Cross-Site Scripting (XSS) prevention refers to the development practices and security controls that stop attackers from injecting malicious scripts into web pages viewed by other users. XSS vulnerabilities allow attackers to execute arbitrary JavaScript in victims' browsers, enabling session hijacking, credential theft, defacement, and malware distribution through trusted web applications.
XSS prevention requires a multi-layered approach. Output encoding transforms user-supplied data into safe representations before rendering in HTML, JavaScript, CSS, or URL contexts -- converting characters like angle brackets and quotes into their entity equivalents so browsers display them as text rather than interpreting them as code. Context-aware encoding is critical because data safe in an HTML context may be dangerous in a JavaScript or URL context. Content Security Policy (CSP) headers instruct browsers to restrict script execution to approved sources, blocking inline scripts and eval() that XSS payloads commonly exploit. Input validation rejects or sanitizes data that does not match expected patterns, though it should supplement rather than replace output encoding. Modern frontend frameworks including React, Angular, and Vue automatically encode output by default, significantly reducing XSS risk when developers avoid bypassing these protections. HTTPOnly cookie flags prevent JavaScript from accessing session tokens, limiting the impact of successful XSS. Trusted Types API provides a browser-native mechanism to prevent DOM-based XSS by requiring sanitization of strings assigned to dangerous DOM sinks.
XSS is one of the most prevalent web application vulnerabilities, appearing in applications of all sizes and complexity levels. Successful XSS attacks compromise user sessions, steal credentials, redirect users to malicious sites, and distribute malware through trusted domains. Stored XSS is particularly dangerous because it persists in the application database and executes for every user who views the affected content.
CDA prioritizes XSS prevention within VSD as a fundamental web security control. Theater missions implement CSP headers, audit template rendering for proper encoding, and train development teams on context-specific output encoding -- ensuring XSS defenses match the actual rendering contexts used in each application.