SEC Consult Vulnerability Lab Security Advisory < 20200827-0 >
=======================================================================
              title: Multiple Vulnerabilities
            product: ZTE mobile Hotspot MS910S
 vulnerable version: DL_MF910S_CN_EUV1.00.01
      fixed version: -
         CVE number: CVE-2019-3422
             impact: High
           homepage: https://www.zte.com.cn
              found: 2019-09-25
                 by: Ying Shen
                     T. Weber (Office Vienna)
                     SEC Consult Vulnerability Lab

                     An integrated part of SEC Consult
                     Europe | Asia | North America

                     https://www.sec-consult.com

=======================================================================

Vendor description:
-------------------
"ZTE Corporation is a global leader in telecommunications and information
technology. Founded in 1985 and listed on both the Hong Kong and Shenzhen Stock
Exchanges, the company has been committed to providing integrated end-to-end
innovations to deliver excellence and value to consumers, carriers, businesses
and public sector customers from over 160 countries around the world to enable
increased connectivity and productivity."

Source: https://www.zte.com.cn/global/about/corporate_information


Business recommendation:
------------------------
The vendor recommends to change the hardware and use a newer product.

SEC Consult recommends to remove the device from productive environments.


Vulnerability overview/description:
-----------------------------------
1) Hard-coded Administrator Password
The hard-coded administrator password was found in the ZTE mobile hotspot
MS910S firmware version "CN_EUV1.00.01", which is available at.
http://devicedownload.zte.com.cn/support/product/201701161506340/soft/20170116151106465.zip

2) Known BusyBox Vulnerabilities
The used BusyBox toolkit in version 1.15.0 is outdated and contains multiple
known vulnerabilities. The outdated version was found by IoT Inspector.
One of the discovered vulnerabilities (CVE-2017-16544) was verified by using
the MEDUSA scalable firmware run-time.

3) Known Backdoor in GoAhead Webserver
An unusual "telnetd" port was identified on an emulated device which led to the
assumption that a backdoor can be opened via the GoAhead web-server.
This conclusion was done because of a blog post from another researcher:
http://blog.asiantuntijakaveri.fi/2017/03/backdoor-and-root-shell-on-zte-mf286.html

By partially reverse engineering the binaries of the GoAhead webserver, the
functionality described in the corresponding blog post can be underpinned.

Proof of concept:
-----------------
1) Hard-coded Administrator Password
The firmware file (ZTE_MF910SV1.0.1B09.bin) is using the JFFS2 filesystem which
was extracted. The hardcoded password can be found in the /etc/shadow file
within the firmware:

/_DL_MF910S_CN_EUV1.00.01.exe.extracted/Data/version/_ZTE_MF910SV1.0.1B09.bin.extracted/jffs2-root/fs_1/etc/shadow

The file content is shown below:
-------------------------------------------------------------------------------
root:$1$$zdlNHiCDxYDfeF4MZL.H3/:10933:0:99999:7:::
Admin:$1$$zdlNHiCDxYDfeF4MZL.H3/:10933:0:99999:7:::
bin::10933:0:99999:7:::
daemon::10933:0:99999:7:::
adm::10933:0:99999:7:::
lp:*:10933:0:99999:7:::
sync:*:10933:0:99999:7:::
shutdown:*:10933:0:99999:7:::
halt:*:10933:0:99999:7:::
uucp:*:10933:0:99999:7:::
operator:*:10933:0:99999:7:::
nobody::10933:0:99999:7:::
ap71::10933:0:99999:7:::
-------------------------------------------------------------------------------
Both the user "root" and "admin" are using the same weak hardcoded password
"5up".

2) Known BusyBox Vulnerabilities
BusyBox version 1.12.0 contains multiple CVEs like:
CVE-2013-1813, CVE-2016-2148, CVE-2016-6301, CVE-2011-2716, CVE-2011-5325,
CVE-2015-9261, CVE-2016-2147 and more.

The BusyBox shell autocompletion vulnerability (CVE-2017-16544) was verified on
an emulated device. A file with the name "\ectest\n\e]55;test.txt\a" was crea-
ted to trigger the vulnerability.

-------------------------------------------------------------------------------
# ls "pressing <TAB>"
test
]55;test.txt
#
-------------------------------------------------------------------------------

3) Known Backdoor in GoAhead Webserver
Starting the telnet daemon on the emulated device leads to a listener on a very
unusual port:
[...]
# telnetd
#netstat -tulen
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State
tcp        0      0 0.0.0.0:4719            0.0.0.0:*               LISTEN
[...]

