lib/goog/html/safestyle.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 SafeStyle type and its builders.
17 *
18 * TODO(user): Link to document stating type contract.
19 */
20
21goog.provide('goog.html.SafeStyle');
22
23goog.require('goog.array');
24goog.require('goog.asserts');
25goog.require('goog.string');
26goog.require('goog.string.Const');
27goog.require('goog.string.TypedString');
28
29
30
31/**
32 * A string-like object which represents a sequence of CSS declarations
33 * ({@code propertyName1: propertyvalue1; propertyName2: propertyValue2; ...})
34 * and that carries the security type contract that its value, as a string,
35 * will not cause untrusted script execution (XSS) when evaluated as CSS in a
36 * browser.
37 *
38 * Instances of this type must be created via the factory methods
39 * ({@code goog.html.SafeStyle.create} or
40 * {@code goog.html.SafeStyle.fromConstant}) and not by invoking its
41 * constructor. The constructor intentionally takes no parameters and the type
42 * is immutable; hence only a default instance corresponding to the empty string
43 * can be obtained via constructor invocation.
44 *
45 * A SafeStyle's string representation ({@link #getSafeStyleString()}) can
46 * safely:
47 * <ul>
48 * <li>Be interpolated as the entire content of a *quoted* HTML style
49 * attribute, or before already existing properties. The SafeStyle string
50 * *must be HTML-attribute-escaped* (where " and ' are escaped) before
51 * interpolation.
52 * <li>Be interpolated as the entire content of a {}-wrapped block within a
53 * stylesheet, or before already existing properties. The SafeStyle string
54 * should not be escaped before interpolation. SafeStyle's contract also
55 * guarantees that the string will not be able to introduce new properties
56 * or elide existing ones.
57 * <li>Be assigned to the style property of a DOM node. The SafeStyle string
58 * should not be escaped before being assigned to the property.
59 * </ul>
60 *
61 * A SafeStyle may never contain literal angle brackets. Otherwise, it could
62 * be unsafe to place a SafeStyle into a &lt;style&gt; tag (where it can't
63 * be HTML escaped). For example, if the SafeStyle containing
64 * "{@code font: 'foo &lt;style/&gt;&lt;script&gt;evil&lt;/script&gt;'}" were
65 * interpolated within a &lt;style&gt; tag, this would then break out of the
66 * style context into HTML.
67 *
68 * A SafeStyle may contain literal single or double quotes, and as such the
69 * entire style string must be escaped when used in a style attribute (if
70 * this were not the case, the string could contain a matching quote that
71 * would escape from the style attribute).
72 *
73 * Values of this type must be composable, i.e. for any two values
74 * {@code style1} and {@code style2} of this type,
75 * {@code goog.html.SafeStyle.unwrap(style1) +
76 * goog.html.SafeStyle.unwrap(style2)} must itself be a value that satisfies
77 * the SafeStyle type constraint. This requirement implies that for any value
78 * {@code style} of this type, {@code goog.html.SafeStyle.unwrap(style)} must
79 * not end in a "property value" or "property name" context. For example,
80 * a value of {@code background:url("} or {@code font-} would not satisfy the
81 * SafeStyle contract. This is because concatenating such strings with a
82 * second value that itself does not contain unsafe CSS can result in an
83 * overall string that does. For example, if {@code javascript:evil())"} is
84 * appended to {@code background:url("}, the resulting string may result in
85 * the execution of a malicious script.
86 *
87 * TODO(user): Consider whether we should implement UTF-8 interchange
88 * validity checks and blacklisting of newlines (including Unicode ones) and
89 * other whitespace characters (\t, \f). Document here if so and also update
90 * SafeStyle.fromConstant().
91 *
92 * The following example values comply with this type's contract:
93 * <ul>
94 * <li><pre>width: 1em;</pre>
95 * <li><pre>height:1em;</pre>
96 * <li><pre>width: 1em;height: 1em;</pre>
97 * <li><pre>background:url('http://url');</pre>
98 * </ul>
99 * In addition, the empty string is safe for use in a CSS attribute.
