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Anti Debugging Tricks Rel.2

Tecniche di Antidebug in assembler

(by inbar raz)

Un mini corso contenente poco pił di una decina di tecniche assembler di antidebug. Sicuramente interessante.
This article is online from 5890 days and has been seen 6091 times

40Hex Number 9 Volume 2 Issue 5 File 004

I picked this up in a collection of clips from the Fidonet 80xxx echo,
figured it might interest someone.


Anti Debugging Tricks
Inbar Raz

Release number 2

Today's anti debugging tricks devide into two categories:

1. Preventive actions;
2. Self-modifying code.

Most debugging tricks, as for today, are used within viruses, in order to
avoid dis-assembly of the virus, as it will be exampled later in this file.
Another big part of anti debugging tricks is found with software protection
programs, what use them in order to make the cracking of the protection

1. Preventive actions:

Preventive actions are, basically, actions that the program takes in order
to make the user unable to dis-assemble the code or trace it while running.

1.1. Interrupt disable:

Interrupt disable is probably the most common form of anti-debugging
trick. It can be done in several ways:

1.1.1. Hardware masking of interrupt:

In order to avoid tracing of a code, one usually disables the
interrupt via the 8259 Interrupt Controller, addressed by read/write
actions to port 21h. The 8259 Interrupt Controller controls the IRQ
lines. This means that any IRQ between 0 and 7 may be disabled by
this action. Bit 0 is IRQ0, bit 1 is IRQ1 etc. Since IRQ1 is the
keyboard interrupt, you may disable the keyboard without the
debugger being able to bypass it.


CS:0100 E421 IN AL,21
CS:0102 0C02 OR AL,02
CS:0104 E621 OUT 21,AL

Just as a side notice, the keyboard may be also disabled by
commanding the Programmable Perepheral Interface (PPI), port 61h.


CS:0100 E461 IN AL,61
CS:0102 0C80 OR AL,80
CS:0104 E661 OUT 61,AL

1.1.2. Software masking of interrupt:

This is quite an easy form of anti-debugging trick. All you have
to do is simply replace the vectors of interrupts debuggers use/any
other interrupt you will not be using or expecting to happen. Do not
forget to restore the original vectors when you are finished.
It is adviseable to use manual change of vector, as shown below,
rather than to change it using interrupt 21h service 25h, because
any debugger that has gained control of interrupt 21h may replace
your vector with the debugger's. The example shows an interception
of interrupt 03h - the breakpoint interrupt.


CS:0100 EB04 JMP 0106
CS:0102 0000 ADD [BX+SI],AL
CS:0104 0000 ADD [BX+SI],AL
CS:0106 31C0 XOR AX,AX
CS:0108 8EC0 MOV ES,AX
CS:010A 268B1E0C00 MOV BX,ES:[000C]
CS:010F 891E0201 MOV [0102],BX
CS:0113 268B1E0E00 MOV BX,ES:[000E]
CS:0118 891E0401 MOV [0104],BX
CS:011C 26C7064C000000 MOV Word Ptr ES:[000C],0000
CS:0123 26C7064E000000 MOV Word Ptr ES:[000E],0000

1.1.3. Vector manipulation

This method involves manipulations of the interrupt vectors,
mainly for proper activation of the algorithm. Such action, as
exampled, may be used to decrypt a code (see also 2.1), using data
stored ON the vectors. Ofcourse, during normal operation of the
program, vectors 01h and 03h are not used, so unless you are trying
to debug such a program, it works fine.


