XSS attack, also known as Cross-Site Scripting (XSS), is a security vulnerability in web applications that enables attackers to execute malicious scripts within the user's browser. Once executed, these scripts can steal user information, alter website content, and inadvertently trick users into performing certain actions.
XSS attacks are typically categorized into three types:
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Stored XSS (Persistent XSS): Malicious scripts are permanently stored on the target server, such as in databases, message forums, or visitor comment sections. When users browse related pages, the scripts execute. For example, an attacker posts a comment with malicious JavaScript code on a social media site; when other users view this comment, the script runs in their browser.
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Reflected XSS: Malicious scripts are not stored on the server but are delivered to users via URLs, emails, or instant messages. After users click a link, the server dynamically generates a page containing the attack code and returns it to the user, causing the malicious script to execute in the browser. For instance, a search engine's results page includes user-input search keywords; if these keywords are not properly handled, an attacker can craft a special URL that, when clicked, executes malicious scripts in the search keyword field.
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DOM-based XSS: In this attack, malicious code is not directly reflected in the server's response but executes on the client side after the page loads in the user's browser due to insecure data flow within the DOM environment. For example, a webpage determines its content based on URL parameters; if these parameters are not properly handled, an attacker can modify the URL parameters to make the page execute malicious scripts.
Defense strategies against XSS attacks include:
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Validate and filter user input to prevent direct output of unprocessed user input.
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Use HTTP-only cookies to prevent JavaScript from accessing sensitive cookies.
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Implement Content Security Policy (CSP) to restrict the types and sources of resources that pages can load and execute.
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Use CSRF tokens for important operations to ensure requests are initiated voluntarily by the user.
This concludes the overview and defense strategies for XSS attacks.