csrf-magic.php

   1 <?php
   2
   3 /**
   4  * @file
   5  *
   6  * csrf-magic is a PHP library that makes adding CSRF-protection to your
   7  * web applications a snap. No need to modify every form or create a database
   8  * of valid nonces; just include this file at the top of every
   9  * web-accessible page (or even better, your common include file included
  10  * in every page), and forget about it! (There are, of course, configuration
  11  * options for advanced users).
  12  *
  13  * This library is PHP4 and PHP5 compatible.
  14  */
  15
  16 // CONFIGURATION:
  17
  18 /**
  19  * By default, when you include this file csrf-magic will automatically check
  20  * and exit if the CSRF token is invalid. This will defer executing
  21  * csrf_check() until you're ready.  You can also pass false as a parameter to
  22  * that function, in which case the function will not exit but instead return
  23  * a boolean false if the CSRF check failed. This allows for tighter integration
  24  * with your system.
  25  */
  26 $GLOBALS['csrf']['defer'] = false;
  27
  28 /**
  29  * This is the amount of seconds you wish to allow before any token becomes
  30  * invalid; the default is two hours, which should be more than enough for
  31  * most websites.
  32  */
  33 $GLOBALS['csrf']['expires'] = 7200;
  34
  35 /**
  36  * Callback function to execute when there's the CSRF check fails and
  37  * $fatal == true (see csrf_check). This will usually output an error message
  38  * about the failure.
  39  */
  40 $GLOBALS['csrf']['callback'] = 'csrf_callback';
  41
  42 /**
  43  * Whether or not to include our JavaScript library which also rewrites
  44  * AJAX requests on this domain. Set this to the web path. This setting only works
  45  * with supported JavaScript libraries in Internet Explorer; see README.txt for
  46  * a list of supported libraries.
  47  */
  48 $GLOBALS['csrf']['rewrite-js'] = false;
  49
  50 /**
  51  * A secret key used when hashing items. Please generate a random string and
  52  * place it here. If you change this value, all previously generated tokens
  53  * will become invalid.
  54  */
  55 $GLOBALS['csrf']['secret'] = '';
  56 // nota bene: library code should use csrf_get_secret() and not access
  57 // this global directly
  58
  59 /**
  60  * Set this to false to disable csrf-magic's output handler, and therefore,
  61  * its rewriting capabilities. If you're serving non HTML content, you should
  62  * definitely set this false.
  63  */
  64 $GLOBALS['csrf']['rewrite'] = true;
  65
  66 /**
  67  * Whether or not to use IP addresses when binding a user to a token. This is
  68  * less reliable and less secure than sessions, but is useful when you need
  69  * to give facilities to anonymous users and do not wish to maintain a database
  70  * of valid keys.
  71  */
  72 $GLOBALS['csrf']['allow-ip'] = true;
  73
  74 /**
  75  * If this information is available, use the cookie by this name to determine
  76  * whether or not to allow the request. This is a shortcut implementation
  77  * very similar to 'key', but we randomly set the cookie ourselves.
  78  */
  79 $GLOBALS['csrf']['cookie'] = '__csrf_cookie';
  80
  81 /**
  82  * If this information is available, set this to a unique identifier (it
  83  * can be an integer or a unique username) for the current "user" of this
  84  * application. The token will then be globally valid for all of that user's
  85  * operations, but no one else. This requires that 'secret' be set.
  86  */
  87 $GLOBALS['csrf']['user'] = false;
  88
  89 /**
  90  * This is an arbitrary secret value associated with the user's session. This
  91  * will most probably be the contents of a cookie, as an attacker cannot easily
  92  * determine this information. Warning: If the attacker knows this value, they
  93  * can easily spoof a token. This is a generic implementation; sessions should
  94  * work in most cases.
  95  *
  96  * Why would you want to use this? Lets suppose you have a squid cache for your
  97  * website, and the presence of a session cookie bypasses it. Let's also say
  98  * you allow anonymous users to interact with the website; submitting forms
  99  * and AJAX. Previously, you didn't have any CSRF protection for anonymous users
 100  * and so they never got sessions; you don't want to start using sessions either,
 101  * otherwise you'll bypass the Squid cache. Setup a different cookie for CSRF
 102  * tokens, and have Squid ignore that cookie for get requests, for anonymous
 103  * users. (If you haven't guessed, this scheme was(?) used for MediaWiki).
