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Cross-site scripting

Cross-site scripting (XSS) is a vulnerability where hackers insert malicious scripts inside web applications with the aim of executing them in a user’s browser.

Wiz Experts Team
5 min read

What is cross-site scripting (XSS)? 

Cross-site scripting (XSS) is a vulnerability where hackers insert malicious scripts inside web applications with the aim of executing them in a user’s browser. When victims interact with the compromised web page, the injected script executes within their browsers, potentially leading to the theft of session cookies, the theft of personal data, or the facilitation of further attacks. XSS attacks not only jeopardize individual user security but also pose a broader risk to the overall integrity of web applications.

Possible impacts of cross-site scripting attacks

Although in the majority of cases the malicious code is written in JavaScript, an XSS attack can be triggered through any client-side language. Regardless of how it’s carried out, the risks of XSS include:

  • Captured user keystrokes: Invasive XSS attacks can intercept and capture the keystrokes of a user, compromising sensitive information such as passwords or personal details entered on an affected website.

  • Redirection to malicious websites: Malicious actors can exploit XSS to redirect users to harmful websites, potentially leading to further attacks, phishing, or the download of malware onto the user's system.

  • Web browser exploits: XSS attacks may enable the execution of web browser–based exploits, causing disruptions like browser crashes. This disruption can impact the user experience and potentially serve as a precursor to more advanced attacks.

  • Cookie information theft: By exploiting XSS vulnerabilities, attackers can gain unauthorized access to users' session cookies. This compromise puts the victim's account at risk because attackers can access sensitive information and perform activities on their behalf by impersonating the user using cookies.

3 types of cross-site scripting attacks

There are three main types of XSS attacks:

1. Stored XSS

Stored XSS, also called second-order XSS or persistent XSS, is a type of XSS attack where the malicious script injected by an attacker is permanently stored on a target server. In this scenario, the injected script is saved within the server's database or file system, awaiting retrieval and execution when a user accesses a specific page or interacts with the compromised content.

2. Reflected XSS

Unlike stored XSS, where the injected script is permanently stored on the server, in reflected XSS, the script is embedded in the URL or a form and then reflected in the server's response to that specific request. Reflected XSS is the most common form of cross-site scripting. Reflected XSS attacks often exploit vulnerabilities in search queries, form submissions, or URL parameters. Their impact can range from displaying fake login pages for stealing user credentials or acting on the user’s behalf without their consent.

3. DOM-based XSS

DOM-based cross-site scripting (XSS) is a more advanced type of XSS attack where the vulnerability is exploited on the client side rather than the server side. In this case, the malicious actor can examine and modify Document Object Model (DOM) data to create a harmful URL that’s designed to deceive a user into clicking it. After the user clicks the link, the attacker gains the ability to access sensitive information, such as the user's active session details and keystrokes, among other potential security breaches.

Examples of XSS attacks

Let’s take a closer look at XSS attacks by looking at examples of some of the most common types of attacks in action.

Example 1: Unvalidated input from comments on a social media website

Most social media platforms allow users to post comments on each other's profiles. Imagine a platform that lacks appropriate validation or sanitization of user inputs before displaying them; this platform would open up users to attacks when their session cookies are stolen and then actions are performed on their behalf.

In this scenario, an attacker could post a comment containing malicious JavaScript code, which might look like this: 

    // Malicious JavaScript code to steal user's session cookie

When other users view this comment, the malicious script present in it is executed on their browsers and sends the cookies and possibly other sensitive data to the hacker’s server. The hacker can then impersonate the compromised users.

Example 2: Improper validation of a user’s search query in a URL

Most websites with a search feature display the query in the URL on the browser. Let’s imagine a query that’s displayed without any proper validation. In this case, an attacker could craft a phishing link that contains malicious code, enticing users to click on it.

Here’s an example URL:

https://vulnerable-site.com/search?query=<script>alert('You've been hacked');</script>

Example 3: A banking website that dynamically updates details

Online banking applications often update information like account details and transactions dynamically using JavaScript. For this example XSS attack, let’s imagine that an application displays these details without sufficient input validation, creating a potential DOM-based XSS vulnerability.

An attacker can exploit this vulnerability by crafting a malicious link that, when clicked by a victim with an active banking session, triggers the execution of JavaScript code on the victim's browser.

A malicious link might look like this:


Here, when the victim's browser loads the account summary page, the malicious stealCredentials()function inside the script tag is executed within the context of the DOM. This could lead to the theft of sensitive financial information, session tokens, or even unauthorized transactions if the hacker’s payload is sophisticated.

How to find XSS vulnerabilities

Generally, a web application is susceptible to cross-site scripting (XSS) when it returns input from requests to the client without any validation. Discovering cross-site scripting (XSS) vulnerabilities involves a combination of manual testing, automated scanning, and thorough code analysis. Here are some approaches to finding XSS vulnerabilities:

Manual testing

  • Inspect input fields and forms: Manually inspect web application input fields, such as search boxes, login forms, and comment sections, to check for proper input validation and output encoding.

  • Check URLs and parameters: Examine URLs and parameters for potential injection points, especially in GET and POST API requests.

  • Verify cookie handling: Investigate how the application handles cookies since XSS attacks may involve stealing session cookies.

Automated scanning

  • Use security scanning tools: Utilize automated security scanning tools such as Burp Suite, ZAP, or Acunetix for dynamic analysis to identify potential XSS vulnerabilities.

  • Leverage web application scanners: Employ web application scanners that specialize in XSS detection to automate the process and identify common vulnerabilities.

Static code analysis

  • Prioritize code review: Perform a thorough code review, focusing on how user inputs are handled. Look for instances where user inputs are directly echoed in the application without proper validation or encoding.

  • Take advantage of static analysis tools: Use static code analysis tools to scan the source code for potential XSS vulnerabilities. Tools like ESLint (for JavaScript), Bandit (for Python), or SonarQube can help identify insecure coding practices.

Bug bounty programs

  • Participate in bug bounty programs: Consider participating in bug bounty programs or engaging security professionals to perform penetration testing. These measures can provide an invaluable external perspective on potential vulnerabilities.

Regularly testing and securing your web applications against XSS vulnerabilities is essential to maintaining a robust security posture. Combine automated tools with manual testing and ongoing security practices to stay ahead of emerging threats.

Preventing XSS attacks

Here are essential steps to prevent XSS attacks before they happen to you:

  • Leverage escape input: Securing user input through escaping is crucial for preventing cross-site scripting (XSS) vulnerabilities. This method involves ensuring the security of received data before presenting it to the user, preventing the interpretation of key characters as executable code. By doing so, it mitigates the risk of the browser misinterpreting characters used to signal executable code, rendering them as escaped and averting their execution.

  • Validate input: Validate user inputs thoroughly on both the client and server sides. Confirm that inputs adhere to anticipated formats, and discard any data that fails to meet the specified validation criteria.

  • Encode output: Encode user inputs before displaying them. This ensures that any potentially malicious scripts entered by users are treated as plain text and not executed when rendered.

  • Implement Content Security Policy (CSP): Implement Content Security Policy (CSP) headers to specify and enforce the allowed sources for resources, including scripts. CSP effectively reduces the impact of cross-site scripting (XSS) attacks by ensuring that only trusted sources are allowed to execute the scripts.

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