What is cross-site scripting?

Cross site scripting definition: What is it?

Cross-site scripting (XSS) attacks are a type of malware attack that injects malware code into otherwise normal and secure websites. Hackers use flaws or vulnerabilities in a web application (script) to send malicious JavaScript to users on the websites. Users trigger the malware when they click on the application inside the website or when the web page itself is loaded.

Organisations and companies running scripts on their websites can leave the door open to XSS attacks if they display content from users from untrusted sources without proper validation.

The definition of cross-site scripting (XSS) attacks

XSS attacks happen when an attacker tricks a web application into sending data in a form that a user’s browser can execute, often in the form of malicious JavaScript code. Most often this is a combination of HTML code and XSS provided by the cybercriminal, but XSS can also be used to deliver malicious downloads, plugins or media content.

An attacker is able to trick a web application in this way when the web application (script) allows data from an untrusted source - such as data entered into a form by users or passed to an API endpoint by client software - to be displayed to users without being properly processed.

Because XSS can allow hackers to insert malicious code into users’ browsers and gain access to some types of data, such as session cookies, an XSS vulnerability can allow an attacker to take data from users and dynamically include it on web pages, taking control of a website or application.

Malicious content from XSS attacks can be executed immediately whenever a page loads or a specific action occurs. These attacks are designed to steal personal data from users of a web application and can be especially dangerous since they appear on reputable and trusted websites, affecting all visitors.

More simply put, cybercriminals insert malicious scripts into a website’s content, which are then included in the dynamic content delivered to the user’s browser. Most browsers may not know that the malicious scripts are dangerous and therefore execute them.

The term “cross-site scripting” refers to scripts across websites that are being attacked.

Initially, attacks involved JavaScript only, but now all client-side languages can be affected, such as ActiveX, Flash and HTML.

How Cross-Site Scripting Works

Cross-site scripting (XSS) is a security flaw that enables attackers to insert harmful code into a web page, which is then executed by the user's browser. This typically occurs when a web application fails to properly validate or sanitize user input, creating an opportunity for attackers to inject harmful scripts.

Here’s a step-by-step breakdown of how a cross-site scripting attack unfolds:

1. Identifying a vulnerability: The attacker finds a flaw in the web application that permits the injection of malicious code. This could be a form field, a URL parameter, or any other input mechanism.

2. Crafting the payload: The attacker creates a malicious payload, often a piece of JavaScript, designed to exploit the identified vulnerability.

3. Injecting the payload: The attacker injects this malicious code into the web application through the vulnerable input point.

4. Processing the input: The web application processes the input and inadvertently includes the malicious code in the web page.

5. Executing the payload: When the user loads the compromised web page, their browser executes the malicious code, leading to potential security breaches such as data theft, session hijacking, or malware distribution.

By understanding this process, developers can better appreciate the importance of validating and sanitizing user input to prevent XSS vulnerabilities.

Types of XSS attacks

Cross-site scripting attacks come in several forms, each with its unique characteristics and methods of exploitation. The three most common types are:

1. Reflected XSS: This attack occurs when malicious code is injected into a web page and immediately reflected back to the user’s browser. For example, an attacker might craft a URL containing a malicious script and trick the user into clicking it. The script is then executed in the user’s browser, often appearing as a legitimate part of the web page.

2. Stored XSS: Also known as persistent XSS, this attack involves injecting malicious code that is stored on the server and served to users whenever they access the affected page. Common injection points include comment sections, user profiles, or any other data storage mechanism. Once stored, the malicious script is executed whenever a user views the compromised content.

3. DOM-based XSS: This type of attack manipulates the Document Object Model (DOM) of a web page directly in the user’s browser. Unlike reflected or stored XSS, DOM-based XSS does not involve sending malicious code to the server. Instead, the attacker exploits vulnerabilities in client-side scripts to execute malicious code, making it harder to detect and mitigate.

Understanding these types of XSS attacks helps in identifying potential vulnerabilities and implementing appropriate security measures.

Types of XXS attacks

The three most common types of XXS attacks are reflected XXS, persistent XXS and DOM-based XXS.

Reflected XSS attacks

This is one of the most commonly used cross-site scripting attacks. It takes place, for example, when the user sends a request to a website’s server. Instead of getting a response from the server itself, the user may come into contact with a malicious script that will be immediately reflected back to the user's browser and contain malware. The script may be designed to look like a normal error message or perhaps a search result.

The user will often click on the link or message and the user’s device will then be infected with the malware as the click triggers the execution of the malicious code. Then, all the information that the user enters will be sent to the hacker. This could be search engine addresses, login details, personal data, etc. Learn more about what personal data includes and why it’s a prime target for cybercriminals.

DOM-based XSS attack

This is a type of attack that relies on the DOM (Document Object Model). The Document Object Model (DOM) is the data representation of the objects that comprise the structure and content of a document on the Web.

The DOM allows the user to access the complete content of the website without having to interact with the server in question. In DOM attacks, the hacker focuses on the victim’s browser, manipulating the user's browser directly.

