|
|
|  |
|
|
|
|
F-Secure Malware Information Pages: Trojan
|
|
|
| Radar |
 |
|
|
|
Summary
|
Trojan (Generic Description)
A trojan is usually a standalone program that performs destructive or other malicious or annoying actions when it's run (direct action trojan), after some period of time (time bomb) or in certain conditions (condition-triggered trojan).
Description continues in the Details Section. |
|
|
|
Disinfection
|
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.
If Windows 95, 98 and ME 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. 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.
Malware Disinfection Tools
F-Secure provides disinfection tools for certain malware. These tools can be downloaded from this webpage:
http://www.f-secure.com/download-purchase/tools.shtml
ftp://ftp.f-secure.com/anti-virus/tools/
Windows System Restore Issues
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.f-secure.com/v-descs/sfc_dis.shtml
Windows XP:
http://www.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.
Failed Disinfection
In some cases F-Secure Anti-Virus might not disinfect a system automatically. In this case please visit our Support pages:
http://support.f-secure.com/enu/home/virusproblem/howtoclean/ |
|
|
|
Detailed Description
|
Destructive actions can vary from erasing or modifying the contents of files or sectors on a hard or floppy drive to a complete destruction of data on an entire media. Some trojans can destroy Flash Bios of certain types of motherboards and that will make an affected computer unbootable. Certain trojans can disable keyboard or mouse or only specific keys on a keyboard. Other trojans can disable certain features of an operating system. A few trojans perform annoying actions like changing position of icons on a desktop or showing video effects that makes a system hardly usable.
Backdoor is a hacker's remote access trojan. 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.
An IRC backdoor is usually a standalone file that copies its file to Windows or Windows System folder and creates a Registry key to start that file during every Windows session. Also some IRC backdoors modify WIN.INI and SYSTEM.INI files or copy themselves to startup folders for different users. Some IRC backdoors replace INI scripts of an IRC client (mostly mIRC).
When an IRC backdoor is run, it established connection to an IRC server or waits until a user connects to IRC (mIRC script-based backdoor). A backdoor then creates a bot in a specific channel on a specific IRC server. An IRC bot acts as a backdoor server interface. An IRC client in this case acts as a backdoor client. A hacker can give commands to an IRC bot using IRC interface.
Most of advanced IRC backdoors allow to get a limited access to an infected system and to modify, upload, download and run files. Some IRC backdoors have additional functionalities that allow a hacker to perform malicious actions in IRC channels and in some cases can allow an attacker to completely take over an IRC channel. |
|
|
|
F-Secure Corporation |
|
|
|
|
|
Last Modified: January 01, 2006
|
|
|
|
|
