// Copyright 2013 The Closure Library Authors. All Rights Reserved.
//
// 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.

/**
 * @fileoverview The SafeUrl type and its builders.
 *
 * TODO(user): Link to document stating type contract.
 */

goog.provide('goog.html.SafeUrl');

goog.require('goog.asserts');
goog.require('goog.i18n.bidi.Dir');
goog.require('goog.i18n.bidi.DirectionalString');
goog.require('goog.string.Const');
goog.require('goog.string.TypedString');



/**
 * A string that is safe to use in URL context in DOM APIs and HTML documents.
 *
 * A SafeUrl is a string-like object that carries the security type contract
 * that its value as a string will not cause untrusted script execution
 * when evaluated as a hyperlink URL in a browser.
 *
 * Values of this type are guaranteed to be safe to use in URL/hyperlink
 * contexts, such as, assignment to URL-valued DOM properties, or
 * interpolation into a HTML template in URL context (e.g., inside a href
 * attribute), in the sense that the use will not result in a
 * Cross-Site-Scripting vulnerability.
 *
 * Note that, as documented in {@code goog.html.SafeUrl.unwrap}, this type's
 * contract does not guarantee that instances are safe to interpolate into HTML
 * without appropriate escaping.
 *
 * Note also that this type's contract does not imply any guarantees regarding
 * the resource the URL refers to.  In particular, SafeUrls are <b>not</b>
 * safe to use in a context where the referred-to resource is interpreted as
 * trusted code, e.g., as the src of a script tag.
 *
 * Instances of this type must be created via the factory methods
 * ({@code goog.html.SafeUrl.fromConstant}, {@code goog.html.SafeUrl.sanitize}),
 * etc and not by invoking its constructor.  The constructor intentionally
 * takes no parameters and the type is immutable; hence only a default instance
 * corresponding to the empty string can be obtained via constructor invocation.
 *
 * @see goog.html.SafeUrl#fromConstant
 * @see goog.html.SafeUrl#from
 * @see goog.html.SafeUrl#sanitize
 * @constructor
 * @final
 * @struct
 * @implements {goog.i18n.bidi.DirectionalString}
 * @implements {goog.string.TypedString}
 */
goog.html.SafeUrl = function() {
  /**
   * The contained value of this SafeUrl.  The field has a purposely ugly
   * name to make (non-compiled) code that attempts to directly access this
   * field stand out.
   * @private {string}
   */
  this.privateDoNotAccessOrElseSafeHtmlWrappedValue_ = '';

  /**
   * A type marker used to implement additional run-time type checking.
   * @see goog.html.SafeUrl#unwrap
   * @const
   * @private
   */
  this.SAFE_URL_TYPE_MARKER_GOOG_HTML_SECURITY_PRIVATE_ =
      goog.html.SafeUrl.TYPE_MARKER_GOOG_HTML_SECURITY_PRIVATE_;
};


/**
 * The innocuous string generated by goog.html.SafeUrl.sanitize when passed
 * an unsafe URL.
 *
 * about:invalid is registered in
 * http://www.w3.org/TR/css3-values/#about-invalid.
 * http://tools.ietf.org/html/rfc6694#section-2.2.1 permits about URLs to
 * contain a fragment, which is not to be considered when determining if an
 * about URL is well-known.
 *
 * Using about:invalid seems preferable to using a fixed data URL, since
 * browsers might choose to not report CSP violations on it, as legitimate
 * CSS function calls to attr() can result in this URL being produced. It is
 * also a standard URL which matches exactly the semantics we need:
 * "The about:invalid URI references a non-existent document with a generic
 * error condition. It can be used when a URI is necessary, but the default
 * value shouldn't be resolveable as any type of document".
 *
 * @const {string}
 */
goog.html.SafeUrl.INNOCUOUS_STRING = 'about:invalid#zClosurez';


/**
 * @override
 * @const
 */
goog.html.SafeUrl.prototype.implementsGoogStringTypedString = true;


