Oporto is a Windows virus. It stays in memory only until the host process (infected program process) terminates. The virus infects PE (Portable Executable) files, the infection is of appending type. Upon infection the virus increases the size of the last file section, writes its code there and modifies necessary fields in PE header (affected section characteristics). To get control when an infected file is run, the virus replaces first bytes of program's startup code with a short routine that passes control to virus body. With this trick the virus doesn't have to modify program's entry point address. Before the virus passes control to the host program the replaced area is restored.
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When an infected file is run, the virus searches for PE EXE files in current directory, then in Windows or Windows system directories, and infects these files. Then the virus hooks up to 15 Windows file access functions (file searching, opening, etc.) and stays in Windows memory as a part of host program. When hooked functions are accessed the virus searches for more PE EXE files on the disk and infects them.
The virus is able to hook Windows functions only in case the host program uses them (the program has imports of these functions from Windows kernel). The 'life-time' of the resident virus copy fully depends on the life-time of the host program: when it is terminated, the resident virus code is terminated too.
The virus has a payload: it deletes anti-virus data file ANTI-VIR.DAT if found. On 24th of September the virus displays a MessageBox and halts the system:
Technical Details: Eugene Kaspersky, AVP Team