I have been nagged a lot regarding guest posts, and almost 90% of them are related to some news, social media bullshit and half baked security crescendo. Until recently, I was contacted by amiable folks at
Infosec Institute with a good article on
Anti Debugging. This is an article by
Dejan Lukan, a security researcher at
Infosec Institute, in which
he discusses the
Anti Debugging techniques in an objective and direct manner. I loved the implementation part, reminded me of my rev days (you can learn about
how to reverse Winrar or just have a look at
a real noobs guide to reverse some more stuff) , and more importantly Dejan explains how to stop (read : slow down) people from reversing your code. Hope you will enjoy it.
Before we begin, we must mention that it’s impossible to
completely prevent reversing. What is possible is that we can place as many
obstacles on the way as we want to make the process slow enough that reverse
engineers will give up. Actually there are hardware implementations where you
can buy a black box that attaches to your computer which can do the encryption/decryption
for you, but this is far from being used in everyday life.
Techniques to Harden Reverse Engineering
The most basic approaches to harden the reverse engineering of
programs are the following [1]:
- Eliminating Symbolic Information
- Obfuscating the Program
- Embedding Antidebugger Code
When eliminating symbolic information, we’re taking the textual
information from the program, which means we’re striping all symbolic
information from the program executable. In bytecode programs, the executable
often contains large amounts of internal symbolic information such as class names,
class member names, the names of instantiated global objects. By removing every
symbol from the executable or by renaming every symbol, the reverser is faced
with a bigger problem than usual because symbol names alone can often be used
to gather enough information about what the function does, which simplifies the
reverse engineering part.
This can easily be done in C/C++ programs where we only have to
append a few compiler flags to the command line that actually compiles the
program into the executable. It’s much harder with programming languages like
Java and .NET, where those symbols are used internally to reference variables,
functions, etc. This is also the reason why Java and .NET programs can easily
be converted into a pretty good source code of the original program. We can
still strip the symbols from such programs by renaming all the symbols from
their meaningful names into meaningless representations, which effectively does
the job.
Besides stripping the executable symbols, we can also obfuscate
the program. When obfuscating a program, we’re basically changing the code of
the program without actually changing the logic behind it, so the program does
the same as before but its code is far less readable. Here we have two
techniques that can achieve that:
- Encoding: With encoding, we must add the decoding instructions
that decode the whole program before it’s being run. This can be done by
appending the decoding instruction at the end of the program and changing the
entry point to point to the decoding instructions. When the program is run, the
decoding instructions are executed first, which decodes the whole program into
its original form. After that, we must jump to the start of the program and
actually run the original instructions as if the encoding didn’t even happen.
- Packing: When packing the executable, we’re basically reducing
the size of the executable as well as encrypting it. When such a program is
run, it must first be decoded in memory and then run.
- By obfuscating the program with nonstandard encoders/packers, we
can greatly complicate the task of reverse engineering the executable, but at
the end, a persistent reverse engineer will nevertheless be able to bypass that
and get the non-obfuscated version of the executable, which can easily be
reversed.
Last but not least, we can use an antidebugger code, where we
can include a code into the executable that can detect if the program is
currently being debugged. If that happens, the program terminates itself
prematurely without actually executing the functions that would normally be
executed if it wasn’t running under a debugger.
Antidebugging
Before discussing how anti-debugging tricks do their magic, we
must first talk about how the debugger is able to debug the program. We know
that we can stop and resume the program with the use of either software or
hardware breakpoints.
When using software breakpoints, we’re replacing the instruction
on which we’ve set the breakpoint with the INT 3 instruction (at least on the
x86 architecture), which is a special software interrupt. In this case, we’re
passing the value 3 to the instruction INT, which means that we’re generating
the software interrupt 3. This causes the function pointed to by the 3rd vector
in the interrupt address table (IAT) to be executed. I guess we’re all familiar
with the INT 80 interrupt that makes a system call on Linux systems.
