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Discover the hidden dangers of XSS attacks and learn how to protect yourself in our latest video! Watch now to dive into the different types of XSS, real-world attack examples, and expert insights on securing your website and data.

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Overview of Cross-Site Scripting (XSS)

Cross-Site Scripting (XSS) is a security vulnerability that occurs when an attacker injects malicious scripts into trusted websites or web applications. These scripts are executed in the browsers of users who visit the compromised site, leading to various harmful consequences, such as session hijacking, data theft, or defacement. XSS can be used to manipulate the content of a webpage, steal sensitive information, or perform actions on behalf of the user without their consent.

XSS attacks target both users and websites, making it a critical vulnerability to address in web security. The attack typically involves injecting JavaScript code, which is then executed by the victim's browser as part of the webpage content. As the victim interacts with the page, the malicious code can steal credentials, modify page content, or redirect the user to phishing sites, among other potential threats.

Types of XSS

• Stored XSS: This type of XSS occurs when the attacker injects a malicious script that is permanently stored on the target server, such as in a database or in user-generated content like comments or forums. When users view the compromised content, the script is executed in their browsers. This is one of the most dangerous types of XSS because the attack remains active until the stored content is removed.
• Reflected XSS: In reflected XSS, the malicious script is not stored on the server but is instead reflected back to the user through a URL or HTTP request. This type of XSS usually occurs when a user clicks on a malicious link or submits a form containing malicious input. The script is then executed in the browser as part of the server's response to the user's request.
• DOM-Based XSS: This type of XSS occurs when the vulnerability exists in the client-side JavaScript code, specifically in how the application processes user input. The malicious script is executed on the victim's browser as a result of a manipulation of the Document Object Model (DOM). Unlike other types of XSS, the server does not process the malicious input; instead, the attack is triggered by how the client-side code handles the input.
• Self-XSS: In self-XSS, the attacker tricks the victim into executing malicious code on their own browser. This often involves social engineering, where the attacker convinces the victim to paste malicious JavaScript into the browser's console. While this attack doesn't target a server, it can still lead to the theft of session cookies or other sensitive data.
• Mutated XSS: Mutated XSS occurs when malicious input is sanitized by the web server, but the sanitized input is later manipulated or "mutated" by the client-side JavaScript. As a result, the input is transformed into executable code once it reaches the browser, even though it was originally considered safe. This type of XSS relies on the interplay between server-side filtering and client-side processing.

How to Perform an XSS Attack

To perform a Stored XSS attack, an attacker must first identify an input field on a website that does not properly sanitize user input (such as a comment section or user profile). The attacker would then inject a malicious script, such as a simple JavaScript payload, into the field. When other users load the page containing the malicious script, their browsers execute the script, potentially leading to data theft or session hijacking.

For Reflected XSS, the attacker typically crafts a malicious link containing a script in the URL query string. When a victim clicks on the link, the server reflects the script back in the response without proper sanitization, causing the victim's browser to execute the script. This attack is often used in phishing campaigns, where the malicious URL is sent via email or social media, tricking the user into executing the attack unknowingly.

How to Defend Against XSS

• Input Validation and Sanitization: Ensure all user input is properly validated and sanitized before being processed or stored. Use a robust whitelist approach that allows only specific, safe characters and values, and reject any suspicious input. Input should never be trusted, especially when it's coming from untrusted sources like user-generated content.
• Output Encoding: When displaying user input on webpages, ensure it is properly encoded to prevent the browser from interpreting it as executable code. For example, encoding characters like <, >, &, and ", so they are treated as data rather than HTML or JavaScript. This prevents attackers from injecting malicious code into the page.
• Use Content Security Policy (CSP): Implement a Content Security Policy to restrict which resources can be loaded and executed by the browser. This helps prevent malicious scripts from running, even if an attacker manages to inject them. A well-configured CSP can mitigate the impact of XSS by limiting the sources from which scripts can be loaded.