Cross-Site Scripting (XSS)

Key Concepts

Cross-Site Scripting (XSS) is a type of security vulnerability typically found in web applications. XSS attacks enable attackers to inject malicious scripts into web pages viewed by other users. This can lead to the theft of sensitive information such as cookies, session tokens, or other confidential data.

Types of XSS

• Stored XSS: The malicious script is permanently stored on the target servers and is executed when users request the infected page.
• Reflected XSS: The malicious script is reflected off the web server, typically through a URL parameter, and executed in the user's browser.
• DOM-based XSS: The vulnerability exists in client-side code rather than server-side code. The malicious script manipulates the Document Object Model (DOM) of the page.

Detailed Explanation

Stored XSS

Stored XSS occurs when an attacker manages to store a malicious script in the application's database. When other users access the infected page, the script is executed in their browsers. This type of XSS is particularly dangerous because it can affect a large number of users.

Example: An attacker posts a comment on a blog containing a malicious script. When other users view the comment, the script executes, potentially redirecting them to a malicious website.

Reflected XSS

Reflected XSS happens when an attacker sends a URL containing a malicious script to a victim. When the victim clicks on the link, the script is executed in their browser. This type of XSS is often used in phishing attacks.

Example: An attacker sends an email with a link to a vulnerable search page. The URL includes a malicious script as a search parameter. When the victim clicks the link, the script executes in their browser.

DOM-based XSS

DOM-based XSS vulnerabilities occur when the client-side code manipulates the DOM in an unsafe way. The malicious script is executed locally in the user's browser, not on the server.

Example: An attacker crafts a URL that, when clicked, modifies the DOM of a page to include a malicious script. When the page is loaded, the script executes in the user's browser.

Examples and Analogies

Stored XSS: The Persistent Virus

Think of stored XSS as a virus that infects a shared computer. Once the computer is infected, everyone who uses it is at risk. Similarly, once a web page is infected with stored XSS, all users who access it are vulnerable.

Reflected XSS: The Poisoned Candy

Consider reflected XSS as a poisoned candy given to a child. The child (victim) eats the candy (clicks the link) and becomes sick (script executes). The attacker relies on the victim to take the malicious action.

DOM-based XSS: The Hidden Trap

Imagine DOM-based XSS as a hidden trap in a video game. The trap is triggered when the player (user) performs a specific action (clicks a link or interacts with a page element). The trap affects the player locally, without involving the game server.

Understanding these types of XSS is crucial for identifying and mitigating security vulnerabilities in web applications. By recognizing how attackers can inject and execute malicious scripts, you can better protect your applications and users from potential threats.