Title:           Microsoft DHCP INFORM Configuration Overwrite
Version:         1.0
Issue type:      Protocol Security Flaw
Affected vendor: Microsoft
Release date:    28/05/2014
Discovered by:   Laurent Gaffié
Advisory by:     Laurent Gaffié
Issue status:    Patch not available
===============================================================================

Summary
-------

A vulnerability in Windows DHCP (http://www.ietf.org/rfc/rfc2131.txt) was
found on Windows OS versions
ranging from Windows 2000 through to Windows server 2003.  This
vulnerability allows an attacker to remotely
overwrite DNS, Gateway, IP Addresses, routing, WINS server, WPAD, and
server configuration with no user
interaction. Successful exploitation of this issue will result in a remote
network configuration
overwrite. Microsoft acknowledged the issue but has indicated no plans to
publish a patch to resolve it.


Technical details
-----------------

Windows 2003/XP machines are sending periodic DHCP INFORM requests and are
not checking if the DHCP INFORM answer (DHCP ACK) is from the registered
DHCP server/relay-server. Any local system may respond to these requests
and overwrite a Windows 2003/XP network configuration by sending a properly
formatted unicast reply.

Impact
------

Successful attempts will overwrite DNS, WPAD, WINS, gateway, and/or routing
settings on the target system.

Affected products
-----------------

Windows:
- 2000
- XP
- 2003

Proof of concept
----------------
The DHCP.py utility found within the Responder toolkit can be used to
exploit this vulnerability.

git clone https://github.com/Spiderlabs/Responder

Solution
--------
Set a DWORD registry key "UseInform" to "0" in each subfolder found in
HKLM\SYSTEM\CCS\Services\TCP\Interfaces\

Response timeline
-----------------
* 18/04/2014 - Vendor notified.
* 18/04/2014 - Vendor acknowledges the advisory ( [MSRC]0050886 )
* 18/04/2014 - Suggested to vendor to run Responder on a A-D environment
while looking at the DHCP issue for education purposes. Since multiple
attempts were
               made to have them be aware that any A-D environment by
default is vulnerable if Responder is running on the subnet. Also, MSRC was
asked what
               code change made this DHCP INFORM issue different on Windows
Vista than Windows Server 2003.
* 21/04/2014 - MSRC answers with an automated response.
* 08/05/2014 - Request for a reply.
* 14/05/2014 - MSRC reply and refuses to share their view on the code
change, however they mention that 'The product team is investigating
whether the RFC for
               a DHCPINFORM message is properly implemented'.
* 14/05/2014 - An email was sent to notify MSRC that no code change was
requested, but the logic behind it. Also, MSRC was asked if they were
successful with
               Responder.
* 16/05/2014 - MSRC closes [MSRC]0050886 and doesn't provide any info on if
they were successful with Responder in their environment.


References
----------
* Responder: https://github.com/Spiderlabs/Responder
* http://g-laurent.blogspot.ca/
* https://twitter.com/PythonResponder
*
http://blog.spiderlabs.com/2014/02/responder-20-owning-windows-networks-part-3.html