Win32/Parite is a packed, encrypted virus that infects files on the local file system and on writeable network shares. The virus repeatedly performs a sequence whereby it selects an .exe or .scr executable file, infects the file, waits a random time interval, selects and infects another file, and so on.
To each file that it selects for infection, the virus appends a new code section named ".<three random alphabetic characters>", such as .mpg. The virus changes the code-entry point of the file to the new, appended section. The virus contains certain code that it unpacks, re-encrypts, re-packs, and then copies to the new section of the executable file. This copied code consists of certain DLL code and some additional execution instructions.
When a file infected in this way runs, the execution instructions in the appended section of the file unpack and decrypt the DLL code in the appended section and drop it as a new file into the Windows temporary directory. The name of the dropped DLL consists of 4 randomly-chosen alphanumeric characters: the first three characters are alphabetic and the final character is a digit. Although the file is actually a DLL, the file extension is .tmp. So for example, the file could have a name like fia1.tmp.
The virus may also creates a marker in the registry:
Adds value: PINF
To subkey: HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer
This value contains the full path to the dropped DLL. Win32/Parite may check for this registry entry to determine whether the virus is already running on the computer.
The execution instructions in the appended section of the infected file load the dropped DLL. The DLL exports a start function which contains the main virus code. The virus code calls the start function, which injects the DLL into the explorer.exe process. When a certain condition is satisfied, the DLL code runs and infects other .exe and .scr executable files.
Win32/Parite does not run executable files that it infects. When an infected file runs (for example, when a user or the system runs the file), the virus code in the appended section runs and then returns control to the original code-entry point of the executable file. The executable file then runs normally even though it is infected, so there may be no readily apparent indication to a user of any malicious activity.