Cross Site Scripting Prevention Cheat Sheet

Introduction

This cheat sheet provides guidance to prevent XSS vulnerabilities.

Cross-Site Scripting (XSS) is a misnomer. The name originated from early versions of the attack where stealing data cross-site was the primary focus. Since then, it has extended to include injection of basically any content, but we still refer to this as XSS. XSS is serious and can lead to account impersonation, observing user behaviour, loading external content, stealing sensitive data, and more.

This cheatsheet is a list of techniques to prevent or limit the impact of XSS. No single technique will solve XSS. Using the right combination of defensive techniques is necessary to prevent XSS.

Framework Security

Fewer XSS bugs appear in applications built with modern web frameworks. These frameworks steer developers towards good security practices and help mitigate XSS by using templating, auto-escaping, and more. That said, developers need to be aware of problems that can occur when using frameworks insecurely such as:

• escape hatches that frameworks use to directly manipulate the DOM
• React’s dangerouslySetInnerHTML without sanitising the HTML
• React cannot handle javascript: or data: URLs without specialized validation
• Angular’s bypassSecurityTrustAs* functions
• Template injection
• Out of date framework plugins or components
• and more

Understand how your framework prevents XSS and where it has gaps. There will be times where you need to do something outside the protection provided by your framework. This is where Output Encoding and HTML Sanitization are critical. OWASP are producing framework specific cheatsheets for React, Vue, and Angular.

XSS Defense Philosophy

For XSS attacks to be successful, an attacker needs to insert and execute malicious content in a webpage. Each variable in a web application needs to be protected. Ensuring that all variables go through validation and are then escaped or sanitized is known as perfect injection resistance. Any variable that does not go through this process is a potential weakness. Frameworks make it easy to ensure variables are correctly validated and escaped or sanitised.

However, frameworks aren’t perfect and security gaps still exist in popular frameworks like React and Angular. Output Encoding and HTML Sanitization help address those gaps.

Output Encoding

Output Encoding is recommended when you need to safely display data exactly as a user typed it in. Variables should not be interpreted as code instead of text. This section covers each form of output encoding, where to use it, and where to avoid using dynamic variables entirely.

Start with using your framework’s default output encoding protection when you wish to display data as the user typed it in. Automatic encoding and escaping functions are built into most frameworks.

If you’re not using a framework or need to cover gaps in the framework then you should use an output encoding library. Each variable used in the user interface should be passed through an output encoding function. A list of output encoding libraries is included in the appendix.

There are many different output encoding methods because browsers parse HTML, JS, URLs, and CSS differently. Using the wrong encoding method may introduce weaknesses or harm the functionality of your application.

Output Encoding for “HTML Contexts”

“HTML Context” refers to inserting a variable between two basic HTML tags like a <div> or <b>. For example..

<div> $varUnsafe </div>

An attacker could modify data that is rendered as $varUnsafe. This could lead to an attack being added to a webpage.. for example.

<div> <script>alert`1`</script> </div> // Example Attack

In order to add a variable to a HTML context safely, use HTML entity encoding for that variable as you add it to a web template.

Here are some examples of encoded values for specific characters.

If you’re using JavaScript for writing to HTML, look at the .textContent attribute as it is a Safe Sink and will automatically HTML Entity Encode.

&    &
<    &lt;
>    &gt;
"    &quot;
'    &#x27;

Output Encoding for “HTML Attribute Contexts”

“HTML Attribute Contexts” refer to placing a variable in an HTML attribute value. You may want to do this to change a hyperlink, hide an element, add alt-text for an image, or change inline CSS styles. You should apply HTML attribute encoding to variables being placed in most HTML attributes. A list of safe HTML attributes is provided in the Safe Sinks section.

<div attr="$varUnsafe">
<div attr=”*x” onblur=”alert(1)*”> // Example Attack

It’s critical to use quotation marks like " or ' to surround your variables. Quoting makes it difficult to change the context a variable operates in, which helps prevent XSS. Quoting also significantly reduces the characterset that you need to encode, making your application more reliable and the encoding easier to implement.

