lib/goog/html/safescript.js

1// Copyright 2014 The Closure Library Authors. All Rights Reserved.
2//
3// Licensed under the Apache License, Version 2.0 (the "License");
4// you may not use this file except in compliance with the License.
5// You may obtain a copy of the License at
6//
7// http://www.apache.org/licenses/LICENSE-2.0
8//
9// Unless required by applicable law or agreed to in writing, software
10// distributed under the License is distributed on an "AS-IS" BASIS,
11// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12// See the License for the specific language governing permissions and
13// limitations under the License.
14
15/**
16 * @fileoverview The SafeScript type and its builders.
17 *
18 * TODO(xtof): Link to document stating type contract.
19 */
20
21goog.provide('goog.html.SafeScript');
22
23goog.require('goog.asserts');
24goog.require('goog.string.Const');
25goog.require('goog.string.TypedString');
26
27
28
29/**
30 * A string-like object which represents JavaScript code and that carries the
31 * security type contract that its value, as a string, will not cause execution
32 * of unconstrained attacker controlled code (XSS) when evaluated as JavaScript
33 * in a browser.
34 *
35 * Instances of this type must be created via the factory method
36 * {@code goog.html.SafeScript.fromConstant} and not by invoking its
37 * constructor. The constructor intentionally takes no parameters and the type
38 * is immutable; hence only a default instance corresponding to the empty string
39 * can be obtained via constructor invocation.
40 *
41 * A SafeScript's string representation can safely be interpolated as the
42 * content of a script element within HTML. The SafeScript string should not be
43 * escaped before interpolation.
44 *
45 * Note that the SafeScript might contain text that is attacker-controlled but
46 * that text should have been interpolated with appropriate escaping,
47 * sanitization and/or validation into the right location in the script, such
48 * that it is highly constrained in its effect (for example, it had to match a
49 * set of whitelisted words).
50 *
51 * A SafeScript can be constructed via security-reviewed unchecked
52 * conversions. In this case producers of SafeScript must ensure themselves that
53 * the SafeScript does not contain unsafe script. Note in particular that
54 * {@code <} is dangerous, even when inside JavaScript strings, and so should
55 * always be forbidden or JavaScript escaped in user controlled input. For
56 * example, if {@code </script><script>evil</script>"} were
57 * interpolated inside a JavaScript string, it would break out of the context
58 * of the original script element and {@code evil} would execute. Also note
59 * that within an HTML script (raw text) element, HTML character references,
60 * such as "<" are not allowed. See
61 * http://www.w3.org/TR/html5/scripting-1.html#restrictions-for-contents-of-script-elements.
62 *
63 * @see goog.html.SafeScript#fromConstant
64 * @constructor
65 * @final
66 * @struct
67 * @implements {goog.string.TypedString}
68 */
69goog.html.SafeScript = function() {
70 /**
71 * The contained value of this SafeScript. The field has a purposely
72 * ugly name to make (non-compiled) code that attempts to directly access this
73 * field stand out.
74 * @private {string}
75 */
76 this.privateDoNotAccessOrElseSafeScriptWrappedValue_ = '';
77
78 /**
79 * A type marker used to implement additional run-time type checking.
80 * @see goog.html.SafeScript#unwrap
81 * @const
82 * @private
83 */
84 this.SAFE_SCRIPT_TYPE_MARKER_GOOG_HTML_SECURITY_PRIVATE_ =
85 goog.html.SafeScript.TYPE_MARKER_GOOG_HTML_SECURITY_PRIVATE_;
86};
87
88
89/**
90 * @override
91 * @const
92 */
93goog.html.SafeScript.prototype.implementsGoogStringTypedString = true;
94
95
96/**
97 * Type marker for the SafeScript type, used to implement additional
98 * run-time type checking.
99 * @const {!Object}
100 * @private
101 */
102goog.html.SafeScript.TYPE_MARKER_GOOG_HTML_SECURITY_PRIVATE_ = {};
103
104
105/**
106 * Creates a SafeScript object from a compile-time constant string.
107 *
108 * @param {!goog.string.Const} script A compile-time-constant string from which
109 * to create a SafeScript.
110 * @return {!goog.html.SafeScript} A SafeScript object initialized to
111 * {@code script}.
112 */
113goog.html.SafeScript.fromConstant = function(script) {
114 var scriptString = goog.string.Const.unwrap(script);
115 if (scriptString.length === 0) {
116 return goog.html.SafeScript.EMPTY;
117 }
118 return goog.html.SafeScript.createSafeScriptSecurityPrivateDoNotAccessOrElse(
119 scriptString);
120};
121
122
123/**
124 * Returns this SafeScript's value as a string.
