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Understanding XSS Attacks


Cross-Site Scripting (XSS) is a type of security vulnerability that occurs when an attacker injects malicious scripts into web content that is then viewed by other users. These scripts are executed in the context of the victim's web browser, allowing the attacker to steal information, perform actions on behalf of the victim, or manipulate the content of the web page. XSS attacks are a common and significant threat to web applications.

There are three main types of XSS attacks:
  • Stored XSS (Persistent XSS): In this type of attack, the malicious script is permanently stored on the target server, such as in a database or a forum post. When a user views the infected page, the script is executed in their browser. This can have serious consequences as the malicious code affects all users who access the infected resource. An example of a stored XSS attack could be an attacker posting a malicious script in a comment on a social media platform, which then gets displayed to all visitors who view that comment.
  • Reflected XSS: In a reflected XSS attack, the malicious script is not stored on the target server but is instead embedded in a URL or a form input. The attacker tricks a user into clicking on a crafted link or submitting a form that includes the script. When the user interacts with the link or form, the script is reflected back to the user's browser and executed. This type of attack often relies on social engineering to lure users into clicking on malicious links. For example, an attacker could send a user a link with a script as a parameter in the URL, and when the user clicks on it, the script runs in their browser.
  • DOM-based XSS: DOM-based XSS is a more complex type of XSS attack. It occurs when the web application's client-side scripts manipulate the Document Object Model (DOM) to execute a malicious script. The malicious script typically modifies the DOM dynamically on the client side, which can lead to information leakage or other malicious actions. An example of DOM-based XSS could involve an attacker manipulating a website's client-side code to inject a script that steals user data when certain conditions are met, such as when the user clicks a specific button.

Here's a simple example of a reflected XSS attack:

Suppose there is a search page on a website that takes a user's query and displays the results on the same page. The URL structure for the search results looks like this:

https://example.com/search?q=<user_query>

An attacker constructs a malicious link like this:

https://example.com/search?q=<script>alert('XSS Attack');</script>

If a user clicks on this link, the script injected by the attacker will be executed, causing an alert to pop up in the user's browser with the message "XSS Attack." This is a basic example of reflected XSS, where the malicious script is reflected back in the web page's response.


XSS attacks can have severe consequences, including data theft, session hijacking, and the potential compromise of user accounts, so it's crucial for web developers to implement security measures like input validation and output encoding to prevent these vulnerabilities.