Perrun is a proof of concept virus that appends itself to JPEG files. In order to run the viral part it modifies the JPEG handler program in the registry and drops a special extractor to the system that extracts and runs the malicious code. In practice it means that the malicious part in a JPEG file will run only if the system is already infected with the virus. A clean system can not get infected from an "infected" JPEG file since that would need the virus to be active on the system already.
Please note that Perrun can not be activated from infected JPEG files on a clean system.
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The virus modifies local files only. It has no ability to send itself in email or over local network. It has not been reported in the wild yet.
Perrun was written in Visual Basic and it is compressed with the UPX EXE compressor.
When the virus is started in it's EXE form it drops two files:
'extrk.exe' - the program that extracts the virus code from the JPEG files
'reg.mp3' - registry file that adds the extractor program as the handler for opening JPEG files
After this it looks for '*.jpg' files in the current directory and appends itself to them. It does not tuch the file if it has 'alco' as the four last bytes. This way it does not append itself to a file twice.
Since from here the JPEG files are opened with the special extractor the viral code can be started from the JPEG files. The extractor extracts the virus code to 'x.exe' in the current directory and runs it. After that the JPEG file itself is opened with the original handler.