Cross-Site Scripting (XSS)
Cross-Site Scripting, commonly known as XSS, is a type of security vulnerability typically found in web applications. This vulnerability allows attackers to inject malicious scripts into websites viewed by other users. Here's an in-depth look at XSS:
History and Discovery
- Early Discovery: The first known XSS attack was discovered around 2000, but the term "Cross-Site Scripting" was coined later by Microsoft during the release of Internet Explorer 6 in 2001 to differentiate it from a similar-sounding vulnerability, Cross-Site Request Forgery (CSRF).
- Notable Incidents: Over the years, several high-profile incidents have highlighted the dangers of XSS, including attacks on PayPal, eBay, and even major social networking sites like Twitter and Facebook.
Types of XSS Attacks
- Stored XSS: The malicious script is permanently stored on the target server, such as in a database, message forum, or comment field. The script is then executed every time a user loads the affected page.
- Reflected XSS: This attack involves sending a malicious script to a user via a link, which the user must click on. The script is then reflected back to the user's browser, often through a search or error message. It does not get stored on the server.
- DOM-based XSS: This type of XSS occurs when client-side scripts write data to the Document Object Model (DOM) in an unsafe way, allowing the execution of arbitrary JavaScript code. Unlike other types, the server does not directly participate in the attack.
Impact of XSS
- Session Hijacking: Attackers can steal session tokens or cookies to impersonate the victim.
- Defacement: Changing the appearance of a website or displaying unwanted content.
- Phishing: Using the trusted website to conduct phishing attacks.
- Key Logging: Capturing keystrokes to steal sensitive information.
Prevention and Mitigation
- Input Validation: Ensure all user input is validated for type, length, format, and range before being accepted.
- Output Encoding: Encode user-supplied data before it is included in the HTML output. This prevents the browser from interpreting the data as code.
- Content Security Policy (CSP): Implement CSP headers to restrict the sources of content that can be loaded on your site.
- Use HttpOnly Flag: Set the HttpOnly flag on session cookies to prevent client-side script access.
- Sanitization: Use libraries like DOMPurify to sanitize HTML content.
- Security Headers: Utilize headers like X-XSS-Protection to enable the browser's built-in XSS filter.
References
See Also