Overview

Cross-site scripting (XSS) is a type of computer security vulnerability typically found in web applications that enables malicious attackers to inject client-side scripts into web pages viewed by other users. Because HTML documents have a flat, serial structure that mixes control statements, formatting, and the actual content, any non-validated user-supplied data included in the resulting page without proper HTML encoding may lead to markup injection.

What ZK Encodes

All Input Components Block XSS

To prevent a XSS attack, ZK components encode any value that might be input by a user by escaping & and other unsafe characters. For example, the following statement is totally safe even if any_value contains a script like `

<script>
alert('xss')
</script>

<textbox value="${any_value}"/>

Attributes to Generate Texts

Label component’s value and those attributes that generate texts into a page including label, title, tooltiptext, placeholder, name, type, and message like createMessage, emptyMessage. (ZK encodes them with zUtl.encodeXML() at client-side.)

Since ZK implicitly turns an EL expression like ${myMessage} on a zul into a Label, so it’s encoded, too.

What ZK Doesn’t Encode

Components used to Generate HTML Directly

ZK provides several ways to write HTML tags in a zul, including html component, native namespace, and xhtml components. Since their purpose is to allow you to write HTML tags directly, ZK doesn’t encode them.

Comboitem’s content

The org.zkoss.zul.Comboitem’s content attribute is designed to allow applications to generate HTML content directly. In other words, it is not encoded. In most cases we expect these values to come from the server-side. However, if your application takes user input as the content property, you will need to encode it properly. For example, if the value of any_content, in the following example, is generated directly without proper encoding, it may be vulnerable to XSS attacks.

<html>${any_content}</html>

The content is sanitized by default to avoid XSS attack. Please don’t use JavaScript in the content.

Some methods of Clients

As the name says this utility allows more direct client-side access. Thus the methods don’t encode the strings passed into them to allow formatting of the messages at the client-side, e.g.:

Clients.showNotification("Successfully processed: <br/>" + myTextbox.getValue());

When displaying user input using methods such as org.zkoss.zk.ui.util.Clients#showBusy(java.lang.String), org.zkoss.zk.ui.util.Clients#showNotification(java.lang.String), or anything similar; or when using org.zkoss.zk.ui.util.Clients#evalJavaScript(java.lang.String) to dynamically concatenate JS code, user input should be escaped carefully.

Client-side Actions

The client-side action is not encoded and the options are interpreted as a JSON object. In most cases we expect the values to come from the server-side. However, if you allow end-users to specify them (not recommended), you should encode them by yourself.

Page Directive

All attributes of <?page?> not encoded.

Sanitize User Input

As a framework, ZK tries to maintain a good balance between flexibility and default settings. Regarding attributes that are not escaped by default, application developers should use ZK org.zkoss.xml.XMLs#escapeXML(java.lang.String) or Apache Commons Lang’s StringEscapeUtils to sanitize user input if you are taking user input as these attributes.