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packetstormT. WeberPACKETSTORM:158990
HistoryAug 27, 2020 - 12:00 a.m.

ZTE Mobile Hotspot MS910S Backdoor / Hardcoded Password

T. Weber

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`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  
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  
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."  
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.  
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The used BusyBox toolkit in version 1.15.0 is outdated and contains multiple  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 The outdated version was found by IoT Inspector.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:  
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:  
The file content is shown below:  
Both the user "root" and "admin" are using the same weak hardcoded password  
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>"  
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* 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:  
dword_788DC = (int)"/bin/login";  
dword_788E0 = (int)"/etc/";  
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);  
v6 = 4719; <------------------------------------- Port "4719"  
if ( !v4 )  
v2 = sub_65578(v75, v6);  
sub_E394(v2, 1);  
goto LABEL_13;  
dword_788D8 = (int)sub_1C480(0);  
if ( dword_788D8 )  
This led to the assumption that the GoAhead webserver was modified like  
described in the following blog post:  
Inspecting the GoAhead webserver binary reinforces this assumption. The  
pseudocode was generated with Hex-Rays ARM Decompiler, like for the prior  
BusyBox binary:  
int __fastcall sub_21D48(int a1)  
int v1; // r5  
const char *v2; // r4  
v1 = a1;  
v2 = (const char *)sub_17DF0(v1, "change_mode", "");  
cfg_set("login_9527", "1");  
if ( !strcmp("1", v2) )  
cfg_set("change_mode", "1");  
system("mode_change 1");  
else if ( !strcmp("2", v2) ) <--- change mode "2"  
system("telnetd &"); <--------- telnet daemon started  
else if ( !strcmp("3", v2) )  
system(" &");  
else if ( !strcmp("4", v2) )  
system(" &");  
cfg_set("change_mode", "0");  
system("mode_change 0");  
return sub_34374(v1, "success");  
Other scripts could also be started via the webserver, like "".  
This script contains the following lines:  
if ps|grep remserial  
echo "remserial is running.."  
remserial -p 10005 -r -s "115200 raw" /dev/ttyUSB0 &  
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 [email protected]  
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.  
Upgrade to new hardware.  
Advisory URL:  
SEC Consult Vulnerability Lab  
SEC Consult  
Europe | Asia | North America  
About SEC Consult Vulnerability Lab  
The SEC Consult Vulnerability Lab is an integrated part of SEC Consult. It  
ensures the continued knowledge gain of SEC Consult in the field of network  
and application security to stay ahead of the attacker. The SEC Consult  
Vulnerability Lab supports high-quality penetration testing and the evaluation  
of new offensive and defensive technologies for our customers. Hence our  
customers obtain the most current information about vulnerabilities and valid  
recommendation about the risk profile of new technologies.  
Interested to work with the experts of SEC Consult?  
Send us your application  
Interested in improving your cyber security with the experts of SEC Consult?  
Contact our local offices  
Mail: research at sec-consult dot com  
EOF Thomas Weber / @2020