/**
 * Returns this SafeUrl's value a string.
 *
 * IMPORTANT: In code where it is security relevant that an object's type is
 * indeed {@code SafeUrl}, use {@code goog.html.SafeUrl.unwrap} instead of this
 * method. If in doubt, assume that it's security relevant. In particular, note
 * that goog.html functions which return a goog.html type do not guarantee that
 * the returned instance is of the right type. For example:
 *
 * <pre>
 * var fakeSafeHtml = new String('fake');
 * fakeSafeHtml.__proto__ = goog.html.SafeHtml.prototype;
 * var newSafeHtml = goog.html.SafeHtml.htmlEscape(fakeSafeHtml);
 * // newSafeHtml is just an alias for fakeSafeHtml, it's passed through by
 * // goog.html.SafeHtml.htmlEscape() as fakeSafeHtml instanceof
 * // goog.html.SafeHtml.
 * </pre>
 *
 * IMPORTANT: The guarantees of the SafeUrl type contract only extend to the
 * behavior of browsers when interpreting URLs. Values of SafeUrl objects MUST
 * be appropriately escaped before embedding in a HTML document. Note that the
 * required escaping is context-sensitive (e.g. a different escaping is
 * required for embedding a URL in a style property within a style
 * attribute, as opposed to embedding in a href attribute).
 *
 * @see goog.html.SafeUrl#unwrap
 * @override
 */
goog.html.SafeUrl.prototype.getTypedStringValue = function() {
  return this.privateDoNotAccessOrElseSafeHtmlWrappedValue_;
};


/**
 * @override
 * @const
 */
goog.html.SafeUrl.prototype.implementsGoogI18nBidiDirectionalString = true;


/**
 * Returns this URLs directionality, which is always {@code LTR}.
 * @override
 */
goog.html.SafeUrl.prototype.getDirection = function() {
  return goog.i18n.bidi.Dir.LTR;
};


if (goog.DEBUG) {
  /**
   * Returns a debug string-representation of this value.
   *
   * To obtain the actual string value wrapped in a SafeUrl, use
   * {@code goog.html.SafeUrl.unwrap}.
   *
   * @see goog.html.SafeUrl#unwrap
   * @override
   */
  goog.html.SafeUrl.prototype.toString = function() {
    return 'SafeUrl{' + this.privateDoNotAccessOrElseSafeHtmlWrappedValue_ +
        '}';
  };
}


/**
 * Performs a runtime check that the provided object is indeed a SafeUrl
 * object, and returns its value.
 *
 * IMPORTANT: The guarantees of the SafeUrl type contract only extend to the
 * behavior of  browsers when interpreting URLs. Values of SafeUrl objects MUST
 * be appropriately escaped before embedding in a HTML document. Note that the
 * required escaping is context-sensitive (e.g. a different escaping is
 * required for embedding a URL in a style property within a style
 * attribute, as opposed to embedding in a href attribute).
 *
 * Note that the returned value does not necessarily correspond to the string
 * with which the SafeUrl was constructed, since goog.html.SafeUrl.sanitize
 * will percent-encode many characters.
 *
 * @param {!goog.html.SafeUrl} safeUrl The object to extract from.
 * @return {string} The SafeUrl object's contained string, unless the run-time
 *     type check fails. In that case, {@code unwrap} returns an innocuous
 *     string, or, if assertions are enabled, throws
 *     {@code goog.asserts.AssertionError}.
 */
goog.html.SafeUrl.unwrap = function(safeUrl) {
  // Perform additional Run-time type-checking to ensure that safeUrl is indeed
  // an instance of the expected type.  This provides some additional protection
  // against security bugs due to application code that disables type checks.
  // Specifically, the following checks are performed:
  // 1. The object is an instance of the expected type.
  // 2. The object is not an instance of a subclass.
  // 3. The object carries a type marker for the expected type. "Faking" an
  // object requires a reference to the type marker, which has names intended
  // to stand out in code reviews.
  if (safeUrl instanceof goog.html.SafeUrl &&
      safeUrl.constructor === goog.html.SafeUrl &&
      safeUrl.SAFE_URL_TYPE_MARKER_GOOG_HTML_SECURITY_PRIVATE_ ===
          goog.html.SafeUrl.TYPE_MARKER_GOOG_HTML_SECURITY_PRIVATE_) {
    return safeUrl.privateDoNotAccessOrElseSafeHtmlWrappedValue_;
  } else {
    goog.asserts.fail('expected object of type SafeUrl, got \'' +
                      safeUrl + '\'');
    return 'type_error:SafeUrl';

