EXE Infector (generic description).
This type of virus infects EXE files. An EXE file is a binary executable file. EXE files can be 16-bit and 32-bit. 16-bit executable files contain for 16-bit operating systems such as DOS and Windows 3.xx. The 32-bit executable files are used in modern operating systems such as Windows. Both 16-bit and 32-bit executable files have headers. A header is a data area that preceedes an executable code and contains vital information about a file (for example all headers contain entry point addresses - the place where execution of a file starts).
Based on the settings of your F-Secure security product, it will either automatically delete, quarantine or rename the detected program or file, or ask you for a desired action.
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An EXE infector can be prepending (writes itself before the original file), appending (writes itself to the end of the original file), overwriting (overwrites the original file with its own code), inserting (inserts itself into gaps inside the original file), companion (renames the original file and writes itself with the original file's name) and cavity infector (writes itself between file sections of 32-bit file). An EXE infector can be memory resident and non-memory resident. Memory resident viruses stay active in memory, trap one or more system functions (usually interrupt 21h or Windows file system hooks) and infect files while they are accessed. Non-memory resident viruses search for EXE files on a hard disk and infect them.
An EXE infector can be non-encrypted, encrypted or polymorphic. An encrypted or polymorphic virus consists of one or more decryptors and a main code. A decryptor decrypts main virus code before it could be started. Encrypted viruses usually use fixed or variable key decryptors while polymorphic viruses have decryptors that are randomly generated from processor instructions and contain a lot of commands that are not used in decryption process.