Cross-Site Scripting (XSS)
Cross-Site Scripting, commonly known as XSS, is a type of web security vulnerability that allows attackers to inject malicious scripts into otherwise benign and trusted websites. XSS attacks occur when an attacker uses a web application to send malicious code, generally in the form of a browser side script, to different end users. The end user's browser has no way to know that the script should not be trusted, leading to the execution of attacker-controlled code.
Types of XSS Attacks
- Stored XSS (Persistent): The script is permanently stored on the target server, such as in a database or message forum. When a victim requests data containing the script, it is executed in the user's browser.
- Reflected XSS (Non-Persistent): The attacker's payload is part of the user's request to the web application, which then reflects the attack back to the user's browser. This type typically requires tricking the victim into clicking a malicious link.
- DOM-based XSS: The vulnerability exists in client-side scripts where the source of the data is in the DOM, the document object model of a page. The payload is executed as a result of modifying the DOM environment in the victim's browser used by the original client-side script.
History and Evolution
The term Cross-Site Scripting was first coined by Microsoft in the early 2000s. Initially, these attacks were not well understood, and many developers were unaware of the risks. The first documented case of an XSS attack was likely in 1999, but it wasn't until the early 2000s that the vulnerability became widely recognized. Over time, as web applications became more complex and interactive, the potential for XSS attacks grew. This led to:
- Development of tools like BeEF (The Browser Exploitation Framework) for testing and exploiting XSS vulnerabilities.
- Introduction of security features in browsers like Content Security Policy (CSP) to mitigate XSS risks.
- Enhancements in web application frameworks to help developers avoid common XSS pitfalls.
Impact of XSS
XSS vulnerabilities can lead to various security issues:
- Session Hijacking: An attacker can steal user cookies to impersonate the user.
- Keylogging: Capture keystrokes to steal credentials.
- Phishing Attacks: Display fraudulent forms to trick users into providing sensitive information.
- Website Defacement: Alter the appearance of the website.
- Malware Distribution: Force-download malware onto the user's system.
Prevention and Mitigation
To prevent XSS attacks:
- Validate and Sanitize Input: Ensure all user inputs are sanitized to remove potentially malicious scripts.
- Use HTTP-only Cookies: Set cookies with the HttpOnly flag to prevent access by client-side script.
- Implement Content Security Policy (CSP): This helps to specify which sources of content are allowed to be loaded on your site, reducing the risk of XSS.
- Output Encoding: Encode data before rendering it in HTML to prevent script execution.
- Web Application Firewalls (WAFs): Use WAFs to detect and block common XSS attack patterns.
External Resources
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