If you’re using JavaScript for writing to a HTML Attribute, look at the .setAttribute and [attribute] methods which will automatically HTML Attribute Encode. Those are Safe Sinks as long as the attribute name is hardcoded and innocuous, like id or class. Generally, attributes that accept JavaScript, such as onClick, are NOT safe to use with untrusted attribute values.

Output Encoding for “JavaScript Contexts”

“JavaScript Contexts” refer to placing variables into inline JavaScript which is then embedded in an HTML document. This is commonly seen in programs that heavily use custom JavaScript embedded in their web pages.

The only ‘safe’ location for placing variables in JavaScript is inside a “quoted data value”. All other contexts are unsafe and you should not place variable data in them.

Examples of “Quoted Data Values”

<script>alert('$varUnsafe’)</script>
<script>x=’$varUnsafe’</script>
<div onmouseover="'$varUnsafe'"</div>

Encode all characters using the \xHH format. Encoding libraries often have a EncodeForJavaScript or similar to support this function.

Please look at the OWASP Java Encoder JavaScript encoding examples for examples of proper JavaScript use that requires minimal encoding.

For JSON, verify that the Content-Type header is application/json and not text/html to prevent XSS.

Output Encoding for “CSS Contexts”

“CSS Contexts” refer to variables placed into inline CSS. This is common when you want users to be able to customize the look and feel of their webpages. CSS is surprisingly powerful and has been used for many types of attacks. Variables should only be placed in a CSS property value. Other “CSS Contexts” are unsafe and you should not place variable data in them.

<style> selector { property : $varUnsafe; } </style>
<style> selector { property : "$varUnsafe"; } </style>
<span style="property : $varUnsafe">Oh no</span>

If you’re using JavaScript to change a CSS property, look into using style.property = x. This is a Safe Sink and will automatically CSS encode data in it.

// Add CSS Encoding Advice

Output Encoding for “URL Contexts”

“URL Contexts” refer to variables placed into a URL. Most commonly, a developer will add a parameter or URL fragment to a URL base that is then displayed or used in some operation. Use URL Encoding for these scenarios.

<a href="http://www.owasp.org?test=$varUnsafe">link</a >

Encode all characters with the %HH encoding format. Make sure any attributes are fully quoted, same as JS and CSS.

Common Mistake

There will be situations where you use a URL in different contexts. The most common one would be adding it to an href or src attribute of an <a> tag. In these scenarios, you should do URL encoding, followed by HTML attribute encoding.

url = "https://site.com?data=" + urlencode(parameter)
<a href='attributeEncode(url)'>link</a>

If you’re using JavaScript to construct a URL Query Value, look into using window.encodeURIComponent(x). This is a Safe Sink and will automatically URL encode data in it.

Dangerous Contexts

Output encoding is not perfect. It will not always prevent XSS. These locations are known as dangerous contexts. Dangerous contexts include:

<script>Directly in a script</script>
<!-- Inside an HTML comment -->
<style>Directly in CSS</style>
<div ToDefineAnAttribute=test />
<ToDefineATag href="/test" />

Other areas to be careful of include:

• Callback functions
• Where URLs are handled in code such as this CSS { background-url : “javascript:alert(xss)”; }
• All JavaScript event handlers (onclick(), onerror(), onmouseover()).
• Unsafe JS functions like eval(), setInterval(), setTimeout()

Don’t place variables into dangerous contexts as even with output encoding, it will not prevent an XSS attack fully.

HTML Sanitization

Sometimes users need to author HTML. One scenario would be allow users to change the styling or structure of content inside a WYSIWYG editor. Output encoding here will prevent XSS, but it will break the intended functionality of the application. The styling will not be rendered. In these cases, HTML Sanitization should be used.

