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Recovering from a system compromise.
What to do if you've been hacked.
If you find you've been hacked, simply deleting the Trojan horse or closing
the open share is often not enough. Using the initial security breach as an
entry point, an attacker could easily have created other backdoors into your
system or even modified the actual operating system itself. Because of this
there is only one real way to secure a system which has been compromised and
that is to reinstall it from a known-good source. This document describes the
steps involved in recovering a typical windows system from a security compromise.
Step 1 : Isolate the affected machine.
You should disconnect any compromised machine from both the internet and any
local network as soon as you realize it's been compromised. This helps limit
the potential damage both to your own systems (remote attackers can no longer
gain access) and to other systems on the internet (your machine cannot be used
to attack others). It's important to physically disconnect the machine from
the network. That's right, unplug the network cable or power off the modem .
Cable and DSL modems in particular often feature 'standby' buttons which claim
to isolate the computer from the network - in several cases this is simply not
true, even with the modem in standby mode the computer is still connected to
the network.
At this point you should consider what other actions you need to take. Do you
for example store bank or credit card details on your PC? If you do, you should
inform the appropriate organizations that your accounts may be compromised at
once. Have you used your credit card number online recently? Again, if you have
you should inform the credit card company that your number may have been compromised.
Any password or secure data stored or used on your PC should be assumed to
have been compromised and changed at once. This includes ISP access passwords,
FTP, email and website passwords as well as any other service you use which
requires a secure login.
Step 2 : Find out how serious the problem is.
If you only have one computer you can safely skip this section, those with home
networks should read on.
A compromised machine on a network can lead to the compromise of all other machines
connected to that network. The risk of this happening depends on a number of
things, including :
The length of time the security breach has gone undetected.
Be honest with yourself and assume the worst case scenario is true when evaluating
this. When did you first suspect something might be wrong? When did you last
scan your network for viruses and Trojan horses? When did you last verify that
your files hadn't been tampered with? The longer a compromised machine has been
on a network the greater the chances of other machines on the network being
affected are.
The type of network you run.
If all machines on your network have unrestricted access to and from the compromised
machine, the chances of a network-wide security breach increase dramatically.
On the other hand, if you restrict access between machines either by using desktop
firewall products or by means of username/password authentication the risk falls.
The presence (or absence) of anti-virus and desktop firewall software.
If each machine runs properly maintained, independent anti-virus and desktop
firewall software the risk of a network-wide security breach falls sharply.
Step 3 : Begin the cleanup.
Locate the original software distribution disks for your operating system, any
drivers you need for your system and any license information you'll need during
the installation. You will be performing a clean install on the affected machines,
so you will loose any data stored on them unless you have backups. If you haven't
got recent backups, follow the procedure below :
Start up the compromised machine without connecting to any network.
Copy any data files you wish to keep to floppy disks or cd-r media, if at all
possible in non-executable form (ie. save word files as rich text since it can't
contain macro viruses). DO NOT COPY PROGRAM FILES!
Label this media clearly as potentially infected and store it safely.
You are now ready to begin rebuilding your machine. To be absolutely sure that
your system does not remain compromised, follow the steps below before installing
your operating system.
Restart your PC in DOS mode (NT/Win2k users should boot from the cd-rom or
setup disks)
Use the FDISK command to delete all partitions on the disk (NT/2k users should
follow the appropriate prompts in the setup program)
Power cycle your PC with the setup disk in the floppy drive or CD-Rom drive
as appropriate (switch off, wait 10 seconds, switch on). This applies to all
versions of windows including NT and win2k (power cycle after removing the partitions,
don't worry about still being in the setup utility) and ensures that any memory-resident
or boot sector virus is removed.
Reload your operating system & required drivers from the original disks.
At this point you'll have a working system with no software installed other
than the operating system & drivers. Assuming you used only original media,
the system will be free of any Trojan horse or virus but may not be secure.
Step 4 : Secure your system and load additional software.
You now need to obtain and apply the latest security patches for your operating
system. Ideally you should download these from their source using another machine
and apply them from disk. If that is not possible, connect your rebuilt system
to the internet for the minimum period possible to obtain the patches you need.
Apply them at once. You should be aware that this opens your system to potential
compromise while you are downloading the patches so keep the connection as short
as possible. Windows 98,ME and 2000 users can use the 'Windows Update' function
to automatically update their systems.
Once your system is updated, you can begin installing additional software.
Be sure only to use software you know has not been tampered with, ideally from
original distribution media. If necessary, download a fresh copy from the source
and use that. Install software in a logical order, beginning with security-related
products (anti-virus, firewall etc.).
Step 5 : Finishing off
Once you've installed and configured all your software you are ready to begin
restoring the data from backups. Before doing so, you may wish to make an image
copy of your system using a utility such as Norton's ghost. This will allow
you to quickly restore the machine to a known clean state in the event of future
compromise. If you do this, store the image on non-volatile media such as CD-Rom.
You may also wish to take a 'fingerprint' of the files installed on your machine
to enable comparison in future. See 'Attack Mitigation' for details on this.
When you eventually restore the data, do so gradually especially if you copied
the files from an infected machine. Virus scan each one first and discard any
with unexpected macros.
That's it, your machine is now rebuilt and ready to reconnect to the network
and the internet. It's been a lot of work but you now know for sure that your
machine is virus-free and reasonably secure against attack in future.
Attack Mitigation
There are a number of steps you can take to limit the damage done by a system
compromise. Not all apply to all systems and some require additional software
but they can make you life considerably easier if you are unfortunate enough
to be hacked.
File Signatures
Keeping a database of file signatures can help you pinpoint any files which
change unexpectedly. This is often one of the first signs of a security breach.
You can get free file signature checkers from a number of sources, we suggest
WinTerrogate (all versions of windows, basic but effective) from http://winfingerprint.sourceforge.net
or LANGuard File Integrity Checker (NT/2000 only, more advanced) from http://www.gfi.com/languard
Image Files
Taking an image of your disk regularly can dramatically reduce the amount of
work involved in recovering from a security breach. The best known tool for
doing this is Norton's GHOST although there are other options. You should keep
two or three images files on non-volatile media and update them regularly.
Keep the data on a separate partition.
Keeping your data on a separate partition (ideally on a separate disk) will
reduce the amount of work needing done if you have to rebuild the system. It
also makes backing up much easier and can improve overall system performance.
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