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[mediawiki.git] / includes / libs / IEUrlExtension.php

<?php
/**
 * Checks for validity of requested URL's extension.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program; if not, write to the Free Software Foundation, Inc.,
 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 * http://www.gnu.org/copyleft/gpl.html
 *
 * @file
 */

/**
 * Internet Explorer derives a cache filename from a URL, and then in certain
 * circumstances, uses the extension of the resulting file to determine the
 * content type of the data, ignoring the Content-Type header.
 *
 * This can be a problem, especially when non-HTML content is sent by MediaWiki,
 * and Internet Explorer interprets it as HTML, exposing an XSS vulnerability.
 *
 * Usually the script filename (e.g. api.php) is present in the URL, and this
 * makes Internet Explorer think the extension is a harmless script extension.
 * But Internet Explorer 6 and earlier allows the script extension to be
 * obscured by encoding the dot as "%2E".
 *
 * This class contains functions which help in detecting and dealing with this
 * situation.
 *
 * Checking the URL for a bad extension is somewhat complicated due to the fact
 * that CGI doesn't provide a standard method to determine the URL. Instead it
 * is necessary to pass a subset of $_SERVER variables, which we then attempt
 * to use to guess parts of the URL.
 */
class IEUrlExtension {
        /**
         * Check a subset of $_SERVER (or the whole of $_SERVER if you like)
         * to see if it indicates that the request was sent with a bad file
         * extension. Returns true if the request should be denied or modified,
         * false otherwise. The relevant $_SERVER elements are:
         *
         *   - SERVER_SOFTWARE
         *   - REQUEST_URI
         *   - QUERY_STRING
         *   - PATH_INFO
         *
         * If the a variable is unset in $_SERVER, it should be unset in $vars.
         *
         * @param array $vars A subset of $_SERVER.
         * @param array $extWhitelist Extensions which are allowed, assumed harmless.
         * @return bool
         */
        public static function areServerVarsBad( $vars, $extWhitelist = [] ) {
                // Check QUERY_STRING or REQUEST_URI
                if ( isset( $vars['SERVER_SOFTWARE'] )
                        && isset( $vars['REQUEST_URI'] )
                        && self::haveUndecodedRequestUri( $vars['SERVER_SOFTWARE'] ) )
                {
                        $urlPart = $vars['REQUEST_URI'];
                } elseif ( isset( $vars['QUERY_STRING'] ) ) {
                        $urlPart = $vars['QUERY_STRING'];
                } else {
                        $urlPart = '';
                }

                if ( self::isUrlExtensionBad( $urlPart, $extWhitelist ) ) {
                        return true;
                }

                // Some servers have PATH_INFO but not REQUEST_URI, so we check both
                // to be on the safe side.
                if ( isset( $vars['PATH_INFO'] )
                        && self::isUrlExtensionBad( $vars['PATH_INFO'], $extWhitelist ) )
                {
                        return true;
                }

                // All checks passed
                return false;
        }

        /**
         * Given a right-hand portion of a URL, determine whether IE would detect
         * a potentially harmful file extension.
         *
         * @param string $urlPart The right-hand portion of a URL
         * @param array $extWhitelist An array of file extensions which may occur in this
         *    URL, and which should be allowed.
         * @return bool
         */
        public static function isUrlExtensionBad( $urlPart, $extWhitelist = [] ) {
                if ( strval( $urlPart ) === '' ) {
                        return false;
                }

                $extension = self::findIE6Extension( $urlPart );
                if ( strval( $extension ) === '' ) {
                        // No extension or empty extension
                        return false;
                }

                if ( in_array( $extension, [ 'php', 'php5' ] ) ) {
                        // Script extension, OK
                        return false;
                }
                if ( in_array( $extension, $extWhitelist ) ) {
                        // Whitelisted extension
                        return false;
                }

                if ( !preg_match( '/^[a-zA-Z0-9_-]+$/', $extension ) ) {
                        // Non-alphanumeric extension, unlikely to be registered.
                        // The regex above is known to match all registered file extensions
                        // in a default Windows XP installation. It's important to allow
                        // extensions with ampersands and percent signs, since that reduces
                        // the number of false positives substantially.
                        return false;
                }

                // Possibly bad extension
                return true;
        }

        /**
         * Returns a variant of $url which will pass isUrlExtensionBad() but has the
         * same GET parameters, or false if it can't figure one out.
         * @param $url
         * @param $extWhitelist array
         * @return bool|string
         */
        public static function fixUrlForIE6( $url, $extWhitelist = [] ) {
                $questionPos = strpos( $url, '?' );
                if ( $questionPos === false ) {
                        $beforeQuery = $url . '?';
                        $query = '';
                } elseif ( $questionPos === strlen( $url ) - 1 ) {
                        $beforeQuery = $url;
                        $query = '';
                } else {
                        $beforeQuery = substr( $url, 0, $questionPos + 1 );
                        $query = substr( $url, $questionPos + 1 );
                }

