ASP.NET Reflected XSS

What this means

Reflected Cross-Site Scripting (XSS) is a security vulnerability where a cyber attacker injects malicious scripts into a web application. When other users load affected pages, the attacker’s scripts run, allowing them to:

• steal cookies and session tokens
• change the contents of the web page through Document Object Model (DOM) manipulation
• redirect the browser to another page

XSS vulnerabilities typically occur when an application takes user input and outputs it to a page without validation.

Why this is a problem

XSS is an issue in ASP.NET because it compromises the security and integrity of web sites.

Some of the security risks of XSS in ASP.NET applications are:

• session hijacking – attackers can steal authentication cookies, allowing them to impersonate legitimate users
• data theft – malicious scripts can access or modify sensitive information displayed on web pages
• defacement or malware injection – scripts can alter page content or redirect users to harmful sites
• bypass of access controls – some exploits use XSS to escalate privileges or gain unauthorised access

How to check if the problem is there

To check if there’s a problem, you can use:

• browser developer tools – inspect the response source code to see if user input appears in the HTML output without escaping
• automated security scanners – submit various inputs using scanners such as BURP Suite or OWASP ZAP to see how the application responds
• penetration testing frameworks – use tools such as XSStrike or XSSer to check for vulnerabilities

How to fix this

To protect against this vulnerability, web developers should:

• use Content Security Policy (CSP) – by configuring a CSP header in your application you can restrict the execution of inline scripts
• use a Web Application Firewall (WAF) or Next Generation Firewall (NGFW) to detect and block malicious requests
• enable input validation – the web application should validate any input to the application before it is used