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F-Secure Malware Information Pages: Backdoor
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Summary
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Backdoor (Generic Description)
Backdoor is a hacker's remote access tool. Usually a backdoor is
a standalone file that installs itself to system and then remains
active there listening to specific network ports for specific
commands. A typical backdoor consists of 2 parts - client and
server. Some backdoor packages have configuration utilities that
allow a hacker to configure server parts to their needs. A few
backdoors have special scanner utilities to locate victim
computers where server parts are installed.
There also exist IRC backdoors. These backdoors are controlled
via bots that they create in specific channels on selected IRC
channels. These channels are usually invite-only, so they can be
only accessed by hackers who use these backdoors.
A server part of a typical backdoor is usually installed on a
computer which is going to be accessed. Hackers use different
tricks to infect users with server parts of backdoors - they send
them in trojan dropper packages, give fancy names to server files
and send them in e-mails. Some worms and viruses drop backdoors
to infected systems.
When a typical backdoor is run, it copies its file to Windows or
Windows System folder and creates a Registry key to start that
file during every Windows session. Also some backdoors modify
WIN.INI and SYSTEM.INI files or copy themselves to startup
folders for different users. After installation some backdoors
can show fake error messages. Modern backdoors usually send a
notification to specific e-mail, ICQ or MSN account when they are
activated. They report infected computer's IP address and some
other info. After a backdoor is installed, it starts to listen to
certain network ports for specific commands coming from a client
part.
A client part of a backdoor is used to control a server part that
is installed on a victim's computer. Client parts usually have a
well-designed GUI (Graphical User Interface) to make
communications with servers easy.
The most advanced backdoors allow a hacker to get full control
over an infected system. They include such features as sending
and receiving files, browsing through victim's hard and network
drives, getting system information, receiving a screenshot from a
victim's computer, communicating with an infected user, change
date/time and settings of operating system, play tricks (like
open/close CD-ROM tray) and so on. Some backdoors even allow a
hacker to listen and see what happens at a remote computer it
it's equipped with a microphone and a webcam. Simple backdoors
only allow upload, download and run files on victim's computers.
Most famous backdoors: SubSeven, NetBus, Deep Throat, Back
Orifice, Bionet, Master's Paradise.
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Disinfection
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Automatic Disinfection
Usually standalone malware (backdoors, worms, trojans, etc.) is
automatically removed by F-Secure Anti-Virus (FSAV) starting from
version 5.40. Malware files get automatically renamed by FSAV, so
they can not be started any more. In some rare cases, when
automatic disinfection is not possible, a user can select
disinfection action by him/herself to make FSAV rename or delete
an infected file. In some special cases it is recommended to use
specific disinfection tools provided by F-Secure. They can be
downloaded from our ftp site:
ftp://ftp.f-secure.com/anti-virus/tools/
F-Secure Anti-Virus can be purchased from our webshop or from our
authorised distributors. A trial version F-Secure Anti-Virus,
limited to 30 days, can be downloaded from our website:
http://www.f-secure.com/download-purchase/
All the latest versions of FSAV can download anti-virus database
updates automatically. However, these updates can be also
downloaded and installed manually from our web or ftp sites:
http://www.f-secure.com/download-purchase/updates.shtml
Manual Disinfection
To manually disinfect standalone malware (backdoors, worms,
trojans, etc.) it's usually enough to delete all infected files
from a computer and to restart it. Active malware files are
usually locked by operating system so different disinfection
approaches are required for different operating systems.
Please note that manual disinfection is a risky process, so it is
recommended only for advanced users.
Windows 95, 98, ME
If Windows 9x operating system is used, it is recommended to
restart a computer from a bootable system diskette and to delete
an infected file from command prompt. For example if a malicious
file named ABC.EXE is located in Windows folder, it is usually
enough to type the following command at command prompt:
DEL C:\WINDOWS\ABC.EXE
and to press Enter. After that an infected file will be gone.
Windows NT, 2000, XP
If Windows NT, 2000 or XP is used, a malicious file has to be
renamed with a different extension (for example .VIR) and then a
system has to be restarted. After restart a renamed malicious
file will no longer be active and it can be easily deleted
manually.
System Restore issue
If Windows ME or XP is used, it is recommended to disable System
Restore feature of these operating systems to prevent a computer
from re-infection by an already removed malware. The fact is that
System Restore feature of these operating systems might save an
infected file into the special folder and copy it back to a hard
drive it every time it's been renamed or deleted by F-Secure
Anti-Virus or by a user. Instructions on how to disable System
Restore feature are here:
Windows ME:
http://www.europe.f-secure.com/v-descs/sfc_dis.shtml
Windows XP:
http://www.europe.f-secure.com/v-descs/sfc_dis1.shtml
It is recommended to re-enable System Restore after disinfection
in order to restore stable system configuration in the future,
if any crash or incompatibility issue occurs.
Contacting F-Secure for help
If you have problems with disinfection, please consult a computer
technician or send a message (and a sample) to our Viruslab. We
have guidelines for sending virus samples, hoaxes and
virus-related questions to F-Secure Viruslab published here:
http://support.f-secure.com/enu/home/virusproblem/sample/ |
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F-Secure Corporation |
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Last Modified: January 01, 2006
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