100 *
101 * The following example values do NOT comply with this type's contract:
102 * <ul>
103 * <li><pre>background: red</pre> (missing a trailing semi-colon)
104 * <li><pre>background:</pre> (missing a value and a trailing semi-colon)
105 * <li><pre>1em</pre> (missing an attribute name, which provides context for
106 * the value)
107 * </ul>
108 *
109 * @see goog.html.SafeStyle#create
110 * @see goog.html.SafeStyle#fromConstant
111 * @see http://www.w3.org/TR/css3-syntax/
112 * @constructor
113 * @final
114 * @struct
115 * @implements {goog.string.TypedString}
116 */
117goog.html.SafeStyle = function() {
118 /**
119 * The contained value of this SafeStyle. The field has a purposely
120 * ugly name to make (non-compiled) code that attempts to directly access this
121 * field stand out.
122 * @private {string}
123 */
124 this.privateDoNotAccessOrElseSafeStyleWrappedValue_ = '';
125
126 /**
127 * A type marker used to implement additional run-time type checking.
128 * @see goog.html.SafeStyle#unwrap
129 * @const
130 * @private
131 */
132 this.SAFE_STYLE_TYPE_MARKER_GOOG_HTML_SECURITY_PRIVATE_ =
133 goog.html.SafeStyle.TYPE_MARKER_GOOG_HTML_SECURITY_PRIVATE_;
134};
135
136
137/**
138 * @override
139 * @const
140 */
141goog.html.SafeStyle.prototype.implementsGoogStringTypedString = true;
142
143
144/**
145 * Type marker for the SafeStyle type, used to implement additional
146 * run-time type checking.
147 * @const
148 * @private
149 */
150goog.html.SafeStyle.TYPE_MARKER_GOOG_HTML_SECURITY_PRIVATE_ = {};
151
152
153/**
154 * Creates a SafeStyle object from a compile-time constant string.
155 *
156 * {@code style} should be in the format
157 * {@code name: value; [name: value; ...]} and must not have any < or >
158 * characters in it. This is so that SafeStyle's contract is preserved,
159 * allowing the SafeStyle to correctly be interpreted as a sequence of CSS
160 * declarations and without affecting the syntactic structure of any
161 * surrounding CSS and HTML.
162 *
163 * This method performs basic sanity checks on the format of {@code style}
164 * but does not constrain the format of {@code name} and {@code value}, except
165 * for disallowing tag characters.
166 *
167 * @param {!goog.string.Const} style A compile-time-constant string from which
168 * to create a SafeStyle.
169 * @return {!goog.html.SafeStyle} A SafeStyle object initialized to
170 * {@code style}.
171 */
172goog.html.SafeStyle.fromConstant = function(style) {
173 var styleString = goog.string.Const.unwrap(style);
174 if (styleString.length === 0) {
175 return goog.html.SafeStyle.EMPTY;
176 }
177 goog.html.SafeStyle.checkStyle_(styleString);
178 goog.asserts.assert(goog.string.endsWith(styleString, ';'),
179 'Last character of style string is not \';\': ' + styleString);
180 goog.asserts.assert(goog.string.contains(styleString, ':'),
181 'Style string must contain at least one \':\', to ' +
182 'specify a "name: value" pair: ' + styleString);
183 return goog.html.SafeStyle.createSafeStyleSecurityPrivateDoNotAccessOrElse(
184 styleString);
185};
186
187
188/**
189 * Checks if the style definition is valid.
190 * @param {string} style
191 * @private
192 */
193goog.html.SafeStyle.checkStyle_ = function(style) {
194 goog.asserts.assert(!/[<>]/.test(style),
195 'Forbidden characters in style string: ' + style);
196};
197
198
199/**
200 * Returns this SafeStyle's value as a string.
201 *
202 * IMPORTANT: In code where it is security relevant that an object's type is
203 * indeed {@code SafeStyle}, use {@code goog.html.SafeStyle.unwrap} instead of
204 * this method. If in doubt, assume that it's security relevant. In particular,
205 * note that goog.html functions which return a goog.html type do not guarantee
206 * the returned instance is of the right type. For example:
207 *
208 * <pre>
209 * var fakeSafeHtml = new String('fake');
210 * fakeSafeHtml.__proto__ = goog.html.SafeHtml.prototype;
211 * var newSafeHtml = goog.html.SafeHtml.htmlEscape(fakeSafeHtml);
212 * // newSafeHtml is just an alias for fakeSafeHtml, it's passed through by
213 * // goog.html.SafeHtml.htmlEscape() as fakeSafeHtml
214 * // instanceof goog.html.SafeHtml.
215 * </pre>
216 *
217 * @see goog.html.SafeStyle#unwrap
218 * @override
219 */
220goog.html.SafeStyle.prototype.getTypedStringValue = function() {
221 return this.privateDoNotAccessOrElseSafeStyleWrappedValue_;
222};
223
224
225if (goog.DEBUG) {
226 /**
227 * Returns a debug string-representation of this value.