 104  */
 105 $GLOBALS['csrf']['key'] = false;
 106
 107 /**
 108  * The name of the magic CSRF token that will be placed in all forms, i.e.
 109  * the contents of <input type="hidden" name="$name" value="CSRF-TOKEN" />
 110  */
 111 $GLOBALS['csrf']['input-name'] = '__csrf_magic';
 112
 113 /**
 114  * Set this to false if your site must work inside of frame/iframe elements,
 115  * but do so at your own risk: this configuration protects you against CSS
 116  * overlay attacks that defeat tokens.
 117  */
 118 $GLOBALS['csrf']['frame-breaker'] = true;
 119
 120 /**
 121  * Whether or not CSRF Magic should be allowed to start a new session in order
 122  * to determine the key.
 123  */
 124 $GLOBALS['csrf']['auto-session'] = true;
 125
 126 /**
 127  * Whether or not csrf-magic should produce XHTML style tags.
 128  */
 129 $GLOBALS['csrf']['xhtml'] = true;
 130
 131 // FUNCTIONS:
 132
 133 // Don't edit this!
 134 $GLOBALS['csrf']['version'] = '1.0.4';
 135
 136 /**
 137  * Rewrites <form> on the fly to add CSRF tokens to them. This can also
 138  * inject our JavaScript library.
 139  */
 140 function csrf_ob_handler($buffer, $flags) {
 141     // Even though the user told us to rewrite, we should do a quick heuristic
 142     // to check if the page is *actually* HTML. We don't begin rewriting until
 143     // we hit the first <html tag.
 144     static $is_html = false;
 145     if (!$is_html) {
 146         // not HTML until proven otherwise
 147         if (stripos($buffer, '<html') !== false) {
 148             $is_html = true;
 149         } else {
 150             return $buffer;
 151         }
 152     }
 153     $tokens = csrf_get_tokens();
 154     $name = $GLOBALS['csrf']['input-name'];
 155     $endslash = $GLOBALS['csrf']['xhtml'] ? ' /' : '';
 156     $input = "<input type='hidden' name='$name' value=\"$tokens\"$endslash>";
 157     $buffer = preg_replace('#(<form[^>]*method\s*=\s*["\']post["\'][^>]*>)#i', '$1' . $input, $buffer);
 158     if ($GLOBALS['csrf']['frame-breaker']) {
 159         $buffer = str_ireplace('</head>', '<script type="text/javascript">if (top != self) {top.location.href = self.location.href;}</script></head>', $buffer);
 160     }
 161     if ($js = $GLOBALS['csrf']['rewrite-js']) {
 162         $buffer = str_ireplace(
 163             '</head>',
 164             '<script type="text/javascript">'.
 165                 'var csrfMagicToken = "'.$tokens.'";'.
 166                 'var csrfMagicName = "'.$name.'";</script>'.
 167             '<script src="'.$js.'" type="text/javascript"></script></head>',
 168             $buffer
 169         );
 170         $script = '<script type="text/javascript">CsrfMagic.end();</script>';
 171         $buffer = str_ireplace('</body>', $script . '</body>', $buffer, $count);
 172         if (!$count) {
 173             $buffer .= $script;
 174         }
 175     }
 176     return $buffer;
 177 }
 178
 179 /**
 180  * Checks if this is a post request, and if it is, checks if the nonce is valid.
 181  * @param bool $fatal Whether or not to fatally error out if there is a problem.
 182  * @return True if check passes or is not necessary, false if failure.
 183  */
 184 function csrf_check($fatal = true) {
 185     if ($_SERVER['REQUEST_METHOD'] !== 'POST') return true;
 186     csrf_start();
 187     $name = $GLOBALS['csrf']['input-name'];
 188     $ok = false;
 189     $tokens = '';
 190     do {
 191         if (!isset($_POST[$name])) break;
 192         // we don't regenerate a token and check it because some token creation
 193         // schemes are volatile.
