

In summary, while the Web Application Firewall is critical for safeguarding web applications against specific technical threats, it does not provide protection against social engineering, thereby making it the least effective choice for that particular concern.