Cracking The Code: Strategies For Xss Bug Bounty Write-ups And $$$$$ – Sophisticated XSS attacks can bypass filters through techniques such as encoding HTML objects, manipulating JavaScript strings, and evading event handling.
Cross-Site Scripting (XSS) is a common security vulnerability in web applications. It allows attackers to insert malicious client-side scripts into web pages viewed by other users. While basic XSS attacks are well understood, advanced XSS attacks can be more complex and difficult to detect or prevent. This article will explore some advanced XSS attacks, providing examples to illustrate their potential impact and some WAF XSS Filter Avoidance Bypass techniques.
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DOM-based XSS is a type of XSS attack where the vulnerability is in the client-side script rather than the server-side script. The Document Object Model (DOM) is a programming interface for web documents. It represents the structure of the document and allows the program to manage the structure, style and content of the document.
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In this example, JavaScript uses the ‘document.write’ function to display a welcome message that includes the username. The name is taken from the URL after the `name=` parameter. If an attacker modifies the URL to include a script in the `name=` parameter, the script will be executed.
Blind XSS is a persistent variant of XSS attacks, where the payload is stored on the server and retrieved later, often on another page or by another user. Attackers don’t get immediate feedback; They have to wait until the payload is fired.
An mXSS attack occurs when a web application’s output code process causes unwanted code execution. These attacks use the way browsers parse HTML and are often used to bypass XSS mitigation techniques.
In this example, the browser’s HTML parser might interpret the input in a way that causes the ‘error’ event handler to run, causing a warning.
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Self-XSS is a social engineering attack where victims are tricked into running malicious code in their own browser console.
A stored XSS attack involves the attacker injecting a Persistent (persistent) stored script on the target server. When the victim requests the stored data, a malicious script is sent and executed in their browser.
This script sends the user’s cookie to the attacker’s server, which may allow the attacker to impersonate the user.
In a mirrored XSS attack, a malicious script is embedded in the URL. The script is sent to the server as part of the request and then returned as a response to the server where it is executed in the user’s browser.
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Scalable Vector Graphics (SVG) files may contain JavaScript, which can cause XSS attacks if not properly sanitized.
AJAX (Asynchronous JavaScript and XML) is a technique for creating interactive web applications. However, if not handled properly, it can open new avenues for XSS attacks.
If the part of the URL after the hash (`#`) is controlled by an attacker, he can insert a script that will be executed when the AJAX request completes.
Flash applications can also be vulnerable to XSS attacks, especially if they rely on data obtained from the DOM or other sources.
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The HTML5 method `window.postMessage` allows windows to communicate with each other regardless of their source domain. However, if not validated properly, it can lead to XSS attacks.
An attacker can send a malicious message to a window that does not properly authenticate the source of the message:
If the receiving window inserts text content into the DOM without clearing it, the script will be executed.
JSONP (JSON with Padding) is a method used to bypass the same origin rule and allow data retrieval from servers under other domains. However, if the server does not validate the callback properly, it can cause an XSS attack.
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Modeling mechanisms are often used to create the HTML that is sent to the client. If user input is entered into a template in an unsafe manner, it can lead to an XSS attack.
Rich text editors (RTEs) allow users to format and format text in web applications. However, if not configured or modified correctly, RTE can be vulnerable to XSS attacks.
If the RTE allows scripting, the attacker script will be executed when the content is displayed.
The file upload function can be a potential entry point for an XSS attack if the uploaded file is not properly cleaned.
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If the server does not check the file type or content correctly, it can be stored and served to other users, which causes the script to run.
Third-party libraries and plugins are commonly used in web development. However, if this library is exposed or not maintained, it can introduce an XSS vulnerability into your application.
If the library does not clean up the input properly, the script will be executed when the content is displayed.
A Single Page Application (SPA) is a web application that dynamically updates content without requiring a full page load. However, if not implemented properly, SPAs can be vulnerable to XSS attacks.
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If the SPA inserts user input into the DOM without clearing it, the script will be executed.
Web sockets enable real-time communication between clients and servers. However, if not properly secured, web sockets can be vulnerable to XSS attacks.
If the server does not validate or clean up the received message correctly, the script is executed on the client side.
Web APIs (application programming interfaces) allow software applications to communicate with each other. If the API does not validate and sanitize user input, it can introduce an XSS vulnerability.
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If the API does not properly sanitize the response before sending it to the client, the script will be executed.
Web analytics and tracking scripts are commonly used to collect information about website visitors. However, if this script is not executed correctly or securely, it can introduce an XSS vulnerability.
If the tracking script does not properly clean the collected data, the script will run when the data is processed.
XSS filtering avoidance techniques aim to bypass filters implemented by web browsers or security tools to detect and prevent XSS attacks. Here are some common techniques used to avoid XSS filtering, along with examples:
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One of the simplest techniques involves encoding special characters using HTML objects. This technique aims to fool the filter by converting symbols into corresponding object representations. For example:
When the payload encoded by the HTML object is shown in the HTML response, the browser decodes the object, causing the script to run.
Another technique involves manipulating JavaScript strings to bypass filters that look for specific keywords or patterns. For example:
In this example, the string “javascript” is separated using the Unicode escape sequence (`u`) to avoid detection by filters. Filters may look for the keyword “javascript”, but evasion techniques overcome this by changing the string.
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XSS filters often target specific event handlers such as `onmouseover`, `onload` or `onclick` to prevent script execution. An attacker can try to bypass this filter by using alternative event handlers or changes to existing ones. For example:
Here, the attacker uses the `onload` event handler in the context of the SVG element. Because filters may not explicitly target SVG elements, payloads may avoid detection.
Filters are sometimes based on sensitive matches to detect harmful words or patterns. By manipulating the registry of certain characters, an attacker can bypass the filter. For example:
Here, the attacker changes the case of each character in the “ tag to `ScRiPt`. Although visually similar, the case is edited through exact matching of filters.
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Confusion involves changing the code to make it harder to filter and analyze. Techniques such as character substitution, concatenation or complex JavaScript tools can be used. Consider the following example:
In this case, the payload is hashed using the `String.fromCharCode()` function, which converts a sequence of Unicode character codes to a string. The obfuscated payload is then executed using `eval()`, the default script implementation.
This payload is designed to be able to interpret both HTML and JavaScript. It triggers an alert box in JavaScript and is treated like an image tag in HTML.
In this example, the payload attempts to extract the first token from the user’s cookie by measuring the response time of an image loaded from a domain controlled by the attacker. The payload then informs various features based on response time.
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This payload injects JavaScript code that modifies all hyperlinks on the vulnerable page, adding attacker-controlled domains to the URLs. When a user clicks on one of these links, an AJAX request is sent to the attacker’s domain, allowing the attacker to collect the user’s browsing activity.
This payload captures every keystroke made by the user and sends the captured data to a domain controlled by the attacker using an image resource (which can be used to exfiltrate the data).
This payload sends the user’s cookie information to an attacker-controlled domain by embedding it in an image resource.
Please note that the examples provided are for educational purposes only and malicious use is illegal and unethical. Performing security testing or penetration testing should always be done with proper permissions and in a controlled environment to identify and fix vulnerabilities in web applications.
Cross Site Scripting (xss) For Beginners
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