XSS Attacks: Meaning, How It Works, and Prevention Methods
| David Tidmarsh
| Network Security
XSS attacks are one of the most common cyberattacks, making them essential knowledge for cybersecurity experts. In this article, we’ll examine what XSS attacks are, why you should know about them, and how IT security professionals can prevent XSS attacks.
What Are XSS Attacks?
A cross-site scripting attack (XSS attack) is a type of cyberattack in which the attacker exploits a web application vulnerability. This vulnerability allows malicious actors to inject their code into websites accessed by other users.
The possible consequences of an XSS attack include:
- Stealing data such as browser cookies or session tokens
- Distributing malware and ransomware
- Changing website content or redirecting users to malicious websites
XSS attacks make use of a common vulnerability known as cross-site scripting. The attacker writes a malicious script or code and submits it to the web application. This could occur via a form asking for user input or several other methods (such as cookies or HTTP headers).
The XSS malware is then stored on the website, where it runs when other users visit it. When a user visits an infected page, their browser downloads and executes the malicious code on the user’s machine, allowing the attacker to access restricted content and sensitive information.
What Are the Different Types of XSS Attacks?
XSS types of attacks come in three main formats, depending on how the malicious code is delivered to the target.
- Stored (persistent) XSS: Malicious code is permanently stored on the website’s server (e.g., in a file or database), allowing it to persist for an extended period. The attacker’s script is executed whenever the code is retrieved as part of the website’s This type of XSS attack is particularly dangerous and long-lasting.
- Reflected (non-persistent) XSS: Malicious code is “reflected” back to the user as an HTTP response (IBM, 2021). Reflected XSS attacks require the attacker to deliver the malicious script via an HTTP request sent to the website’s server; however, the code is not permanently stored on the server.
- DOM-Based XSS: Malicious code is stored as part of the website’s DOM (Mozilla, 2023), representing the website’s structure that allows scripts to modify the site’s Because a DOM-based XSS attack occurs strictly on the victim’s computer, it is difficult or impossible for the website host to detect it.
How Are Websites Vulnerable to XSS Attacks?
According to a study at Norfolk State University, more than 60 percent of websites (Mack et al., 2019) are vulnerable to XSS attacks. The most common XSS vulnerability occurs when web developers fail to validate and sanitize user input, leaving the website open to attack.
Validating and sanitizing input are crucial tasks in web development:
- Validation involves checking input against a set of rules (e.g., checking that a user’s proposed password is long enough).
- Sanitizing involves cleaning input to remove or neutralize potentially harmful elements (e.g., escaping HTML tags to treat them as plaintext).
Websites that fail to validate and sanitize user inputs properly allow attackers to send malicious code that can be used in an XSS attack.
What Are the Potential Consequences of An XSS Attack?
Left unchecked, an XSS attack can be devastating for those affected. Companies that fall victim to an XSS attack may expose their customers to data theft, malware, phishing attempts, and other nasty surprises. This can significantly damage the organization’s reputation, reducing trust and causing business loss.
XSS attacks can also harm the user experience on a company’s website and disrupt normal business operations. Malicious code can launch pop-up windows, redirect users to other sites, or secretly add hidden links or spam content to a web page — potentially harming the company’s search engine rankings.
Preventing and Mitigating XSS Attacks
The good news is that organizations aren’t defenseless against XSS attacks. Below, we’ll discuss some methods of XSS attack prevention.
What Are the Best Practices for Preventing XSS Attacks?
Prevention is key when it comes to thwarting potential cyberattacks. The best practices for preventing XSS attacks include the following:
- Validating and sanitizing all user input on the server, ensuring that it uses the expected format and does not contain malicious code
- Using modern secure web development frameworks with features such as templating and auto-escaping
- Enabling CSP (Content Security Policy) to help detect and mitigate XSS and other types of cyberattacks
- Conducting regular security tests and audits on the website, including vulnerability scanning and penetration testing
How Can Web Developers Validate and Sanitize User Input?
XSS mitigation and prevention depend on the website’s ability to validate and sanitize user input. Best practices here include:
- Confirming the expected data type of each input (e.g., numbers, dates, or strings)
- Using regular expressions or whitelists to restrict inputs to a specific format
- Escaping HTML characters such as “<” and “>” to prevent them from being interpreted as code
- Using CSP headers to control which content can be executed on a page, blocking potentially malicious scripts
Are Security Plugins and Frameworks Effective in Preventing XSS Attacks?
Many security plugins and frameworks can help reduce the likelihood of an XSS attack. These include:
- DOMPurify, an HTML sanitizer to prevent XSS
- Vulnerability scanning tools such as Burp Suite, Acunetix, OWASP ZAP, and
- CSP libraries for different web frameworks, such as helmet-csp for Node.js and Django-csp for Django.
- Modern web frameworks such as Angular and React with certain built-in XSS
Examples of High-Profile XSS Attacks
Businesses of all sizes and industries have fallen victim to XSS attacks, which are one of the most common web vulnerabilities.
In 2018, a high-profile XSS vulnerability attack on Fortnite (Ng, 2019) affected players of the massively popular online game. Epic Games, Fortnite’s parent company, discovered that the vulnerability could allow hackers to compromise users’ accounts, eavesdrop on conversations with friends, and even buy in-game items.
A major XSS attack against eBay (Kovacs, 2016), an auction website, occurred between 2015 and 2016. Researchers at MIT demonstrated how attackers could exploit an XSS vulnerability to redirect victims to a different website and launch a phishing attack.
The Evolving Landscape of XSS Attacks
Despite new techniques to counteract them, XSS attacks remain relevant in today’s IT security landscape. Security experts such as ethical hackers and penetration testers should be familiar with XSS in cybersecurity.
Although websites are getting better at thwarting XSS attacks, malicious actors have a variety of techniques up their sleeves. These include DOM-based attacks, advanced payloads, and the use of artificial intelligence and machine learning to enhance attack strategies.
Fortunately, regular security testing and education about XSS vulnerabilities can go a long way in preventing organizations from falling victim to these attacks.
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Understanding XSS attacks and how to defend against them is a critical topic in network security. If you’re interested in a career in the network security field, it’s an excellent idea to obtain a certification. Being certified shows that you have the right combination of theoretical knowledge and practical skills.
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References
- (2021, March 03). HTTP responses. https://www.ibm.com/docs/en/cics-ts/5.2?topic=protocol-http-responses
- (2023, November 29). Introduction to the DOM. https://developer.mozilla.org/en-US/docs/Web/API/Document_Object_Model/Introduction
- Mack, J., Hu, Y.-H., & Hoppa, M. A. (2019). A Study of Existing Cross-Site Scripting Detection and Prevention Techniques Using XAMPP and VirtualBox. Department of Computer Science Norfolk State University, Norfolk, Virginia, USA. https://digitalcommons.odu.edu/cgi/viewcontent.cgi?article=1459&context=vjs
- Ng, A. (2019, January 16). Fortnite had a security vulnerability that let hackers take over accounts.https://www.cnet.com/tech/gaming/fortnite-had-a-security-vulnerability-that-let-hackers-take-over-accounts/
- Kovacs, E. (2016, January 12). XSS Flaw Exposed eBay Users to Phishing Attacks.https://www.securityweek.com/xss-flaw-exposed-ebay-users-phishing-attacks/
