Cross-Site Scripting (XSS) attacks are widespread throughout the internet. They can be used to impersonate users, steal private information, deface a website, and more. So what exactly is an XSS attack? Why is it a security threat? And how can you prevent cross-site scripting attacks?

What Is a Cross-Site Scripting Attack?

Cross-Site Scripting, commonly known as XSS, is a web application security vulnerability that allows attackers to inject arbitrary client-side code or scripts into webpages.

When an unsuspecting user interacts with the infected web page, the injected code is triggered, and the victim is hit with an attack that is potentially capable of stealing sensitive information or carrying out unsolicited transactions at the expense of the victim.

A malicious script can masquerade as a user, steal information and cookies, manipulate site content, deface a website, and redirect victims to malicious sites.

XSS attacks aren't new. They first started surfacing in the 1990s and since then have been consistently listed as one of the top 10 web security risks by the Open Web Application Security Project (OWASP), with over 274,000 reported occurrences in 2021.

How Do XSS Attacks Work?

An XSS attack revolves around the inability of a website to sanitize input data. This means that the website is not properly stripping the HTML content from the input it accepts.

Why is this bad? Your web browser can't tell the difference between the original website code and injected code. So, if an attacker were to feed in malicious scripts (JavaScript, VBScript, etc.), it would get passed off as valid input, and the browser will render it alongside the code of the website itself.

Cross-site scripting attacks are carried out in a multitude of ways, all of which depend on various factors such as the endpoint, how the script is triggered, whether the payload is reflected only on the client-side or can be stored in the server-side database, and more.

Types of Cross-Site Scripting Attacks

man programming a website

Cross-site scripting(XSS) attacks are classified into three main types, based on how the script is executed and how the attack is delivered:

  1. Reflected XSS.
  2. Stored XSS.
  3. DOM-based XSS.

So what do these XSS attacks mean?

Reflected Cross-Site Scripting (XSS)

reflected xss attack

Reflect cross-site scripting is a type of XSS attack where injected code is reflected off the server in any response that includes the input data as a part of the request made to the web server. Due to improper input validation, the browser executes the script and returns its output to the client.

It is the most basic variant of all XSS attacks and is comparatively less of a threat to a website than its counterpart, stored XSS. But it remains a threat and one that you should be cautious about.

Reflected XSS attacks are generally propagated via emails and other forms of social engineering or phishing campaigns.

Stored Cross-Site Scripting (XSS)

image of a server panel

Stored cross-site scripting is a type of XSS attack where malicious code can be embedded or stored in the database of a web server. This is the deadliest type of XSS attack as it potentially compromises the security of the entire website's userbase. Users who visit the web page embedded with malicious code will be hit by the payload.

For example, if a popular website's comment section were vulnerable to stored XSS, an attacker could plant malicious code in their comments. Each time a viewer would come across the comment, their browser would execute the malicious code, jeopardizing their system and data.

A sub-category of stored XSS is blind XSS attack. In this variant, an attacker injects code as input, the input data is stored in the backend and then reflected to the victim from in another part of the web application.

For instance, if an attacker injects malicious code into a website's contact or feedback forms, the injected code will be executed when the admin of the website inspects the input data.

DOM-Based Cross-Site Scripting (XSS)

inspecting the DOM

A Document Object Model (DOM)-based cross-site scripting attack is a client-side attack wherein malicious code is first written into the DOM of the browser and then executed by the browser. This happens due to improper sanitization of input by the website.

In a vulnerable web application, the attacker can use a script to access the DOM and manipulate its content. Like reflected XSS, DOM-based XSS attacks are also delivered via social engineering. The victim is tricked into clicking a malicious URL which leads them to the vulnerable page where the script is executed by the browser.

How to Detect and Prevent XSS Vulnerabilities in Your Site

Cross-site scripting(XSS) vulnerabilities are not easy to detect and patch—a significant reason they are still as prevalent today as they were decades ago. However, following solid development practices and prioritizing security can save your site from devastating cyberattacks and breaches, potentially costing you millions.

Here are some pointers on preventing XSS attacks as a developer:

  • As a ground rule, don't trust user input. Any user input that is rendered with the HTML introduces a risk of an XSS attack.
  • Implement proper escaping or encoding techniques to escape any script fed into input fields.
  • Sanitize user input to strip off any injected scripts.
  • Set the HttpOnly flag for cookies to make them inaccessible via client-side scripting.
  • Enforce Content Security Policy (CSP) to inhibit script injection into the web page from unauthenticated sources.

As a user browsing the web, you can only do so much to protect yourself from a looming XSS attack other than being vigilant of the links you click on and the sites you visit.

Best Web Application Security Practices

As an online business owner, you should always be on the lookout for defensive technologies to secure your business from threats and patch any loopholes within the internal structure. As an emerging business, if you cannot invest in upgrading your organization's security stature, adopting the best security practices will protect you to some extent.