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F-Secure Malware Information Pages: Virus:W32/Virut
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Summary
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"Virut" is a family of polymorphic memory-resident appending file infectors that have Entry Point Obscuring (EPO) capabilities.
Viruses belonging to this family infect files with .EXE and .SCR extensions. All viruses belonging to the Virut family also contain an IRC-based backdoor that provides unauthorized access to infected computers. |
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Disinfection
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All the recent versions of F-Secure Anti-Virus should be able to remove the Virut infection. Below is the recommended disinfection procedure:
- Disable network sharing or disconnect infected computers from the local network and Internet.
- Set the F-Secure Anti-Virus real-time scanner (on-access scanner) disinfection action to "Disinfect Automatically". This is a very important step as the virus, being active in memory, will try to infect more files on the hard disk while it is being scanned. So newly-infected files need to be disinfected immediately.
- Scan ALL files (not just selected ones) on all hard drives with F-Secure Anti-Virus and the latest updates.
- Disinfect all infected files, delete all files that can't be disinfected. If disinfection of many infected files fails, then do not delete the files yet - send a few of them (not more that 5-8) to F-Secure Security Response Labs for examination. It might be that disinfection of that particular Virut variant is not available yet. In this case we will add disinfection shortly.
- Restart the computers after disinfection is complete. Restarting is important - it will destroy the active virus' code that was injected into system processes.
- Scan all files again to ensure that no more infected files are left on the hard drive(s). Repeat disinfection procedure if necessary.
- Disinfect all other computers connected to the same local network before enabling sharing or re-connecting to the network.
Note: If infected files are found in System Restore folder, it is recommended to disable the System Restore feature and restart the system. Instructions can be found here:
Note: If infected files are found in Recycle Bin folder, emptying the Recycle Bin is required. Instructions can be found here:
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Detailed Description
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Virut is a polymorphic appending file infector with EPO (Entry Point Obscuring) capabilities. The virus uses several infection methods:
1. The virus relocates a certain amount of bytes from the entry point of the original file and writes its initial decryptor there. So when an infected file is run, the virus code gets control first. The initial decryptor then decrypts a small part of the virus's body that is appended to the end of the infected file and passes control to it.
2. The virus appends its code to the end of the file and changes the entry point address of the original program so it points to the start of the appended viral code, where the decryptor is located. This is the most common way of infecting files for appending parasitic infectors.
3. The virus writes its initial code into a gap (empty space) in the end of the original file's code section and redirects the entry point address to that code. The initial code decrypts a small part of the virus body and passes control to it. Then the main decryptor takes control and decrypts the rest of the virus body.
The virus patches the first found API call (from the entry point address) in the original program so that instead of the API it calls the initial virus decryptor. That decryptor may be located in the end of the code section as said above.
The virus checks whether or not it is already active and if it is, depending on the infection methods described earlier, it either relocates the original file's data back to its place and passes control to it or returns control to the original file's entry point address or calls the replaced API itself and then passes control to the original program. If the virus is not yet active, the second decryptor decrypts the rest of the virus body and initiates installation cycle.
During the installation cycle the virus injects its code into a system process, hooks a few low-level Windows API calls and stays resident in memory. When a file with .EXE or .SCR extension is opened or run, the virus tries to infect it with one of its four methods.
The virus contains an IRC-based backdoor. The backdoor connects to the pre-defined IRC server (ircd.zief.pl in the last variants) and joins the "virtu" channel. The author of the virus can give commands to all or to specific bots created by the virus in the channel. The bot is quite primitive - it allows a hacker to download and run files from Internet.
Some of Virut variants contain the following text strings:
- O noon of life! O time to celebrate!
- O summer garden!
- Relentlessly happy and expectant, standing: -
- Watching all day and night, for friends I wait:
- Where are you, friends? Come! It is time! It's late!
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F-Secure Corporation |
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Last Modified: October 11, 2007
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