Because this seems to be not configured on the system by any file, the BusyBox
binary was inspected.

The pseudocode snippet of BusyBox' telnetd function that was generated by Hex-
Rays ARM Decompiler, indicated that this port was hard-coded:
<snip>
dword_788DC = (int)"/bin/login";
  dword_788E0 = (int)"/etc/issue.net";
  v3 = sub_5DBFC(a2, "f:l:Kip:b:F", &dword_788E0, &dword_788DC, &v76, &v75);
  v4 = v3 & 8;
  v5 = v3;
  if ( !(v3 & 8) && !(v3 & 0x40) )
    sub_64A10(v3 & 8);
  if ( (v5 & 0x48) != 64 )
  {
    openlog((const char *)dword_798DC, 1, 24);
    dword_78630 = 2;
  }
  if ( v5 & 0x10 )
    v6 = sub_64FF8(v76);
  else
    v6 = 4719; <------------------------------------- Port "4719"
  if ( !v4 )
  {
    v2 = sub_65578(v75, v6);
    sub_E394(v2, 1);
    sub_D9B0(v2);
    goto LABEL_13;
  }
  dword_788D8 = (int)sub_1C480(0);
  if ( dword_788D8 )
  {
<snip>

This led to the assumption that the GoAhead webserver was modified like
described in the following blog post:
http://blog.asiantuntijakaveri.fi/2017/03/backdoor-and-root-shell-on-zte-mf286.html

Inspecting the GoAhead webserver binary reinforces this assumption. The
pseudocode was generated with Hex-Rays ARM Decompiler, like for the prior
BusyBox binary:
<snip>
int __fastcall sub_21D48(int a1)
{
  int v1; // r5
  const char *v2; // r4

  v1 = a1;
  cfg_get("debug_level");
  v2 = (const char *)sub_17DF0(v1, "change_mode", "");
  cfg_set("login_9527", "1");
  if ( !strcmp("1", v2) )
  {
    cfg_set("change_mode", "1");
    cfg_get("debug_level");
    system("mode_change 1");
  }
  else if ( !strcmp("2", v2) ) <--- change mode "2"
  {
    cfg_get("debug_level");
    system("telnetd &"); <--------- telnet daemon started
  }
  else if ( !strcmp("3", v2) )
  {
    cfg_get("debug_level");
    system("rem_start.sh &");
  }
  else if ( !strcmp("4", v2) )
  {
    cfg_get("debug_level");
    system("rem_kill.sh &");
  }
  else
  {
    cfg_set("change_mode", "0");
    cfg_get("debug_level");
    system("mode_change 0");
  }
  return sub_34374(v1, "success");
}
<snip>

Other scripts could also be started via the webserver, like "rem_start.sh".
This script contains the following lines:

#!/bin/sh
if ps|grep remserial
then
echo "remserial is running.."
else
remserial -p 10005 -r 192.168.1.10 -s "115200 raw" /dev/ttyUSB0 &
fi

That means, that a serial console with the speed of 115200 Baud on port 10005
is started.

Vulnerable / tested versions:
-----------------------------
The following firmware has been tested, which was the latest version available
during the time of the test:
* DL_MF910S_CN_EUV1.00.01


Vendor contact timeline:
------------------------
2019-09-30: Contacting vendor through psirt@zte.com.cn
2019-10-10: Vendor provides initial contact.
2019-10-14: Vendor confirmed receive of the advisory.
2019-10-15: ZTE confirmed the hard-coded administrator password issue. The
            GoAhead webserver backdoor is still analyzed.
2019-11-05: ZTE released a Security Bullentin that the product MF910S is
            end-of-service*. SEC Consult Vulnerability Lab added a grace
            time of 9 months so that the hardware can be changed.
2020-08-27: Release of security advisory.

* http://support.zte.com.cn/support/news/LoopholeInfoDetail.aspx?newsId=1011722


Solution:
---------
Upgrade to new hardware.


Workaround:
-----------
None.


Advisory URL:
-------------
https://www.sec-consult.com/en/vulnerability-lab/advisories/index.html


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

SEC Consult Vulnerability Lab

SEC Consult
Europe | Asia | North America

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EOF Thomas Weber / @2020