228 *
229 * To obtain the actual string value wrapped in a SafeStyle, use
230 * {@code goog.html.SafeStyle.unwrap}.
231 *
232 * @see goog.html.SafeStyle#unwrap
233 * @override
234 */
235 goog.html.SafeStyle.prototype.toString = function() {
236 return 'SafeStyle{' +
237 this.privateDoNotAccessOrElseSafeStyleWrappedValue_ + '}';
238 };
239}
240
241
242/**
243 * Performs a runtime check that the provided object is indeed a
244 * SafeStyle object, and returns its value.
245 *
246 * @param {!goog.html.SafeStyle} safeStyle The object to extract from.
247 * @return {string} The safeStyle object's contained string, unless
248 * the run-time type check fails. In that case, {@code unwrap} returns an
249 * innocuous string, or, if assertions are enabled, throws
250 * {@code goog.asserts.AssertionError}.
251 */
252goog.html.SafeStyle.unwrap = function(safeStyle) {
253 // Perform additional Run-time type-checking to ensure that
254 // safeStyle is indeed an instance of the expected type. This
255 // provides some additional protection against security bugs due to
256 // application code that disables type checks.
257 // Specifically, the following checks are performed:
258 // 1. The object is an instance of the expected type.
259 // 2. The object is not an instance of a subclass.
260 // 3. The object carries a type marker for the expected type. "Faking" an
261 // object requires a reference to the type marker, which has names intended
262 // to stand out in code reviews.
263 if (safeStyle instanceof goog.html.SafeStyle &&
264 safeStyle.constructor === goog.html.SafeStyle &&
265 safeStyle.SAFE_STYLE_TYPE_MARKER_GOOG_HTML_SECURITY_PRIVATE_ ===
266 goog.html.SafeStyle.TYPE_MARKER_GOOG_HTML_SECURITY_PRIVATE_) {
267 return safeStyle.privateDoNotAccessOrElseSafeStyleWrappedValue_;
268 } else {
269 goog.asserts.fail(
270 'expected object of type SafeStyle, got \'' + safeStyle + '\'');
271 return 'type_error:SafeStyle';
272 }
273};
274
275
276/**
277 * Package-internal utility method to create SafeStyle instances.
278 *
279 * @param {string} style The string to initialize the SafeStyle object with.
280 * @return {!goog.html.SafeStyle} The initialized SafeStyle object.
281 * @package
282 */
283goog.html.SafeStyle.createSafeStyleSecurityPrivateDoNotAccessOrElse =
284 function(style) {
285 return new goog.html.SafeStyle().initSecurityPrivateDoNotAccessOrElse_(style);
286};
287
288
289/**
290 * Called from createSafeStyleSecurityPrivateDoNotAccessOrElse(). This
291 * method exists only so that the compiler can dead code eliminate static
292 * fields (like EMPTY) when they're not accessed.
293 * @param {string} style
294 * @return {!goog.html.SafeStyle}
295 * @private
296 */
297goog.html.SafeStyle.prototype.initSecurityPrivateDoNotAccessOrElse_ = function(
298 style) {
299 this.privateDoNotAccessOrElseSafeStyleWrappedValue_ = style;
300 return this;
301};
302
303
304/**
305 * A SafeStyle instance corresponding to the empty string.
306 * @const {!goog.html.SafeStyle}
307 */
308goog.html.SafeStyle.EMPTY =
309 goog.html.SafeStyle.createSafeStyleSecurityPrivateDoNotAccessOrElse('');
310
311
312/**
313 * The innocuous string generated by goog.html.SafeUrl.create when passed
314 * an unsafe value.
315 * @const {string}
316 */
317goog.html.SafeStyle.INNOCUOUS_STRING = 'zClosurez';
318
319
320/**
321 * Mapping of property names to their values.
322 * @typedef {!Object<string, goog.string.Const|string>}
323 */
324goog.html.SafeStyle.PropertyMap;
325
326
327/**
328 * Creates a new SafeStyle object from the properties specified in the map.
329 * @param {goog.html.SafeStyle.PropertyMap} map Mapping of property names to
330 * their values, for example {'margin': '1px'}. Names must consist of
331 * [-_a-zA-Z0-9]. Values might be strings consisting of
332 * [-,.'"%_!# a-zA-Z0-9], where " and ' must be properly balanced.
