What type of network security device is a web application firewall designed to protect against?

A web application firewall (WAF) is a specialized security device or solution designed to protect web applications by filtering and monitoring HTTP traffic between a web application and the Internet. The primary focus of a WAF is to protect against specific vulnerabilities and threats that are commonly associated with web applications. Among these vulnerabilities, SQL injection and Cross-Site Scripting (XSS) are particularly significant as they exploit how web applications interact with databases and user input.

SQL injection allows attackers to execute arbitrary SQL code on the database through input fields, leading to data exposure or corruption. XSS attacks enable the injection of malicious scripts into web pages viewed by other users, which can lead to theft of session cookies, redirection to malicious sites, or other harmful actions against users. A WAF operates by detecting and blocking these kinds of attacks, ensuring that only legitimate requests reach the web application.

While other types of network security devices address different types of threats — for instance, preventing Denial of Service attacks, blocking unauthorized access, or stopping malware spread through email — these are not the primary focus of a WAF. Thus, option B reflects the correct understanding of a web application firewall's role in protecting against common web-based threats that directly target the application layer.