  }
};


/**
 * Creates a SafeUrl object from a compile-time constant string.
 *
 * Compile-time constant strings are inherently program-controlled and hence
 * trusted.
 *
 * @param {!goog.string.Const} url A compile-time-constant string from which to
 *         create a SafeUrl.
 * @return {!goog.html.SafeUrl} A SafeUrl object initialized to {@code url}.
 */
goog.html.SafeUrl.fromConstant = function(url) {
  return goog.html.SafeUrl.createSafeUrlSecurityPrivateDoNotAccessOrElse(
      goog.string.Const.unwrap(url));
};


/**
 * A pattern that recognizes a commonly useful subset of URLs that satisfy
 * the SafeUrl contract.
 *
 * This regular expression matches a subset of URLs that will not cause script
 * execution if used in URL context within a HTML document. Specifically, this
 * regular expression matches if (comment from here on and regex copied from
 * Soy's EscapingConventions):
 * (1) Either a protocol in a whitelist (http, https, mailto).
 * (2) or no protocol.  A protocol must be followed by a colon. The below
 *     allows that by allowing colons only after one of the characters [/?#].
 *     A colon after a hash (#) must be in the fragment.
 *     Otherwise, a colon after a (?) must be in a query.
 *     Otherwise, a colon after a single solidus (/) must be in a path.
 *     Otherwise, a colon after a double solidus (//) must be in the authority
 *     (before port).
 *
 * The pattern disallows &, used in HTML entity declarations before
 * one of the characters in [/?#]. This disallows HTML entities used in the
 * protocol name, which should never happen, e.g. "h&#116;tp" for "http".
 * It also disallows HTML entities in the first path part of a relative path,
 * e.g. "foo&lt;bar/baz".  Our existing escaping functions should not produce
 * that. More importantly, it disallows masking of a colon,
 * e.g. "javascript&#58;...".
 *
 * @private
 * @const {!RegExp}
 */
goog.html.SAFE_URL_PATTERN_ = /^(?:(?:https?|mailto):|[^&:/?#]*(?:[/?#]|$))/i;


/**
 * Creates a SafeUrl object from {@code url}. If {@code url} is a
 * goog.html.SafeUrl then it is simply returned. Otherwise the input string is
 * validated to match a pattern of commonly used safe URLs. The string is
 * converted to UTF-8 and non-whitelisted characters are percent-encoded. The
 * string wrapped by the created SafeUrl will thus contain only ASCII printable
 * characters.
 *
 * {@code url} may be a URL with the http, https, or mailto scheme,
 * or a relative URL (i.e., a URL without a scheme; specifically, a
 * scheme-relative, absolute-path-relative, or path-relative URL).
 *
 * {@code url} is converted to UTF-8 and non-whitelisted characters are
 * percent-encoded. Whitelisted characters are '%' and, from RFC 3986,
 * unreserved characters and reserved characters, with the exception of '\'',
 * '(' and ')'. This ensures the the SafeUrl contains only ASCII-printable
 * characters and reduces the chance of security bugs were it to be
 * interpolated into a specific context without the necessary escaping.
 *
 * If {@code url} fails validation or does not UTF-16 decode correctly
 * (JavaScript strings are UTF-16 encoded), this function returns a SafeUrl
 * object containing an innocuous string, goog.html.SafeUrl.INNOCUOUS_STRING.
 *
 * @see http://url.spec.whatwg.org/#concept-relative-url
 * @param {string|!goog.string.TypedString} url The URL to validate.
 * @return {!goog.html.SafeUrl} The validated URL, wrapped as a SafeUrl.
 */
goog.html.SafeUrl.sanitize = function(url) {
  if (url instanceof goog.html.SafeUrl) {
    return url;
  }
  else if (url.implementsGoogStringTypedString) {
    url = url.getTypedStringValue();
  } else {
    url = String(url);
  }
  if (!goog.html.SAFE_URL_PATTERN_.test(url)) {
    url = goog.html.SafeUrl.INNOCUOUS_STRING;
  } else {
    url = goog.html.SafeUrl.normalize_(url);
  }
  return goog.html.SafeUrl.createSafeUrlSecurityPrivateDoNotAccessOrElse(url);
};


