Windows TFTPD Blocking

When huge mass of PCs are affected by a network worm in a large corporation, some percentage of them belongs to traveling personnel so disinfection is difficult.

Infection is accomplished via Microsoft RPC overflow on pert 137, so we can do little to prevent it (other then enabling automatic patch downloads on desktops, which is not a NTI/R domain, anyway).  The start of payload is

 length = 50

000 : 80 B0 00 00 00 01 00 00 00 00 00 00 20 43 4B 41   ............ CKA
010 : 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41 41   AAAAAAAAAAAAAAAA
020 : 41 41 41 41 41 41 41 41 41 41 41 41 41 00 00 21   AAAAAAAAAAAAA..!
030 : 00 01
 

If the worm body is transfered from PC to Pc via TFTP an effective measure is to blocking TFTP on site routers for DHCP segments only (TFTP traffic "to" DHCP segments should be allowed as to measure who else is affected by the worm). Doing it one site a time and proceeding slowly so that the whole change took enough time for us to understand consequences. VPN are very important source of infection here and  probably the candidate No.1 for such measure.

Anyway, finding and blocking important for fiunctioning of network worm parts on site routers is a very effective and relative gageneral strategy. It definitly helps to prevent infection of servers from desktops and thus negative effect on the unpatched Windows servers (automatic updates are rarely enabled on servers, and that's probably for a reason).

This is not limited to worms which use TFTP as transport mechanism.


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[Aug 28, 2003] Enabling the TFTP Server in Windows 2000 WiredEuclid.com Knowledge for the Connected World

The Trivial File Transfer Protocol (tftp) is used most often for updating firmware in routers and in some boot from network environments, most often in the Unix world. Additionally, some IP Telephony vendors are using it to load boot images for their phones.

Unfortunately, most vendors provide a userland, interactive tftp server piece, which requires an actively logged in user to run - which is a security risk. The few commercial packages that support tftp aren't free.

Fortunately, Microsoft includes a free one, but doesn't have any information on setting it up or configuring it. ... However, I came across a Reader-to-Reader suggestion from Windows 2000 Magazine (http://www.win2000mag.com) that included instructions for installing and configuring the tftpd.exe daemon which is shipped with Windows 2000 Server. The following instructions are from the article, which is available in its entirety here. http://www.win2000mag.net/Articles/Index.cfm?ArticleID=26648

Copy the tftpd.exe file from \%systemroot%\system32\dllcache
to \%systemroot%\system32.

Then, use the Microsoft Windows 2000 Server Resource Kit's Instsrv utility to create the service as follows.

instsrv tftpd c:\winnt\system32\tftpd.exe

To start the service, enter

net start tftpd

You have now created a TFTPD server on Win2K. To test the server, enter

tftp -i put <filename>

The service will create a directory named tftpdroot in \%SystemDrive%, and the file whose name you entered above will be in this directory. This is where the first issue arose. We already had an existing data directory that needed to be available via tftp, and we do not serve any content off the system drives of any servers – it is both a security and common sense issue. Therefore, the service needed to be configurable.

Because this isn’t a supported service, finding documentation on it wasn’t going to be easy, and after a few minutes with Google, I determined I’d have to create my own documentation. Since it only makes sense that a Microsoft application would look to the registry for configuration data, I grabbed my copy of RegMon, the free registry monitor from Systernals, and got busy.

After tweaking RegMon’s filter to only show the tftpd.exe process, and setting a highlight on the service’s registry key (HKLM\CurrentControlSet\Services\TFTPD), I started the service. Looking through the dozen or so highlighted registry accesses revealed the start of something good – an OpenKey attempt for this key:

HKLM\CurrentControlSet\Services\TFTPD\Parameters

A quick glance in RegEdt32 showed that key didn’t exist, so let’s add it. Many Windows services have a Parameters key, so this is a good sign.

After adding the key, a stop and restart of tftpd shows a sequence of QueryValue commands – attempts to read a set of values. The values it is looking for were:


HKLM\CurrentControlSet\Services\TFTPD\Parameters\directory
HKLM\CurrentControlSet\Services\TFTPD\Parameters\clients
HKLM\CurrentControlSet\Services\TFTPD\Parameters\masters
HKLM\CurrentControlSet\Services\TFTPD\Parameters\readable
HKLM\CurrentControlSet\Services\TFTPD\Parameters\writable

 

BINGO! The first parameter tftpd is looking for is called directory. Let’s add a value of type REG_SZ with the value data set to “D:\tftpdata” (our tftp data directory). Following a restart of the tftpd service, we’ll retest (as described above) and we got it – the service now points to our data directory, and the file we just uploaded. Unfortunately, none of the other values are documented either, so they are probably best left alone.

W2K Standalone TFTP Dameon

The Windows 2000 Standalone TFTP Daemon

The Trivial File Transfer Protocol (TFTP) services is used widely on TCP/IP networks today. Examples of usage are; firmware updates for switches and routers, remote booting and the Remote installation Service in Windows 2000/Server 2003.

Windows 2000/Server 2003 features a TFTP service as part of the Remote Installation Service. There is no way to install the TFTP Daemon, as it is called, without installing RIS as well. Installing RIS means adding at least one Windows Pro image to your server and running the SIS Growler and the BINL services as well. You can elect not to configure RIS, but then you get constant errors in your event logs about the RIS subsystem not being configured.

I have created a stand alone version of the Microsoft TFTP Service which you can download here. I can be run on all Windows 2000 or later operating systems. Needless to say you have to have a license for Windows 2000 Server to use this service. Even if you run it on Windows 2000 Professional.

The current version of the package is 1.1.
Download it here: TFTPDaemon_v1.1.zip

Filename Info Author
tftpd.exe The executable file that runs the TFTP service. This file is copied from a Windows 2000 Server with SP3. (When SP4 come out I will replace this file with the newer version.) Microsoft Corp.
InstallTFPTDaemon_vX.X.vbs Installation script for the service. MDS
RemoveTFTPService_vX.X.vbs Uninstaller for the service. MDS
TFTPAdmin_vX.X.vbs Admin script to change the TFTP Root directory. MDS

Features/Limitations

Usage Areas

Installation

  1. Extract the files in the archive to a folder of you choice.
  2. Run InstallTFPTDaemon.vbs.
    You will be prompted for the TFTP Root directory; the folder must exist before you select it. The script will not create it, but it will check if the folder exists. Then tftp.exe will be copied to your %systemroot%\system32 directory, the service will be installed in the registry and the service started. You will be prompted with info whether the service was installed successfully or not.
  3. Finished!

Administration

If you need to change the TFTP Root directory you use the TFTPAdmin script. It will read the current TFTP root directory and prompt you for a new one, and the restart the service. There is no checking of what you enter in the new folder field (I haven't programmed it yet). Be careful to enter an existing directory.

Uninstallation

If you whish to remove the TFTP service you can run RemoveTFTService. It will stop the service, unregister the service from registry and delete the tftpd.exe file. You will be prompted with status of the operation.

Future plans

Support/Info

Support or info is me and you can reach me at mds(at)tiscali.no



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