 194         $tokens = $_POST[$name];
 195         if (!csrf_check_tokens($tokens)) break;
 196         $ok = true;
 197     } while (false);
 198     if ($fatal && !$ok) {
 199         $callback = $GLOBALS['csrf']['callback'];
 200         if (trim($tokens, 'A..Za..z0..9:;,') !== '') $tokens = 'hidden';
 201         $callback($tokens);
 202         exit;
 203     }
 204     return $ok;
 205 }
 206
 207 /**
 208  * Retrieves a valid token(s) for a particular context. Tokens are separated
 209  * by semicolons.
 210  */
 211 function csrf_get_tokens() {
 212     $has_cookies = !empty($_COOKIE);
 213
 214     // $ip implements a composite key, which is sent if the user hasn't sent
 215     // any cookies. It may or may not be used, depending on whether or not
 216     // the cookies "stick"
 217     $secret = csrf_get_secret();
 218     if (!$has_cookies && $secret) {
 219         // :TODO: Harden this against proxy-spoofing attacks
 220         $ip = ';ip:' . csrf_hash($_SERVER['IP_ADDRESS']);
 221     } else {
 222         $ip = '';
 223     }
 224     csrf_start();
 225
 226     // These are "strong" algorithms that don't require per se a secret
 227     if (session_id()) return 'sid:' . csrf_hash(session_id()) . $ip;
 228     if ($GLOBALS['csrf']['cookie']) {
 229         $val = csrf_generate_secret();
 230         setcookie($GLOBALS['csrf']['cookie'], $val);
 231         return 'cookie:' . csrf_hash($val) . $ip;
 232     }
 233     if ($GLOBALS['csrf']['key']) return 'key:' . csrf_hash($GLOBALS['csrf']['key']) . $ip;
 234     // These further algorithms require a server-side secret
 235     if (!$secret) return 'invalid';
 236     if ($GLOBALS['csrf']['user'] !== false) {
 237         return 'user:' . csrf_hash($GLOBALS['csrf']['user']);
 238     }
 239     if ($GLOBALS['csrf']['allow-ip']) {
 240         return ltrim($ip, ';');
 241     }
 242     return 'invalid';
 243 }
 244
 245 function csrf_flattenpost($data) {
 246     $ret = array();
 247     foreach($data as $n => $v) {
 248         $ret = array_merge($ret, csrf_flattenpost2(1, $n, $v));
 249     }
 250     return $ret;
 251 }
 252 function csrf_flattenpost2($level, $key, $data) {
 253     if(!is_array($data)) return array($key => $data);
 254     $ret = array();
 255     foreach($data as $n => $v) {
 256         $nk = $level >= 1 ? $key."[$n]" : "[$n]";
 257         $ret = array_merge($ret, csrf_flattenpost2($level+1, $nk, $v));
 258     }
 259     return $ret;
 260 }
 261
 262 /**
 263  * @param $tokens is safe for HTML consumption
 264  */
 265 function csrf_callback($tokens) {
 266     // (yes, $tokens is safe to echo without escaping)
 267     header($_SERVER['SERVER_PROTOCOL'] . ' 403 Forbidden');
 268     $data = '';
 269     foreach (csrf_flattenpost($_POST) as $key => $value) {
 270         if ($key == $GLOBALS['csrf']['input-name']) continue;
 271         $data .= '<input type="hidden" name="'.htmlspecialchars($key).'" value="'.htmlspecialchars($value).'" />';
 272     }
 273     echo "<html><head><title>CSRF check failed</title></head>
 274         <body>
 275         <p>CSRF check failed. Your form session may have expired, or you may not have
 276         cookies enabled.</p>
 277         <form method='post' action=''>$data<input type='submit' value='Try again' /></form>
 278         <p>Debug: $tokens</p></body></html>
 279 ";
 280 }
 281
 282 /**
 283  * Checks if a composite token is valid. Outward facing code should use this
 284  * instead of csrf_check_token()
 285  */
 286 function csrf_check_tokens($tokens) {
 287     if (is_string($tokens)) $tokens = explode(';', $tokens);
 288     foreach ($tokens as $token) {
 289         if (csrf_check_token($token)) return true;
 290     }
 291     return false;
 292 }
 293
 294 /**
 295  * Checks if a token is valid.
