public interface SafeStyles
extends java.io.Serializable

An object that implements this interface encapsulates zero or more CSS properties that are guaranteed to be safe to use (with respect to potential Cross-Site-Scripting vulnerabilities) in a CSS (Cascading Style Sheet) attribute context. A CSS attribute context can be inside of a CSS rule in a style element, or inside the style attribute of a DOM element.

Note on usage: SafeStyles should be used to ensure user input is not executed in the browser. SafeStyles should not be used to sanitize input before sending it to the server: The server cannot rely on the type contract of SafeStyles values received from clients, because a malicious client could provide maliciously crafted serialized forms of implementations of this type that violate the type contract.

All implementing classes must maintain the class invariant (by design and implementation and/or convention of use), that invoking asString() on any instance will return a string that is safe to assign to a CSS attribute in a browser, in the sense that doing so must not cause execution of script in the browser. Generally, SafeStyles should be of the form cssPropertyName:value;, where neither the name nor the value contain malicious scripts.

SafeStyles may never contain literal angle brackets. Otherwise, it could be unsafe to place a SafeStyles into a <style> tag (where it can't be HTML escaped). For example, if the SafeStyles containing "font: 'foo <style><script>evil</script>'" is used in a style sheet in a <style> tag, this could then break out of the style context into HTML.

SafeStyles may contain literal single or double quotes, and as such the entire style string must be escaped when used in a style attribute (if this were not the case, the string could contain a matching quote that would escape from the style attribute).

Furthermore, values of this type must be composable, i.e. for any two values A and B of this type, A.asString() + B.asString() must itself be a value that satisfies the SafeStyles type constraint. This requirement implies that for any value A of this type, A.asString() must not end in a "CSS value" or "CSS name" context. For example, a value of background:url(" or font- would not satisfy the SafeStyles contract. This is because concatenating such strings with a second value that itself does not contain unsafe CSS can result in an overall string that does. For example, if javascript:evil())" is appended to background:url(", the resulting string may result in the execution of a malicious script.

The following example values comply with this type's contract:

• width: 1em;
• height:1em;
• width: 1em;height: 1em;
• background:url('http://url');

In addition, the empty string is safe for use in a CSS attribute.

The following example values do not comply with this type's contract:

• background: red (missing a trailing semi-colon)
• background: (missing a value and a trailing semi-colon)
• 1em (missing an attribute name, which provides context for the value)

All implementations must implement equals() and hashCode() to behave consistently with the result of asString().equals() and asString.hashCode().

Implementations must not return null from asString().