333 * Other values must be wrapped in goog.string.Const. Null value causes
334 * skipping the property.
335 * @return {!goog.html.SafeStyle}
336 * @throws {Error} If invalid name is provided.
337 * @throws {goog.asserts.AssertionError} If invalid value is provided. With
338 * disabled assertions, invalid value is replaced by
339 * goog.html.SafeStyle.INNOCUOUS_STRING.
340 */
341goog.html.SafeStyle.create = function(map) {
342 var style = '';
343 for (var name in map) {
344 if (!/^[-_a-zA-Z0-9]+$/.test(name)) {
345 throw Error('Name allows only [-_a-zA-Z0-9], got: ' + name);
346 }
347 var value = map[name];
348 if (value == null) {
349 continue;
350 }
351 if (value instanceof goog.string.Const) {
352 value = goog.string.Const.unwrap(value);
353 // These characters can be used to change context and we don't want that
354 // even with const values.
355 goog.asserts.assert(!/[{;}]/.test(value), 'Value does not allow [{;}].');
356 } else if (!goog.html.SafeStyle.VALUE_RE_.test(value)) {
357 goog.asserts.fail(
358 'String value allows only [-,."\'%_!# a-zA-Z0-9], got: ' + value);
359 value = goog.html.SafeStyle.INNOCUOUS_STRING;
360 } else if (!goog.html.SafeStyle.hasBalancedQuotes_(value)) {
361 goog.asserts.fail('String value requires balanced quotes, got: ' + value);
362 value = goog.html.SafeStyle.INNOCUOUS_STRING;
363 }
364 style += name + ':' + value + ';';
365 }
366 if (!style) {
367 return goog.html.SafeStyle.EMPTY;
368 }
369 goog.html.SafeStyle.checkStyle_(style);
370 return goog.html.SafeStyle.createSafeStyleSecurityPrivateDoNotAccessOrElse(
371 style);
372};
373
374
375/**
376 * Checks that quotes (" and ') are properly balanced inside a string. Assumes
377 * that neither escape (\) nor any other character that could result in
378 * breaking out of a string parsing context are allowed;
379 * see http://www.w3.org/TR/css3-syntax/#string-token-diagram.
380 * @param {string} value Untrusted CSS property value.
381 * @return {boolean} True if property value is safe with respect to quote
382 * balancedness.
383 * @private
384 */
385goog.html.SafeStyle.hasBalancedQuotes_ = function(value) {
386 var outsideSingle = true;
387 var outsideDouble = true;
388 for (var i = 0; i < value.length; i++) {
389 var c = value.charAt(i);
390 if (c == "'" && outsideDouble) {
391 outsideSingle = !outsideSingle;
392 } else if (c == '"' && outsideSingle) {
393 outsideDouble = !outsideDouble;
394 }
395 }
396 return outsideSingle && outsideDouble;
397};
398
399
400// Keep in sync with the error string in create().
401/**
402 * Regular expression for safe values.
403 *
404 * Quotes (" and ') are allowed, but a check must be done elsewhere to ensure
405 * they're balanced.
406 *
407 * ',' allows multiple values to be assigned to the same property
408 * (e.g. background-attachment or font-family) and hence could allow
409 * multiple values to get injected, but that should pose no risk of XSS.
410 * @const {!RegExp}
411 * @private
412 */
413goog.html.SafeStyle.VALUE_RE_ = /^[-,."'%_!# a-zA-Z0-9]+$/;
414
415
416/**
417 * Creates a new SafeStyle object by concatenating the values.
418 * @param {...(!goog.html.SafeStyle|!Array<!goog.html.SafeStyle>)} var_args
419 * SafeStyles to concatenate.
420 * @return {!goog.html.SafeStyle}
421 */
422goog.html.SafeStyle.concat = function(var_args) {
423 var style = '';
424
425 /**
426 * @param {!goog.html.SafeStyle|!Array<!goog.html.SafeStyle>} argument
427 */
428 var addArgument = function(argument) {
429 if (goog.isArray(argument)) {
430 goog.array.forEach(argument, addArgument);
431 } else {
432 style += goog.html.SafeStyle.unwrap(argument);
433 }
434 };
435
436 goog.array.forEach(arguments, addArgument);
437 if (!style) {
438 return goog.html.SafeStyle.EMPTY;
439 }
440 return goog.html.SafeStyle.createSafeStyleSecurityPrivateDoNotAccessOrElse(
441 style);
442};