125 *
126 * IMPORTANT: In code where it is security relevant that an object's type is
127 * indeed {@code SafeScript}, use {@code goog.html.SafeScript.unwrap} instead of
128 * this method. If in doubt, assume that it's security relevant. In particular,
129 * note that goog.html functions which return a goog.html type do not guarantee
130 * the returned instance is of the right type. For example:
131 *
132 * <pre>
133 * var fakeSafeHtml = new String('fake');
134 * fakeSafeHtml.__proto__ = goog.html.SafeHtml.prototype;
135 * var newSafeHtml = goog.html.SafeHtml.htmlEscape(fakeSafeHtml);
136 * // newSafeHtml is just an alias for fakeSafeHtml, it's passed through by
137 * // goog.html.SafeHtml.htmlEscape() as fakeSafeHtml
138 * // instanceof goog.html.SafeHtml.
139 * </pre>
140 *
141 * @see goog.html.SafeScript#unwrap
142 * @override
143 */
144goog.html.SafeScript.prototype.getTypedStringValue = function() {
145 return this.privateDoNotAccessOrElseSafeScriptWrappedValue_;
146};
147
148
149if (goog.DEBUG) {
150 /**
151 * Returns a debug string-representation of this value.
152 *
153 * To obtain the actual string value wrapped in a SafeScript, use
154 * {@code goog.html.SafeScript.unwrap}.
155 *
156 * @see goog.html.SafeScript#unwrap
157 * @override
158 */
159 goog.html.SafeScript.prototype.toString = function() {
160 return 'SafeScript{' +
161 this.privateDoNotAccessOrElseSafeScriptWrappedValue_ + '}';
162 };
163}
164
165
166/**
167 * Performs a runtime check that the provided object is indeed a
168 * SafeScript object, and returns its value.
169 *
170 * @param {!goog.html.SafeScript} safeScript The object to extract from.
171 * @return {string} The safeScript object's contained string, unless
172 * the run-time type check fails. In that case, {@code unwrap} returns an
173 * innocuous string, or, if assertions are enabled, throws
174 * {@code goog.asserts.AssertionError}.
175 */
176goog.html.SafeScript.unwrap = function(safeScript) {
177 // Perform additional Run-time type-checking to ensure that
178 // safeScript is indeed an instance of the expected type. This
179 // provides some additional protection against security bugs due to
180 // application code that disables type checks.
181 // Specifically, the following checks are performed:
182 // 1. The object is an instance of the expected type.
183 // 2. The object is not an instance of a subclass.
184 // 3. The object carries a type marker for the expected type. "Faking" an
185 // object requires a reference to the type marker, which has names intended
186 // to stand out in code reviews.
187 if (safeScript instanceof goog.html.SafeScript &&
188 safeScript.constructor === goog.html.SafeScript &&
189 safeScript.SAFE_SCRIPT_TYPE_MARKER_GOOG_HTML_SECURITY_PRIVATE_ ===
190 goog.html.SafeScript.TYPE_MARKER_GOOG_HTML_SECURITY_PRIVATE_) {
191 return safeScript.privateDoNotAccessOrElseSafeScriptWrappedValue_;
192 } else {
193 goog.asserts.fail(
194 'expected object of type SafeScript, got \'' + safeScript + '\'');
195 return 'type_error:SafeScript';
196 }
197};
198
199
200/**
201 * Package-internal utility method to create SafeScript instances.
202 *
203 * @param {string} script The string to initialize the SafeScript object with.
204 * @return {!goog.html.SafeScript} The initialized SafeScript object.
205 * @package
206 */
207goog.html.SafeScript.createSafeScriptSecurityPrivateDoNotAccessOrElse =
208 function(script) {
209 return new goog.html.SafeScript().initSecurityPrivateDoNotAccessOrElse_(
210 script);
211};
212
213
214/**
215 * Called from createSafeScriptSecurityPrivateDoNotAccessOrElse(). This
216 * method exists only so that the compiler can dead code eliminate static
217 * fields (like EMPTY) when they're not accessed.
218 * @param {string} script
219 * @return {!goog.html.SafeScript}
220 * @private
221 */
222goog.html.SafeScript.prototype.initSecurityPrivateDoNotAccessOrElse_ = function(
223 script) {
224 this.privateDoNotAccessOrElseSafeScriptWrappedValue_ = script;
225 return this;
226};
227
228
229/**
230 * A SafeScript instance corresponding to the empty string.
231 * @const {!goog.html.SafeScript}
232 */
233goog.html.SafeScript.EMPTY =
234 goog.html.SafeScript.createSafeScriptSecurityPrivateDoNotAccessOrElse('');