TL;DR
Cross-site scripting (XSS) is a widespread injection attack where hackers embed malicious code into legitimate websites, exploiting the browser's trust relationship to steal session data, impersonate users, or exfiltrate information. XSS vulnerabilities stem from unfiltered user inputs and can affect virtually any site using common HTML tags. Developers can reduce risk by adopting context-sensitive server-side output encoding, enabling Content Security Policy (CSP), filtering unexpected inputs, encoding outputs, and restricting JavaScript where possible. Persistent monitoring is essential to catch and stop rogue scripts before they reach visitors.
What is cross-site scripting?
XSS is a form of injection attack. A hacker places malicious code inside some part of a legitimate website or application. The target visits, and the code executes.
At the end of an XSS attack, a hacker has unauthorized access. That person could, for example, grab a user's session cookies. With that bit of data, the hacker could impersonate the victim and do something undesired, like steal money. Thankfully, there are ways to protect your site.
Why are XSS attacks such a widespread threat?
No web designer sets out to create a trap. But unfortunately, XSS attack vulnerabilities are widespread. In fact, some experts say "many" sites include them. If yours does, you put all of your visitors at risk.
How does cross-site scripting exploit browser trust?
Browsers are designed to trust content served from a website the user has already connected with. XSS takes advantage of this trust by injecting malicious code that the browser treats as legitimate, allowing attackers to manipulate interactions between the user and the site.
How does XSS exploit the same-origin policy?
XSS twists the web's same-origin policy. That involves:
- Connection. A device links with a website or app.
- Permission. The website and the device connect through mutual agreement. The device might store cookies, for example.
- Skipping. With the two steps above complete, the device will trust the website with future requests. No more authorization is required.
A hacker implants malignant code somewhere on your site and app. The user's device runs the code without even checking, as it seems like it comes from a valid source.
Problems like this can be remarkably widespread. For example, researchers uncovered a vulnerability inside a WordPress tool installed on about 100,000 websites. The hacker doesn't need to change the code each time. As soon as a visitor taps into an infected site, the script can run.
How do XSS attacks begin?
Devices and websites don't always engage in one-way communication. The two trade data, and if developers don't analyze or block inputs from users, XSS problems begin.
The website includes user input inside the HTML page code. The hacker inserts a malicious string in that vulnerable code. The next visitor can become a victim.
How does XSS differ from a cross-site request forgery (CSRF) attack?
XSS is different from a CSRF. The two attacks differ in what the attacker can see and do:
| Attack Type | Attacker Visibility of Response | Attacker Capability |
|---|---|---|
| Cross-Site Request Forgery (CSRF) | Cannot directly view the server's response | Can entice a user into submitting an unintended request |
| Cross-Site Scripting (XSS) | Can observe the response | Can execute code in the victim's browser and extract data from the site |
What are the two main types of XSS attacks?
Every XSS attack begins with malicious code. But as any developer knows, there are thousands of different code types available.
The Open Worldwide Application Security Project (OWASP) recognizes two main types of XSS attacks:
| Attack Type | Origin of Malicious Code | Mechanism |
|---|---|---|
| Server-based XSS | Included within a Hypertext Transfer Protocol (HTTP) response the server generates | Malicious code is delivered directly from the server to the victim's browser |
| Client-based XSS | Originates from a user input | It updates the Document Object Model (DOM) on the client side |
Some XSS attacks are transient, and they work just once. Others are persistent and can remain stored on the server, on a target's device, or both.
What are the most common cross-site scripting attack vectors?
Vulnerable code is critical in building XSS attacks. Hackers typically look for a common vulnerability.
Tags typically start the attack. These nine tag types are considered vulnerable:
- body
- div
- frame
- img
- input
- link
- object
- script
- table
Why can't developers simply remove vulnerable HTML tags?
You can't build a website without using one or several of these tags. They're incredibly common, and users tend to want the functionality they deliver. You can't protect your site by eliminating them.
How can you reduce your cross-site scripting attack risk?
Any time you allow unfiltered user inputs into your outputs, you raise your risk of an XSS attack. You increase your risk if you use tools like Flash, VBScript (a Microsoft scripting language), and JavaScript.
OWASP says the best way to lower your risk is to use context-sensitive server-side output encoding. In essence, you use a different response to the inputs your visitors give you.
What other techniques can reduce XSS exposure?
Other techniques include:
- Content security. Use the Content Security Policy (CSP) tool. With it enabled, the browser checks scripts before running them. Anything unusual is rejected before it runs.
- Encoding. Ensure that your outputs are encoded, so they don't seem like active content.
- Filters. Determine what inputs seem typical or expected. Reject any user input that seems unusual.
- Headers. Prevent code in HTTP responses that shouldn't include HTML.
- Java. Don't allow JavaScript on your site.
Close monitoring is crucial. Ensure that you know when code heads to your visitors from your website, and shut down anything that seems to run haywire.
Frequently asked questions
What makes a website vulnerable to a cross-site scripting attack?
A site becomes vulnerable to cross-site scripting (XSS) when it accepts user inputs without properly filtering or encoding them before including those inputs in page output. When unvalidated data flows from a user into the rendered page, an attacker can inject a malicious script that the victim's browser will execute as if it were legitimate code from a trusted source.
What does a successful cross-site scripting (XSS) attack mean for end users?
A successful XSS attack can have serious consequences for end users. An attacker who has injected malicious code into a page can grab a user's session cookies, impersonate the victim, steal money, or extract other sensitive data — all without the user's knowledge, since the malicious script runs as if it were legitimate code from a trusted site.
Is removing common HTML tags like <script> or <img> a viable defense against cross-site scripting (XSS)?
Tags such as script, img, input, and div are fundamental building blocks of any modern website. Removing them would break core functionality that users depend on. The solution is not elimination but rather proper encoding, filtering, and security policies that control how these tags behave.
What is the difference between a persistent and a transient cross-site scripting (XSS) attack?
A transient XSS attack executes once and is not stored anywhere — the attack is one-time only. A persistent XSS attack saves the injected code on the server or the victim's device, meaning it can execute repeatedly every time a user visits the affected page, which makes it more dangerous.
What is context-sensitive server-side output encoding, and why does it help?
Rather than treating all user input the same way, context-sensitive encoding applies different sanitization rules depending on where the output will appear — for example, inside HTML, a URL, or a JavaScript block. This approach, recommended by the Open Worldwide Application Security Project (OWASP), ensures that injected strings are rendered as inert text rather than executable code.
How does a Content Security Policy (CSP) protect against cross-site scripting (XSS)?
A CSP instructs the browser to evaluate scripts before running them. If a script does not match the policy's approved sources or patterns, the browser rejects it before execution. This acts as a last line of defense even when other input-filtering measures have been bypassed.
References
Cross-Site Scripting: Many Websites Remain Vulnerable to Common Web Exploit. (November 2015). The State of New Jersey.
XSS Bug in SEOPress WordPress Plugin Allows Site Takeover. (August 2021). Threat Post.
Types of XSS. OWASP.
Cross-Site Scripting (XSS). OWASP.