CS:0100 31C0 XOR AX,AX
CS:0102 8ED0 MOV SS,AX
CS:0104 BC0600 MOV SP,0006
CS:0107 8B0E0211 MOV CX,[1102]
CS:010B 50 PUSH AX
CS:010C 21C8 AND AX,CX
CS:010E 01C5 ADD BP,AX
CS:0110 58 POP AX
CS:0111 E2F8 LOOP 010B

1.1.4. Interrupt replacement

This is a really nasty trick, and it should be used ONLY if you
are ABSOLUTELY sure that your programs needs no more debugging. What
it does is simply copy the vectors of some interrupts you will be
using, say 16h and 21h, onto the vectors of interrupt 01h and 03h,
that do not occure during normal operation of the program. If the
user wants to debug the program, he would have to search for every
occurance of INT 01, and replace it with the appropriate INT


CS:0100 FA CLI
CS:0101 31C0 XOR AX,AX
CS:0103 8EC0 MOV ES,AX
CS:0105 26A18400 MOV AX,ES:[0084]
CS:0109 26A30400 MOV ES:[0004],AX
CS:010D 26A18600 MOV AX,ES:[0086]
CS:0111 26A30600 MOV ES:[0006],AX
CS:0115 B44C MOV AH,4C
CS:0117 CD01 INT 01

1.2. Time watch:

This may be a less common method, but it is usefull against debuggers
that disable all interrupts except for the time that the program is
executed, such as Borland's Turbo Debugger. This method simply retains
the value of the clock counter, updated by interrupt 08h, and waits in an
infinite loop until the value changes. Another example is when you mask
the timer interrupt by ORing the value INed from port 21h with 01h and
then OUTing it back, thus disabling the IRQ0 - Timer interrupt. Note that
this method is usefull only against RUN actions, not TRACE/PROCEED ones.


CS:0100 2BC0 SUB AX,AX
CS:0102 FB STI
CS:0103 8ED8 MOV DS,AX
CS:0105 8A266C04 MOV AH,[046C]
CS:0109 A06C04 MOV AL,[046C]
CS:010E 74F9 JZ 0109

1.3. Fool the debugger:

This is a very nice technique, that works especially and only on those
who use Turbo Debugger or its kind. What you do is init a jump to a
middle of an instruction, whereas the real address actually contains
another opcode. If you work with a normal step debugger such as Debug or
SymDeb, it won't work since the debugger jumps to the exact address of
the jump, and not to the beginning of an instruction at the closest
address, like Turbo Debugger.


CS:0100 E421 IN AL,21
CS:0104 EB02 JMP 0108
CS:0106 C606E62100 MOV Byte Ptr [21E6],00
CS:010B CD20 INT 20

Watch this:

CS:0108 E621 OUT 21,AL

1.4. Cause debugger to stop execution:

This is a technique that causes a debugger to stop the execution of a
certain program. What you need to do is to put some INT 3 instructions
over the code, at random places, and any debugger trying to run will stop
there. Since this techniqu causes the CPU to stop executing the program,
and therefore clear the Prefetch Instruction Queue, it is adviseable to
use this techinque in conjunction with the PIQ trick, 2.2.2. Note that
the example shows how to use these two tricks together.


CS:0100 B97502 MOV CX,0275
CS:0103 BE9001 MOV SI,0190
CS:0106 89F7 MOV DI,SI
CS:0109 C70610013473 MOV Word Ptr [0110],7334
CS:010F CC INT 3
CS:0110 2406 AND AL,06
CS:0113 C70610012406 MOV Word Ptr [0110],0624
CS:0119 E2ED LOOP 0108

1.5. Halt TD386 V8086 mode:

This is a nice way to fool Turbo Debugger's V8086 module (TD386). It is
baed on the fact that TD386 does not use INT 00h to detect division by
zero (or register overrun after division, which is treated by the
processor in the same way as in case of division by zero). When TD386
detects a division fault it aborts, reporting about the faulty
division. In real mode (even under a regular debugger), a faulty DIV
instruction will cause INT 00h to be called. Therefore, pointing INT 00h
to the next instruction, will recover from the faulty DIV.

Note: It is very important to restore INT 00h's vector. Otherwise, the
next call to INT 00h will cause the machine to hang.