                // Multiple question marks cause problems. Encode the second and
                // subsequent question mark.
                $query = str_replace( '?', '%3E', $query );
                // Append an invalid path character so that IE6 won't see the end of the
                // query string as an extension
                $query .= '&*';
                // Put the URL back together
                $url = $beforeQuery . $query;
                if ( self::isUrlExtensionBad( $url, $extWhitelist ) ) {
                        // Avoid a redirect loop
                        return false;
                }
                return $url;
        }

        /**
         * Determine what extension IE6 will infer from a certain query string.
         * If the URL has an extension before the question mark, IE6 will use
         * that and ignore the query string, but per the comment at
         * isPathInfoBad() we don't have a reliable way to determine the URL,
         * so isPathInfoBad() just passes in the query string for $url.
         * All entry points have safe extensions (php, php5) anyway, so
         * checking the query string is possibly overly paranoid but never
         * insecure.
         *
         * The criteria for finding an extension are as follows:
         * - a possible extension is a dot followed by one or more characters not
         *   in <>\"/:|?.#
         * - if we find a possible extension followed by the end of the string or
         *   a #, that's our extension
         * - if we find a possible extension followed by a ?, that's our extension
         *    - UNLESS it's exe, dll or cgi, in which case we ignore it and continue
         *      searching for another possible extension
         * - if we find a possible extension followed by a dot or another illegal
         *   character, we ignore it and continue searching
         *
         * @param string $url URL
         * @return mixed Detected extension (string), or false if none found
         */
        public static function findIE6Extension( $url ) {
                $pos = 0;
                $hashPos = strpos( $url, '#' );
                if ( $hashPos !== false ) {
                        $urlLength = $hashPos;
                } else {
                        $urlLength = strlen( $url );
                }
                $remainingLength = $urlLength;
                while ( $remainingLength > 0 ) {
                        // Skip ahead to the next dot
                        $pos += strcspn( $url, '.', $pos, $remainingLength );
                        if ( $pos >= $urlLength ) {
                                // End of string, we're done
                                return false;
                        }

                        // We found a dot. Skip past it
                        $pos++;
                        $remainingLength = $urlLength - $pos;

                        // Check for illegal characters in our prospective extension,
                        // or for another dot
                        $nextPos = $pos + strcspn( $url, "<>\\\"/:|?*.", $pos, $remainingLength );
                        if ( $nextPos >= $urlLength ) {
                                // No illegal character or next dot
                                // We have our extension
                                return substr( $url, $pos, $urlLength - $pos );
                        }
                        if ( $url[$nextPos] === '?' ) {
                                // We've found a legal extension followed by a question mark
                                // If the extension is NOT exe, dll or cgi, return it
                                $extension = substr( $url, $pos, $nextPos - $pos );
                                if ( strcasecmp( $extension, 'exe' ) && strcasecmp( $extension, 'dll' ) &&
                                        strcasecmp( $extension, 'cgi' ) )
                                {
                                        return $extension;
                                }
                                // Else continue looking
                        }
                        // We found an illegal character or another dot
                        // Skip to that character and continue the loop
                        $pos = $nextPos;
                        $remainingLength = $urlLength - $pos;
                }
                return false;
        }

        /**
         * When passed the value of $_SERVER['SERVER_SOFTWARE'], this function
         * returns true if that server is known to have a REQUEST_URI variable
         * with %2E not decoded to ".". On such a server, it is possible to detect
         * whether the script filename has been obscured.
         *
         * The function returns false if the server is not known to have this
         * behavior. Microsoft IIS in particular is known to decode escaped script
         * filenames.
         *
         * SERVER_SOFTWARE typically contains either a plain string such as "Zeus",
         * or a specification in the style of a User-Agent header, such as
         * "Apache/1.3.34 (Unix) mod_ssl/2.8.25 OpenSSL/0.9.8a PHP/4.4.2"
         *
         * @param $serverSoftware
         * @return bool
         */
        public static function haveUndecodedRequestUri( $serverSoftware ) {
                static $whitelist = [
                        'Apache',
                        'Zeus',
                        'LiteSpeed' ];
                if ( preg_match( '/^(.*?)($|\/| )/', $serverSoftware, $m ) ) {
                        return in_array( $m[1], $whitelist );
                } else {
                        return false;
                }
        }

}