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Home Guide Cross-Site Scripting (XSS) Testing to Prevent XSS attacks

Cross-Site Scripting (XSS) Testing to Prevent XSS attacks

By Sebastian Viquez, BrowserStack Champion -

In the vast digital security ecosystem, a malevolent shadow lurks — Cross-Site Scripting (XSS), an insidious vulnerability that has evolved into a stalwart adversary of web security. It embodies a vulnerability that quietly awaits its chance to inject venom into the fabric of our digital lives.

As the digital landscape continues to flourish, so do the tactics employed by cyber malefactors. XSS, a threat that has persisted for decades, remains as formidable as ever, adapting to the changing face of technology. Its prevalence is a stark reminder that, in this dynamic and interconnected world, security is a never-ending endeavor and a perpetual battle against those who seek to exploit it.

This journey unveils the mechanisms, impacts, and strategies to mitigate this pervasive threat.

Impacts of XSS

To fully understand the magnitude of XSS attacks, here are some of the Potential Impacts of XSS Attacks

  • Data Theft: Attackers can use XSS to steal sensitive information such as login credentials, credit card details, or personal data from unsuspecting users.
  • Identity Theft: By exploiting XSS, attackers can impersonate users, potentially leading to identity theft and unauthorized access to accounts.
  • Financial Loss: XSS can be used to redirect users to fraudulent websites or payment gateways, resulting in financial losses for the users.
  • Website Defacement: XSS attacks can alter the appearance or content of a website, tarnishing the site’s reputation and trustworthiness.
  • Propagation of Malware: XSS can be utilized to deliver and execute malware on users’ devices, creating a pathway for further malicious activities.
  • Session Hijacking: XSS can enable attackers to hijack user sessions, granting them unauthorized access to the application with the victim’s privileges.

Testing For XSS Vulnerabilities

Now, with all the possible harms an XSS attack can bring your application, it is imperative you test your site for these vulnerabilities, and here is how you can do it.

Injecting scripts or HTML tags into input fields and submitting script tags, JavaScript code, or HTML event attributes can help you identify if the application fails to properly sanitize and encode user input.

As with any security testing, please ensure you have proper authorization and only perform tests on systems and environments where you are allowed to test. You can test your website for XSS vulnerabilities with the following approaches

Reflected XSS

In a reflected XSS attack, the injected malicious code is part of a URL or a form input. When a user clicks a malicious link or submits a form, the code is executed in their browser. Few examples below:

1. Simple script alert:


2. Image with an embedded script:

<img src="x" onerror="alert('XSS')">

3. Injecting a malicious link:

<a href="javascript:alert('XSS')">Click me</a>

Stored XSS

In this scenario, an attacker injects malicious code into a web application’s database or storage. When a victim accesses the affected page, the malicious code executes in their browser, potentially leading to account hijacking, data theft, or defacement of the website. For instance:

1. Script that steals user cookies:


2. Injecting a script that redirects users:


3. Injecting an iframe that loads a malicious website:

<iframe src=""></iframe>

DOM-based XSS

This attack injects malicious code that manipulates the Document Object Model (DOM) directly into the victim’s browser. The code modifies the web page’s structure or behavior.

A few samples below:

1. Modifying the DOM using JavaScript:

<script>document.getElementById('myElement').innerHTML = 'XSS';</script>

2. Modifying the URL fragment to execute the script:<script>alert('XSS')</script>

3. Manipulating the URL query parameters:<script>alert('XSS')</script>

XSS via HTML attributes

Attackers can also exploit HTML attributes to inject malicious code. For instance, an attacker might inject a script into an HTML attribute, something like:

1. Injecting a script into an event handler:

<button onclick="alert('XSS')">Click me</button>

2. Injecting a script into an image source:

<img src="javascript:alert('XSS')">

3. Injecting a script into an input value:

<input type="text" value="javascript:alert('XSS')">


CSS can be manipulated by attackers to execute malicious code. They can inject code into CSS properties, potentially affecting the layout or appearance of a webpage and even triggering further actions.

See below:

1. Injecting a script using CSS content property:

<style>.xss:before { content: "XSS"; }</style><div class="xss"></div>

2. Injecting a script using CSS expression:

<style>body { background-image: expression(alert('XSS')); }</style>

Event Handlers 

Event handlers can also be used in similar ways for XSS attacks.

For instance,

  • FSCommand() The attacker can use this when executed from within an embedded Flash object.
  • onAbort() When the user aborts the loading of an image.
  • onActivate() When the object is set as the active element.
  • onAfterUpdate() Activates on data object after updating data in the source object.
  • onBeforeActivate() This fires before the object is set as the active element.
  • onBeforeCopy() The attacker executes the attack string right before a selection is copied to the clipboard.
  • onBeforeCut() The attacker executes the attack string right before a selection is cut.
  • onBeforeDeactivate() Fires right after the active Element is changed from the current object.
  • onBeforeEditFocus() Fires before an object contained in an editable element enters a UI-activated state or when an editable container object is control selected.
  • onBeforePaste() The user needs to be tricked into pasting or forced into it using the exec Command(“Paste”) function.

Preventing XSS Attacks

To prevent XSS attacks you can implement the following security measures:

  • Input Validation and Sanitization: Validate and sanitize user input to ensure it adheres to the expected format and does not contain any malicious code. Use a whitelist approach to allow only specific, safe characters and patterns in input data.
  • Content Security Policy (CSP): Implement a Content Security Policy to define which sources of content are allowed to be loaded and executed on your web page. CSP can help prevent inline scripts and other sources of potential XSS attacks.
  • Escape Output: Encode user-generated data when it’s rendered in HTML, JavaScript, or any other context to ensure it is treated as data rather than code. Use appropriate encoding functions such as htmlspecialchars() in PHP, encodeURIComponent() in JavaScript, or equivalent functions in other languages.
  • Use Frameworks and Libraries: Utilize web frameworks and libraries that have built-in security features for handling user input and output, as many of them include automatic escaping mechanisms.
  • Avoid Inline Scripts: Minimize the use of inline JavaScript in your HTML documents. Instead, use external scripts and event handlers. If you must use inline scripts, sanitize and validate any data before incorporating it into the script.
  • HTTP Only and Secure Cookies: Set the “HttpOnly” and “Secure” flags on cookies to make them inaccessible to JavaScript and enforce HTTPS for secure communication.
  • Same-Site Cookie Attribute: Use the “SameSite” attribute for cookies to control when and how they are sent in cross-origin requests.
  • Security Headers: Implement security headers in your web server configuration to enhance security. Examples include X-XSS-Protection and X-Content-Type-Options headers.

The above are just examples. Remember, XSS testing should be performed responsibly, and it is important to validate and sanitize user input, properly encode output, and implement appropriate security headers (such as Content Security Policy) to mitigate XSS vulnerabilities. It is recommended to use specialized security testing tools and frameworks like OWASP ZAP, Burp Suite, or XSSer for comprehensive testing.

Always remember to go above and beyond, test thoroughly, and be safe!

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