Output encoding is the practice of transforming data into a safe format appropriate for its rendering context before including it in application output. By converting potentially dangerous characters into harmless representations, output encoding prevents user-supplied data from being interpreted as executable code in HTML, JavaScript, CSS, URL, or other output contexts -- providing the primary defense against cross-site scripting and injection vulnerabilities.
Output encoding applies context-specific transformations at the point where data is rendered into output. HTML encoding converts characters like angle brackets, ampersands, and quotes into HTML entities so they display as text rather than being parsed as markup. JavaScript encoding wraps values in safe representations for inclusion in script contexts, preventing string breakout and code injection. URL encoding percent-encodes special characters in URL parameters and path segments. CSS encoding protects against injection in style contexts. Each context requires its own encoding function because characters dangerous in one context may be safe in another. Modern web frameworks automate output encoding by default -- React escapes JSX expressions, Angular sanitizes template bindings, and server-side templating engines encode output unless explicitly bypassed. Developers must understand when they are operating outside auto-encoding protections (using dangerouslySetInnerHTML in React, or bypass directives in Angular) and apply manual encoding in those cases.
Output encoding is the most reliable defense against XSS because it operates at the exact point of vulnerability -- where untrusted data meets an output context. Unlike input validation, which must anticipate all possible attack patterns, output encoding simply ensures that data is always treated as data, never as code. This makes it effective against known and unknown attack patterns alike.
CDA emphasizes output encoding within VSD as the critical counterpart to input validation. Theater missions audit rendering pipelines for proper context-aware encoding, identify framework bypass patterns that reintroduce XSS risk, and ensure development teams understand the encoding requirements for every output context their applications use.