Persistent XSS attack

This is also known as Stored XSS and is an attack that happens when an application or a website’s web server responses have all been infected with malicious code. For example, the malware may be stored in the website’s comment area, where it will be triggered as soon as someone clicks on the comment area.

Persistent XSS is quite harmful as the hacker has already inserted the malware on a website. The victim should not be tricked into clicking on a link. It is enough to use a random function on the web page that is infected with the malware for the attack to happen.

Whereas persistent XSS and reflected XSS attacks show signs of something suspicious on the HTML response page, it is different with DOM XSS attacks. In DOM XSS attacks, you have to look at the website’s code to detect the attack.

The consequences of cross-site scripting attacks

When hackers manage to exploit XSS vulnerabilities, they can gain access to credentials. They can also spread viruses or gain access to the user's computer and view the user's browsing history or remotely control the browser. After gaining control of the victim's system, attackers can exploit other applications.

By exploiting XSS vulnerabilities, an attacker can perform malicious actions, such as:

• Spread malware on a user's device

• Access browsing history and clipboard content

• Remotely control a user's browser

• Access sensitive data

• Scan and exploit various applications

• Redirect the user to a malicious website

• Record user keystrokes

• Obtain cookie information from the user

How to identify script vulnerabilities across websites?

XSS vulnerabilities can occur and be detected if:

• Input coming to web applications is not validated

• Output to the browser is not HTML encoded

Preventing scripting across websites

To minimize cross-site scripting vulnerabilities, website developers or owners can ensure that:

• Update their site and server software to prevent future exploitation of vulnerabilities that could be exploited through an XSS attack.

• All pages on their site that accept user input filter code input, such as HTML and JavaScript.

• Scan for any vulnerabilities in web applications and have them fixed.

Best Practices for Protecting Against XSS

Protecting against cross-site scripting attacks requires a combination of secure coding practices and proactive security measures. Here are some best practices to follow:

1. Validate and sanitize user input: Always validate and sanitize all user inputs to ensure they do not contain malicious code. This includes input from forms, URLs, and any other data entry points.

2. Use secure coding practices: Implement secure coding practices such as input validation and output encoding. This helps prevent malicious code from being injected into the web application.

3. Implement a content security policy (CSP): A CSP defines which sources of content are allowed to be executed within a web page. By restricting the execution of scripts from untrusted sources, CSP can significantly reduce the risk of XSS attacks.

4. Use a web application firewall (WAF): A WAF can detect and block malicious traffic, including attempts to exploit XSS vulnerabilities. Regularly update and configure your WAF to protect against the latest threats.

5. Regularly update and patch software: Keep all software, including web applications and server components, up to date with the latest security patches. This helps close any known vulnerabilities that could be exploited by attackers.

By following these best practices, developers can create more secure web applications and reduce the risk of cross-site scripting attacks.

How users can protect themselves from cross-site scripting attacks

Users can protect themselves from XSS attacks by:

• Disabling scripting on pages where it is not needed, or disabling it altogether.

• Avoid clicking on links from suspicious emails or pop-up advertisements as they may lead the user to compromised websites.

• Go directly to websites by entering the URL in their browser instead of clicking on links.

• Keep their device's software updated with the latest security patches so that there are no security holes. Regularly updating software will reduce the number of vulnerabilities that make a website or applications vulnerable to XSS attacks.

• Review its apps to determine which are necessary and which the user rarely uses. Removing apps that are rarely used reduces the number of potential vulnerabilities that can be exploited by hackers.

• Use high-quality antivirus tools. They block many types of cyber threats such as viruses, spyware and ransomware.

Cyber threats come in many forms, from spyware that secretly monitors your activity to ransomware that locks your files and demands payment. Explore how spyware works and how to prevent it, and understand how ransomware attacks happen and how to defend against them. Learn more about ransomware protection here.

Resources for Cross-Site Scripting Prevention

Several resources are available to help developers prevent cross-site scripting attacks and secure their web applications:

1. OWASP (Open Web Application Security Project): OWASP offers a wealth of resources, including guidelines, tools, and best practices for preventing XSS attacks. Their comprehensive documentation is a valuable reference for developers.

2. Web Application Security Consortium (WASC): WASC provides a range of resources and guidelines focused on web application security, including strategies for mitigating XSS vulnerabilities.

3. Cross-site scripting prevention cheat sheet: This cheat sheet offers practical tips and guidelines for preventing XSS attacks. It covers various techniques for input validation, output encoding, and other security measures.

4. Web application firewall (WAF): Implementing a WAF can help detect and block XSS attacks. Many WAF solutions come with pre-configured rules to protect against common vulnerabilities.

5. Secure coding practices: Adopting secure coding practices, such as input validation and output encoding, is crucial for preventing XSS attacks. Many organizations provide training and resources to help developers write secure code.

By leveraging these resources, developers can enhance their knowledge and skills in preventing cross-site scripting attacks, ultimately creating more secure web applications.