What Is Cross Site Scripting?

Cross site scripting (XSS) is the injection of malicious scripts into otherwise safe, trusted websites. Flaws that allow this to take place are common. The script is then sent to genuine users of the website (rather than the site itself being the target). As those users’ browsers trust the site in question, they will execute the script.

Cross site scripting is enough of a problem to have made it into OWASP’s Top 10 Web Application Security Risks. It occupied position seven in 2017 before the cross site scripting definition was incorporated into the “injection” category and ranked at position three in 2021.

How Does Cross Site Scripting Work?

Bad actors targeting website users with cross site scripting do so by using flaws in web applications that enable them to inject code. These scripts are usually client-side JavaScript, which hackers can inject into sites that display content from users. As the sites are open to content from untrusted sources, hackers can use those that don’t implement proper escaping or validation for cross site scripting attacks.

In addition to using JavaScript, hackers may also use Flash, HTML, or other types of code when carrying out cross site scripting attacks.

The malicious code can execute a wide range of functions. It can redirect victims to websites under the attacker’s control, for example, or share the victim’s private data, such as session information and cookies. Fraudsters also use it with the goal of setting up new passwords on the user’s accounts as part of account takeover fraud scams.

What Are Cross Site Scripting Vulnerabilities and Why Should You Care?

Cross site scripting vulnerabilities occur when a website enables user input but doesn’t sanitize that input before using it as output. Businesses should care about this because those not ensuring their sites are guarded against XSS attacks are vulnerable.

Hackers can use companies’ vulnerable sites to target those businesses’ genuine users, for example by using their browsers to attempt to take over their accounts.

Businesses that allow this to happen by not using the latest security methods – such as real time fraud monitoring and fraud detection tools – are opening themselves up to potential financial and reputational damage.

Types of Cross Site Scripting Attacks

While there are almost infinite variations of cross site scripting attacks, including what they do to users’ machines, the details below cover some of the most common.

Reflected XSS Attacks

Reflected cross site scripting attacks are so named because the malicious injected script reflects off the web server, reaching the victim via email, a webform, or a website that is under the attacker’s control. This can be done using any form of response that includes some or all of the script, such as in response to a search result or as part of an error request.

Stored XSS Attacks

Unlike reflected attacks, stored XSS attacks involve the target server permanently storing the malicious injected script. Attackers use a range of targets to do so, including comment fields and databases, which then send the malicious script to those who use them.

Blind Cross-Site Scripting

A blind XSS attack is when the attacker stores their injected script on the victim’s server, ready for it to execute later. This cross site scripting example is a subtype of stored XSS. With blind cross site scripting, the attacker’s code can be executed at a later time or using a different web app.

DOM Based XSS

DOM based XSS attacks are distinctly different from reflected and stored attacks. That’s because the attacker modifies the document object model (DOM) environment in a DOM based cross site scripting attack, so that their malicious script is contained in the client-side code when a user loads the page.

How to Determine if Your Business Is Vulnerable to Cross Site Scripting (XSS) Attacks

Businesses that are worried about their XSS vulnerabilities are not without options. The first step to take is to determine the ways in which the business’ web apps might be vulnerable. The business should assess whether input and output sanitization is in place as data transfers from one state to another. If not, this should raise a red flag and warrant further investigation into whether the business’ cross site scripting vulnerabilities have been exploited.

Any business can be vulnerable to cross site scripting, meaning every business needs to be aware of this issue and take a proactive approach to limiting its vulnerabilities.

How to Protect Against Cross Site Scripting Attacks

Prevention is always better than cure when it comes to IT security. This is why it’s important for businesses to build solid, secure web applications in the first place, with a firm awareness of how to prevent cross site scripting. That means:

• robust coding with security at its core
• network segmentation
• stringent access control measures

Employee education also plays a major role in cross site scripting prevention, just as it does in keeping businesses safe from malware, phishing, and more.