 296  */
 297 function csrf_check_token($token) {
 298     if (strpos($token, ':') === false) return false;
 299     list($type, $value) = explode(':', $token, 2);
 300     if (strpos($value, ',') === false) return false;
 301     list($x, $time) = explode(',', $token, 2);
 302     if ($GLOBALS['csrf']['expires']) {
 303         if (time() > $time + $GLOBALS['csrf']['expires']) return false;
 304     }
 305     switch ($type) {
 306         case 'sid':
 307             return $value === csrf_hash(session_id(), $time);
 308         case 'cookie':
 309             $n = $GLOBALS['csrf']['cookie'];
 310             if (!$n) return false;
 311             if (!isset($_COOKIE[$n])) return false;
 312             return $value === csrf_hash($_COOKIE[$n], $time);
 313         case 'key':
 314             if (!$GLOBALS['csrf']['key']) return false;
 315             return $value === csrf_hash($GLOBALS['csrf']['key'], $time);
 316         // We could disable these 'weaker' checks if 'key' was set, but
 317         // that doesn't make me feel good then about the cookie-based
 318         // implementation.
 319         case 'user':
 320             if (!csrf_get_secret()) return false;
 321             if ($GLOBALS['csrf']['user'] === false) return false;
 322             return $value === csrf_hash($GLOBALS['csrf']['user'], $time);
 323         case 'ip':
 324             if (!csrf_get_secret()) return false;
 325             // do not allow IP-based checks if the username is set, or if
 326             // the browser sent cookies
 327             if ($GLOBALS['csrf']['user'] !== false) return false;
 328             if (!empty($_COOKIE)) return false;
 329             if (!$GLOBALS['csrf']['allow-ip']) return false;
 330             return $value === csrf_hash($_SERVER['IP_ADDRESS'], $time);
 331     }
 332     return false;
 333 }
 334
 335 /**
 336  * Sets a configuration value.
 337  */
 338 function csrf_conf($key, $val) {
 339     if (!isset($GLOBALS['csrf'][$key])) {
 340         trigger_error('No such configuration ' . $key, E_USER_WARNING);
 341         return;
 342     }
 343     $GLOBALS['csrf'][$key] = $val;
 344 }
 345
 346 /**
 347  * Starts a session if we're allowed to.
 348  */
 349 function csrf_start() {
 350     if ($GLOBALS['csrf']['auto-session'] && !session_id()) {
 351         session_start();
 352     }
 353 }
 354
 355 /**
 356  * Retrieves the secret, and generates one if necessary.
 357  */
 358 function csrf_get_secret() {
 359     if ($GLOBALS['csrf']['secret']) return $GLOBALS['csrf']['secret'];
 360     $dir = dirname(__FILE__);
 361     $file = $dir . '/csrf-secret.php';
 362     $secret = '';
 363     if (file_exists($file)) {
 364         include $file;
 365         return $secret;
 366     }
 367     if (is_writable($dir)) {
 368         $secret = csrf_generate_secret();
 369         $fh = fopen($file, 'w');
 370         fwrite($fh, '<?php $secret = "'.$secret.'";' . PHP_EOL);
 371         fclose($fh);
 372         return $secret;
 373     }
 374     return '';
 375 }
 376
 377 /**
 378  * Generates a random string as the hash of time, microtime, and mt_rand.
 379  */
 380 function csrf_generate_secret($len = 32) {
 381     $r = '';
 382     for ($i = 0; $i < 32; $i++) {
 383         $r .= chr(mt_rand(0, 255));
 384     }
 385     $r .= time() . microtime();
 386     return sha1($r);
 387 }
 388
 389 /**
 390  * Generates a hash/expiry double. If time isn't set it will be calculated
 391  * from the current time.
 392  */
 393 function csrf_hash($value, $time = null) {
 394     if (!$time) $time = time();
 395     return sha1(csrf_get_secret() . $value . $time) . ',' . $time;
 396 }
 397
 398 // Load user configuration
 399 if (function_exists('csrf_startup')) csrf_startup();
 400 // Initialize our handler
 401 if ($GLOBALS['csrf']['rewrite'])     ob_start('csrf_ob_handler');
 402 // Perform check
 403 if (!$GLOBALS['csrf']['defer'])      csrf_check();