CS:0100 31C0 XOR AX,AX
CS:0102 8ED8 MOV DS,AX
CS:0104 C70600001201 MOV WORD PTR [0000],0112
CS:010A 8C0E0200 MOV [0002],CS
CS:010E B400 MOV AH,00
CS:0110 F6F4 DIV AH
CS:0112 B8004C MOV AX,4C00
CS:0115 CD21 INT 21

1.6. Halt any V8086 process:

Another way of messing TD386 is fooling it into an exception.
Unfortunately, this exception will also be generated under any other
program, running at V8086 mode. The exception is exception #13, and its
issued interrupt is INT 0Dh - 13d. The idea is very similar to the
divide by zero trick: Causing an exception, when the exception interrupt
points to somewhere in the program's code. It will always work when the
machine is running in real mode, but never under the V8086 mode.

Note: It is very important to restore the original interrupt vectors.
Otherwise, the next exception will hang the machine.


CS:0100 31C0 XOR AX,AX
CS:0102 8ED8 MOV DS,AX
CS:0104 C70634001301 MOV WORD PTR [0034],0113
CS:010A 8C0E3600 MOV [0036],CS
CS:0113 B8004C MOV AX,4C00
CS:0116 CD21 INT 21

2. Self-modifying code:

2.1. Encryptive/decryptive algorithm:

The first category is simply a code, that has been encrypted, and has
been added with a decryption routine. The trick here is that when a
debugger sets up a breakpoint, it simply places the opcode CCh (INT 03h)
in the desired address, and once that interrupt is executed, the debugger
regains control of things. If you try to set a breakpoint AFTER the
decryption algorithm, what is usually needed, you will end up putting an
opcode CCh in a place where decryption action is taken, therefore losing
your original CCh in favour of whatever the decryption algorithm makes.
The following example was extracted from the Haifa virus. If you try to
set a breakpoint at address CS:0110, you will never reach that address,
since there is no way to know what will result from the change. Note that
if you want to make the tracing even harder, you should start the
decryption of the code from its END, so it takes the whole operation
until the opcode following the decryption routine is decrypted.


CS:0100 BB7109 MOV BX,0971
CS:0103 BE1001 MOV DI,0110
CS:0106 91 XCHG AX,CX
CS:0107 91 XCHG AX,CX
CS:0108 2E803597 XOR Byte Ptr CS:[DI],97
CS:010C 47 INC DI
CS:010E 75F6 JNZ 0106
CS:0110 07 POP ES
CS:0111 07 POP ES

2.2. Self-modifying code:

2.2.1. Simple self-modification:

This method implements the same principle as the encryption
method: Change the opcode before using it. In the following example,
we change the insruction following the call, and therefore, if you
try to trace the entire call ('P'/Debug or F8/Turbo Debugger), you
will not succeed, since the debugger will put its CCh on offset 104h,
but when the routine runs, it overwrites location 104h.


CS:0100 E80400 CALL 0107
CS:0103 CD20 INT 20
CS:0105 CD21 INT 21
CS:0107 C7060301B44C MOV Word Ptr [0103],4CB4
CS:010D C3 RET

Watch this:

CS:0103 B44C MOV AH,4C

2.2.2. Prefetch Instruction Queue (PIQ) manipulation:

This method is a bit similar to (1.3), but it fools ANY debugger,
or any other process that executes one operation at a time. The PIQ
is an area within the CPU, that pre-fethces, ie. takes in advance,
instructions from memory, so when they need to be executed, it
would take less time to get them, since they are already in the CPU.
The PIQ length ranges from 6 or 4 in old computers, up to as high as
25 in new ones. What the trick does is change the FOLLOWING opcode
to something meaningless. If you are debugging, then the change will
take place BEFORE the instructions is executed or fetched. If you
run the program NORMALLY, by the time you change the opcode, it will
have already been fetched.


CS:0100 B97502 MOV CX,0275
CS:0103 BE9001 MOV SI,0190
CS:0106 89F7 MOV DI,SI
CS:0109 C7060F012406 MOV Word Ptr [010F],0624
CS:010F 3473 XOR AL,73
CS:0112 C7060F012406 MOV Word Ptr [010F],0624
CS:0118 E2EE LOOP 0108

Watch this:

CS:010F 2406 AND AL,06


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