/**
 * Normalizes {@code url} the UTF-8 encoding of url, using a whitelist of
 * characters. Whitelisted characters are not percent-encoded.
 * @param {string} url The URL to normalize.
 * @return {string} The normalized URL.
 * @private
 */
goog.html.SafeUrl.normalize_ = function(url) {
  try {
    var normalized = encodeURI(url);
  } catch (e) {  // Happens if url contains invalid surrogate sequences.
    return goog.html.SafeUrl.INNOCUOUS_STRING;
  }

  return normalized.replace(
      goog.html.SafeUrl.NORMALIZE_MATCHER_,
      function(match) {
        return goog.html.SafeUrl.NORMALIZE_REPLACER_MAP_[match];
      });
};


/**
 * Matches characters and strings which need to be replaced in the string
 * generated by encodeURI. Specifically:
 *
 * - '\'', '(' and ')' are not encoded. They are part of the reserved
 *   characters group in RFC 3986 but only appear in the obsolete mark
 *   production in Appendix D.2 of RFC 3986, so they can be encoded without
 *   changing semantics.
 * - '[' and ']' are encoded by encodeURI, despite being reserved characters
 *   which can be used to represent IPv6 addresses. So they need to be decoded.
 * - '%' is encoded by encodeURI. However, encoding '%' characters that are
 *   already part of a valid percent-encoded sequence changes the semantics of a
 *   URL, and hence we need to preserve them. Note that this may allow
 *   non-encoded '%' characters to remain in the URL (i.e., occurrences of '%'
 *   that are not part of a valid percent-encoded sequence, for example,
 *   'ab%xy').
 *
 * @const {!RegExp}
 * @private
 */
goog.html.SafeUrl.NORMALIZE_MATCHER_ = /[()']|%5B|%5D|%25/g;


/**
 * Map of replacements to be done in string generated by encodeURI.
 * @const {!Object<string, string>}
 * @private
 */
goog.html.SafeUrl.NORMALIZE_REPLACER_MAP_ = {
  '\'': '%27',
  '(': '%28',
  ')': '%29',
  '%5B': '[',
  '%5D': ']',
  '%25': '%'
};


/**
 * Type marker for the SafeUrl type, used to implement additional run-time
 * type checking.
 * @const
 * @private
 */
goog.html.SafeUrl.TYPE_MARKER_GOOG_HTML_SECURITY_PRIVATE_ = {};


/**
 * Package-internal utility method to create SafeUrl instances.
 *
 * @param {string} url The string to initialize the SafeUrl object with.
 * @return {!goog.html.SafeUrl} The initialized SafeUrl object.
 * @package
 */
goog.html.SafeUrl.createSafeUrlSecurityPrivateDoNotAccessOrElse = function(
    url) {
  var safeUrl = new goog.html.SafeUrl();
  safeUrl.privateDoNotAccessOrElseSafeHtmlWrappedValue_ = url;
  return safeUrl;
};