includes/libs/IEUrlExtension.php

   1 <?php
   2 /**
   3  * Checks for validity of requested URL's extension.
   4  *
   5  * This program is free software; you can redistribute it and/or modify
   6  * it under the terms of the GNU General Public License as published by
   7  * the Free Software Foundation; either version 2 of the License, or
   8  * (at your option) any later version.
   9  *
  10  * This program is distributed in the hope that it will be useful,
  11  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  12  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  13  * GNU General Public License for more details.
  14  *
  15  * You should have received a copy of the GNU General Public License along
  16  * with this program; if not, write to the Free Software Foundation, Inc.,
  17  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
  18  * http://www.gnu.org/copyleft/gpl.html
  19  *
  20  * @file
  21  */
  22
  23 /**
  24  * Internet Explorer derives a cache filename from a URL, and then in certain
  25  * circumstances, uses the extension of the resulting file to determine the
  26  * content type of the data, ignoring the Content-Type header.
  27  *
  28  * This can be a problem, especially when non-HTML content is sent by MediaWiki,
  29  * and Internet Explorer interprets it as HTML, exposing an XSS vulnerability.
  30  *
  31  * Usually the script filename (e.g. api.php) is present in the URL, and this
  32  * makes Internet Explorer think the extension is a harmless script extension.
  33  * But Internet Explorer 6 and earlier allows the script extension to be
  34  * obscured by encoding the dot as "%2E".
  35  *
  36  * This class contains functions which help in detecting and dealing with this
  37  * situation.
  38  *
  39  * Checking the URL for a bad extension is somewhat complicated due to the fact
  40  * that CGI doesn't provide a standard method to determine the URL. Instead it
  41  * is necessary to pass a subset of $_SERVER variables, which we then attempt
  42  * to use to guess parts of the URL.
  43  */
  44 class IEUrlExtension {
  45         /**
  46          * Check a subset of $_SERVER (or the whole of $_SERVER if you like)
  47          * to see if it indicates that the request was sent with a bad file
  48          * extension. Returns true if the request should be denied or modified,
  49          * false otherwise. The relevant $_SERVER elements are:
  50          *
  51          *   - SERVER_SOFTWARE
  52          *   - REQUEST_URI
  53          *   - QUERY_STRING
  54          *   - PATH_INFO
  55          *
  56          * If the a variable is unset in $_SERVER, it should be unset in $vars.
  57          *
  58          * @param array $vars A subset of $_SERVER.
  59          * @param array $extWhitelist Extensions which are allowed, assumed harmless.
  60          * @return bool
  61          */
  62         public static function areServerVarsBad( $vars, $extWhitelist = [] ) {
  63                 // Check QUERY_STRING or REQUEST_URI
  64                 if ( isset( $vars['SERVER_SOFTWARE'] )
  65                         && isset( $vars['REQUEST_URI'] )
  66                         && self::haveUndecodedRequestUri( $vars['SERVER_SOFTWARE'] )
  67                 ) {
  68                         $urlPart = $vars['REQUEST_URI'];
  69                 } elseif ( isset( $vars['QUERY_STRING'] ) ) {
  70                         $urlPart = $vars['QUERY_STRING'];
  71                 } else {
  72                         $urlPart = '';
  73                 }
  74
  75                 if ( self::isUrlExtensionBad( $urlPart, $extWhitelist ) ) {
  76                         return true;
  77                 }
  78
  79                 // Some servers have PATH_INFO but not REQUEST_URI, so we check both
  80                 // to be on the safe side.
  81                 if ( isset( $vars['PATH_INFO'] )
  82                         && self::isUrlExtensionBad( $vars['PATH_INFO'], $extWhitelist )
  83                 ) {
  84                         return true;
  85                 }
  86
  87                 // All checks passed
  88                 return false;
  89         }
  90
  91         /**
  92          * Given a right-hand portion of a URL, determine whether IE would detect
  93          * a potentially harmful file extension.
  94          *
  95          * @param string $urlPart The right-hand portion of a URL
  96          * @param array $extWhitelist An array of file extensions which may occur in this
  97          *    URL, and which should be allowed.
  98          * @return bool
  99          */
 100         public static function isUrlExtensionBad( $urlPart, $extWhitelist = [] ) {
 101                 if ( strval( $urlPart ) === '' ) {
 102                         return false;
 103                 }
 104
 105                 $extension = self::findIE6Extension( $urlPart );
 106                 if ( strval( $extension ) === '' ) {
 107                         // No extension or empty extension
 108                         return false;
 109                 }
 110
 111                 if ( in_array( $extension, [ 'php', 'php5' ] ) ) {
 112                         // Script extension, OK
 113                         return false;
 114                 }
 115                 if ( in_array( $extension, $extWhitelist ) ) {
 116                         // Whitelisted extension
 117                         return false;
 118                 }
 119
 120                 if ( !preg_match( '/^[a-zA-Z0-9_-]+$/', $extension ) ) {
 121                         // Non-alphanumeric extension, unlikely to be registered.
 122                         // The regex above is known to match all registered file extensions
 123                         // in a default Windows XP installation. It's important to allow
 124                         // extensions with ampersands and percent signs, since that reduces
 125                         // the number of false positives substantially.
 126                         return false;
 127                 }
 128
 129                 // Possibly bad extension
 130                 return true;
 131         }
 132
 133         /**
 134          * Returns a variant of $url which will pass isUrlExtensionBad() but has the
 135          * same GET parameters, or false if it can't figure one out.
