What does XSS mean?

Synonyms: Cross-Site Attack Script Injection

XSS stands for Cross-Site Scripting, a type of security vulnerability commonly found in web applications. In an XSS attack, malicious scripts are injected into otherwise legitimate websites, often through vulnerable input fields or URLs. These scripts are then executed in the browser of unsuspecting users, allowing attackers to steal sensitive information, hijack user sessions, or manipulate website content.

How does XSS work?

In an XSS attack, the attacker finds a way to inject malicious JavaScript into a webpage viewed by other users. This can happen through:

• User input fields: An attacker might insert malicious scripts into form fields (like comments or search boxes), which the website fails to sanitize properly.
• URLs: A website might inadvertently allow attackers to include harmful code in URLs, which then gets executed when users click on those links.

Once the malicious script is executed in the user’s browser, the attacker can:

• Steal cookies (session data) to impersonate the user.
• Redirect the user to phishing sites.
• Modify the page content to display misleading or harmful information.

Types of XSS Attacks

1. Stored XSS (Persistent XSS): The malicious script is permanently stored on the server (e.g., in a database). Whenever a user views the affected page, the script is executed. This is considered the most dangerous form of XSS since it can impact many users.
2. Reflected XSS: The malicious script is not stored on the server. Instead, it is part of the URL or request and is reflected back to the user in the web response. This type of XSS typically affects users who click on malicious links crafted by attackers.
3. DOM-Based XSS: In this variant, the vulnerability lies in the client-side JavaScript code. The attacker manipulates the Document Object Model (DOM) on the user’s browser to execute malicious scripts.

Why is XSS dangerous?

XSS is a critical web security vulnerability because it directly targets users, often without them knowing. Attackers can:

• Steal user credentials: Hijack user sessions and take control of accounts.
• Spread malware: Inject code that redirects users to malicious websites.
• Deface websites: Change content on web pages to show harmful or misleading information.

How to prevent XSS

To protect a website from XSS attacks, developers need to take the following precautions:

• Input sanitization: Ensure that user input is properly filtered and escaped to remove harmful code.
• Content Security Policy (CSP): Implement a CSP to restrict the sources from which scripts can be executed.
• Encoding output: Ensure any dynamic content is safely encoded before being displayed on the webpage.

XSS vulnerabilities are often exploited because many websites don’t properly validate or escape user input. Proper security practices, like regular code reviews and input sanitization, are essential to minimize this risk.

XSS remains one of the most common and dangerous security flaws on the web today, making it a key focus for cybersecurity efforts.