HTML Sanitization will strip dangerous HTML from a variable and return a safe string of HTML. OWASP recommends DOMPurify for HTML Sanitization.

let clean = DOMPurify.sanitize(dirty);

There are some further things to consider:

• If you sanitize content and then modify it afterwards, you can easily void your security efforts.
• If you sanitize content and then send it to a library for use, check that it doesn’t mutate that string somehow. Otherwise, again, your security efforts are void.
• You must regularly patch DOMPurify or other HTML Sanitization libraries that you use. Browsers change functionality and bypasses are being discovered regularly.

Safe Sinks

Security professionals often talk in terms of sources and sinks. If you pollute a river, it’ll flow downstream somewhere. It’s the same with computer security. XSS sinks are places where variables are placed into your webpage.

Thankfully, many sinks where variables can be placed are safe. This is because these sinks treat the variable as text and will never execute it. Try to refactor your code to remove references to unsafe sinks like innerHTML, and instead use textContent or value.

elem.textContent = dangerVariable;
elem.insertAdjacentText(dangerVariable);
elem.className = dangerVariable;
elem.setAttribute(safeName, dangerVariable);
formfield.value = dangerVariable;
document.createTextNode(dangerVariable);
document.createElement(dangerVariable);
elem.innerHTML = DOMPurify.sanitize(dangerVar);

Safe HTML Attributes include: align, alink, alt, bgcolor, border, cellpadding, cellspacing, class, color, cols, colspan, coords, dir, face, height, hspace, ismap, lang, marginheight, marginwidth, multiple, nohref, noresize, noshade, nowrap, ref, rel, rev, rows, rowspan, scrolling, shape, span, summary, tabindex, title, usemap, valign, value, vlink, vspace, width.

For a comprehensive list, check out the DOMPurify allowlist

Other Controls

Framework Security Protections, Output Encoding, and HTML Sanitization will provide the best protection for your application. OWASP recommends these in all circumstances.

Consider adopting the following controls in addition to the above.

• Cookie Attributes - These change how JavaScript and browsers can interact with cookies. Cookie attributes try to limit the impact of an XSS attack but don’t prevent the execution of malicious content or address the root cause of the vulnerability.
• Content Security Policy - An allowlist that prevents content being loaded. It’s easy to make mistakes with the implementation so it should not be your primary defense mechanism. Use a CSP as an additional layer of defense and have a look at the cheatsheet here.
• Web Application Firewalls - These look for known attack strings and block them. WAF’s are unreliable and new bypass techniques are being discovered regularly. WAFs also don’t address the root cause of an XSS vulnerability. In addition, WAFs also miss a class of XSS vulnerabilities that operate exclusively client-side. WAFs are not recommended for preventing XSS, especially DOM-Based XSS.

XSS Prevention Rules Summary

The following snippets of HTML demonstrate how to safely render untrusted data in a variety of different contexts.

Output Encoding Rules Summary

The purpose of output encoding (as it relates to Cross Site Scripting) is to convert untrusted input into a safe form where the input is displayed as data to the user without executing as code in the browser. The following charts details a list of critical output encoding methods needed to stop Cross Site Scripting.

Related Articles

XSS Attack Cheat Sheet:

The following article describes how to exploit different kinds of XSS Vulnerabilities that this article was created to help you avoid:

• OWASP: XSS Filter Evasion Cheat Sheet.

Description of XSS Vulnerabilities:

• OWASP article on XSS Vulnerabilities.

Discussion on the Types of XSS Vulnerabilities:

• Types of Cross-Site Scripting.

How to Review Code for Cross-site scripting Vulnerabilities:

• OWASP Code Review Guide article on Reviewing Code for Cross-site scripting Vulnerabilities.

How to Test for Cross-site scripting Vulnerabilities:

• OWASP Testing Guide article on testing for Cross-Site Scripting vulnerabilities.
• XSS Experimental Minimal Encoding Rules