 136          * @param string $url
 137          * @param array $extWhitelist
 138          * @return bool|string
 139          */
 140         public static function fixUrlForIE6( $url, $extWhitelist = [] ) {
 141                 $questionPos = strpos( $url, '?' );
 142                 if ( $questionPos === false ) {
 143                         $beforeQuery = $url . '?';
 144                         $query = '';
 145                 } elseif ( $questionPos === strlen( $url ) - 1 ) {
 146                         $beforeQuery = $url;
 147                         $query = '';
 148                 } else {
 149                         $beforeQuery = substr( $url, 0, $questionPos + 1 );
 150                         $query = substr( $url, $questionPos + 1 );
 151                 }
 152
 153                 // Multiple question marks cause problems. Encode the second and
 154                 // subsequent question mark.
 155                 $query = str_replace( '?', '%3E', $query );
 156                 // Append an invalid path character so that IE6 won't see the end of the
 157                 // query string as an extension
 158                 $query .= '&*';
 159                 // Put the URL back together
 160                 $url = $beforeQuery . $query;
 161                 if ( self::isUrlExtensionBad( $url, $extWhitelist ) ) {
 162                         // Avoid a redirect loop
 163                         return false;
 164                 }
 165                 return $url;
 166         }
 167
 168         /**
 169          * Determine what extension IE6 will infer from a certain query string.
 170          * If the URL has an extension before the question mark, IE6 will use
 171          * that and ignore the query string, but per the comment at
 172          * isPathInfoBad() we don't have a reliable way to determine the URL,
 173          * so isPathInfoBad() just passes in the query string for $url.
 174          * All entry points have safe extensions (php, php5) anyway, so
 175          * checking the query string is possibly overly paranoid but never
 176          * insecure.
 177          *
 178          * The criteria for finding an extension are as follows:
 179          * - a possible extension is a dot followed by one or more characters not
 180          *   in <>\"/:|?.#
 181          * - if we find a possible extension followed by the end of the string or
 182          *   a #, that's our extension
 183          * - if we find a possible extension followed by a ?, that's our extension
 184          *    - UNLESS it's exe, dll or cgi, in which case we ignore it and continue
 185          *      searching for another possible extension
 186          * - if we find a possible extension followed by a dot or another illegal
 187          *   character, we ignore it and continue searching
 188          *
 189          * @param string $url
 190          * @return mixed Detected extension (string), or false if none found
 191          */
 192         public static function findIE6Extension( $url ) {
 193                 $pos = 0;
 194                 $hashPos = strpos( $url, '#' );
 195                 if ( $hashPos !== false ) {
 196                         $urlLength = $hashPos;
 197                 } else {
 198                         $urlLength = strlen( $url );
 199                 }
 200                 $remainingLength = $urlLength;
 201                 while ( $remainingLength > 0 ) {
 202                         // Skip ahead to the next dot
 203                         $pos += strcspn( $url, '.', $pos, $remainingLength );
 204                         if ( $pos >= $urlLength ) {
 205                                 // End of string, we're done
 206                                 return false;
 207                         }
 208
 209                         // We found a dot. Skip past it
 210                         $pos++;
 211                         $remainingLength = $urlLength - $pos;
 212
 213                         // Check for illegal characters in our prospective extension,
 214                         // or for another dot
 215                         $nextPos = $pos + strcspn( $url, "<>\\\"/:|?*.", $pos, $remainingLength );
 216                         if ( $nextPos >= $urlLength ) {
 217                                 // No illegal character or next dot
 218                                 // We have our extension
 219                                 return substr( $url, $pos, $urlLength - $pos );
 220                         }
 221                         if ( $url[$nextPos] === '?' ) {
 222                                 // We've found a legal extension followed by a question mark
 223                                 // If the extension is NOT exe, dll or cgi, return it
 224                                 $extension = substr( $url, $pos, $nextPos - $pos );
 225                                 if ( strcasecmp( $extension, 'exe' ) && strcasecmp( $extension, 'dll' ) &&
 226                                         strcasecmp( $extension, 'cgi' )
 227                                 ) {
 228                                         return $extension;
 229                                 }
 230                                 // Else continue looking
 231                         }
 232                         // We found an illegal character or another dot
 233                         // Skip to that character and continue the loop
 234                         $pos = $nextPos;
 235                         $remainingLength = $urlLength - $pos;
 236                 }
 237                 return false;
 238         }
 239
 240         /**
 241          * When passed the value of $_SERVER['SERVER_SOFTWARE'], this function
 242          * returns true if that server is known to have a REQUEST_URI variable
 243          * with %2E not decoded to ".". On such a server, it is possible to detect
 244          * whether the script filename has been obscured.
 245          *
 246          * The function returns false if the server is not known to have this
 247          * behavior. Microsoft IIS in particular is known to decode escaped script
 248          * filenames.
 249          *
 250          * SERVER_SOFTWARE typically contains either a plain string such as "Zeus",
 251          * or a specification in the style of a User-Agent header, such as
 252          * "Apache/1.3.34 (Unix) mod_ssl/2.8.25 OpenSSL/0.9.8a PHP/4.4.2"
 253          *
 254          * @param string $serverSoftware
 255          * @return bool
 256          */
 257         public static function haveUndecodedRequestUri( $serverSoftware ) {
 258                 static $whitelist = [
 259                         'Apache',
 260                         'Zeus',
 261                         'LiteSpeed' ];
 262                 if ( preg_match( '/^(.*?)($|\/| )/', $serverSoftware, $m ) ) {
 263                         return in_array( $m[1], $whitelist );
 264                 } else {
 265                         return false;
 266                 }
 267         }
 268
 269 }