How to Protect Against Cross-Site Scripting (XSS) in ASP.NET Core

Cross-Site Scripting (XSS) is a common web security vulnerability that occurs when an attacker injects malicious scripts into web pages viewed by other users. In ASP.NET Core applications, protecting against XSS is critical to safeguarding user data, maintaining trust, and ensuring application integrity. Fortunately, ASP.NET Core provides strong built-in defenses when used correctly.

One of the most important protections is automatic output encoding. Razor views in ASP.NET Core HTML-encode data by default when rendering variables using @Model.Property or @variable. This prevents malicious scripts from being executed in the browser. Developers should avoid bypassing this feature unless absolutely necessary. Methods such as @Html.Raw() should be used with extreme caution and only with fully trusted and sanitized content.

Input validation and sanitization are also essential. While output encoding is the primary defense against XSS, validating user input helps reduce the risk of storing or processing malicious data. ASP.NET Core model validation can enforce length limits, formats, and required fields. For scenarios where rich text input is required, using a trusted HTML sanitizer library helps remove dangerous tags and attributes.

Another effective measure is implementing a Content Security Policy (CSP). CSP is an HTTP response header that restricts which scripts, styles, and other resources a browser is allowed to load. By limiting script sources and disallowing inline scripts, CSP can significantly reduce the impact of XSS vulnerabilities. ASP.NET Core middleware can be used to add and manage CSP headers consistently across the application.

Proper handling of JavaScript contexts is also important. When injecting data into JavaScript, HTML encoding alone is not sufficient. Developers should use JSON serialization methods such as JsonSerializer to safely embed data into scripts, ensuring it is correctly escaped for the JavaScript context.

Additionally, keeping frameworks and dependencies up to date helps protect against known vulnerabilities. Regular security updates to ASP.NET Core and third-party libraries reduce exposure to exploits that could lead to XSS attacks.

In conclusion, protecting against XSS in ASP.NET Core requires a layered approach. By relying on built-in output encoding, validating and sanitizing input, enforcing Content Security Policies, safely handling JavaScript contexts, and maintaining up-to-date dependencies, developers can significantly reduce the risk of XSS and build more secure web applications.

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December 15, 2025