Java tutorial
/* * Copyright 2011 Google Inc. * * Licensed under the Apache License, Version 2.0 (the "License"); you may not * use this file except in compliance with the License. You may obtain a copy of * the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the * License for the specific language governing permissions and limitations under * the License. */ package com.google.gwt.safecss.shared; import 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 * {@code style} element, or inside the {@code style} attribute of a DOM * element. * * <p> * Note on usage: {@link SafeStyles} should be used to ensure user input is not * executed in the browser. {@link SafeStyles} should not be used to sanitize * input before sending it to the server: The server cannot rely on the type * contract of {@link 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. * * <p> * All implementing classes must maintain the class invariant (by design and * implementation and/or convention of use), that invoking {@link #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, {@link SafeStyles} should be of the form * {@code cssPropertyName:value;}, where neither the name nor the value contain * malicious scripts. * * <p> * {@link SafeStyles} may never contain literal angle brackets. Otherwise, it * could be unsafe to place a {@link SafeStyles} into a <style> tag (where * it can't be HTML escaped). For example, if the {@link SafeStyles} containing * "<code>font: 'foo <style><script>evil</script></code>'" is * used in a style sheet in a <style> tag, this could then break out of * the style context into HTML. * * <p> * {@link 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). * * <p> * Furthermore, values of this type must be composable, i.e. for any two values * {@code A} and {@code B} of this type, {@code A.asString() + B.asString()} * must itself be a value that satisfies the {@link SafeStyles} type constraint. * This requirement implies that for any value {@code A} of this type, * {@code A.asString()} must not end in a "CSS value" or "CSS name" context. For * example, a value of {@code background:url("} or {@code font-} would not * satisfy the {@link 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 * {@code javascript:evil())"} is appended to {@code background:url("}, the * resulting string may result in the execution of a malicious script. * * <p> * The following example values comply with this type's contract: * <ul> * <li><code>width: 1em;</code></li> * <li><code>height:1em;</code></li> * <li><code>width: 1em;height: 1em;</code></li> * <li><code>background:url('http://url');</code></li> * </ul> * In addition, the empty string is safe for use in a CSS attribute. * * <p> * The following example values do <em>not</em> comply with this type's contract: * <ul> * <li><code>background: red</code> (missing a trailing semi-colon)</li> * <li><code>background:</code> (missing a value and a trailing semi-colon)</li> * <li><code>1em</code> (missing an attribute name, which provides context for the value)</li> * </ul> * * <p> * All implementations must implement equals() and hashCode() to behave * consistently with the result of asString().equals() and asString.hashCode(). * * <p> * Implementations must not return {@code null} from {@link #asString()}. */ public interface SafeStyles extends Serializable { /* * Notes regarding serialization: * * - It may be reasonable to allow deserialization on the client of objects * serialized on the server (i.e. RPC responses), based on the assumption that * server code is trusted and would not provide a malicious serialized form * (if a MitM were able to modify server responses, the client would be fully * compromised in any case). However, the GWT RPC framework currently does not * seem to provide a facility for restricting deserialization on the Server * only (though this shouldn't be difficult to implement through a custom * SerializationPolicy) * * - Some implementations of SafeStyles would in principle be able to enforce * their class invariant on deserialization. However, the GWT RPC framework * does not provide for an equivalent of readResolve() to enforce the class * invariant on deserialization. */ /** * Returns this object's contained CSS as a string. * * <p> * Based on this class' contract, the returned value will be non-null and a * string that is safe to use in an CSS attribute context. * * @return the contents as a String */ String asString(); /** * Compares this string to the specified object. Must be equal to * asString().equals(). * * @param anObject the object to compare to */ boolean equals(Object anObject); /** * Returns a hash code for this string. Must be equal to * asString().hashCode(). */ int hashCode(); }