The INT 3 instruction temporarily replaces the current
instruction in a running program. This is also a way for the debugger to know
that a software breakpoint has occurred and the program execution should be
stopped. After that, the debugger replaces the INT 3 instruction with the
original instruction so the program can continue without the loss of
instructions, which can otherwise cause abnormal program behavior.
When we use a hardware breakpoint, it’s the processor’s job to
know when the breakpoint has been hit and the program has to be stopped. This
is why the program is not modified when a hardware breakpoint is set.
When the breakpoint is hit, the program is stopped and we can
safely execute instructions in our favorite debugger. At that point, we can run
instructions step-by-step by entering into functions, or by executing them the
same time. If we’re interested in what the function does, we need to enter into
the function; otherwise we can safely ignore the function and step over it.
When stepping through the code, each instruction is executed on its own and
then the program is again stopped, so we’re able to analyze what the
instruction has just done.
When stepping through the code with a debugger, the Trap Flag (TF) in the
EFLAGS register is used. When the TF is enabled, an interrupt will be generated
after every executed instruction, so we get the feeling of stepping though the
program instruction by instruction.
IsDebuggerPresent
The IsDebuggerPresent is a Windows API function, which we can
see on the picture below:
The function doesn’t take any arguments and returns a Boolean
value notifying us whether the program is running under a debugger or not. This
function can be used to trivially detect whether a debugger is being used to
run the program. The function uses the Process Environment Block (PEB) to get
information about whether the user-mode debugger is used.
Let’s create a simple program that prints the number 0 or 1 if
the debugger is present or not. We can do that by first creating an empty
console project under Visual Studio C++ and then changing the code of the main
cpp file into the following:
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|
//
isdebuggerpresent.cpp : Defines the entry point for the console application.
//
#include
"stdafx.h"
#include
<stdio.h>
#include
<Windows.h>
int _tmain(int argc,
_TCHAR* argv[])
{
int
num;
if(IsDebuggerPresent())
{
num
= 0;
}
else
{
num
= 1;
}
printf("Number:
%d\n", num);
/*
wait */
getchar();
return
0;
}
|
The program prints “Number: 0″ if the debugger is present and
“Number: 1″ if the debugger is not. If we run the application under Visual
Studio, the program will display the number 0 because it’s being run under a
debugger. This can be seen on the picture below:
Let’s also run the program under OllyDbg to be sure that the
number 0 is displayed. This can be quickly confirmed by loading the executable
program and running it. On the picture below, we can see that the number 0 was
printed when the program was run under OllyDbg debugger:
But if we run the same program under normal cmd.exe, it will
display the number 1. This can be seen on the picture below:
We can see that the IsDebuggerPresent API function call works as
expected, but that the function call is easy to detect and bypass. This is
because we can quickly find this function call in the executable and delete it
or bypass it. To do this, we can simply open the executable in Ida debugger and
check out the Imports table to verify if that function exists somewhere in
there. We’re right, the function IsDebuggerPresent is listed among all the
imported functions as we can see on the picture below:
This is a clear indication that the executable is using the
function to do something different when the debugger is attached to the
executable. We can also locate the exact instructions that are used to call
that function. The whole Ida graph of the main function that does exactly the
same as the main function from the C++ source code above is presented on the
picture below:
We can see that, at first, we’re initializing the stack for the
function and calling the IsDebuggerPresent function. After that, we’re testing
the returned value in eax against itself to determine whether a true or false
value was returned. If the eax holds a value different than 0 (1 in our case),
then the zero flag will be set and the first box that sets the [ebp+num] to 0
is called. This is exactly what happens now, because we’re running the program
under a debugger, but otherwise the block that sets the [ebp+num] to 1 is
called. After that, we’re just moving the value of [ebp+num] into the register
eax and printing it with the printf function.
If we now set the breakpoint on the call to the
IsDebuggerPresent function and rerun the program, the execution will be stopped
right where we want it. After the breakpoint has been hit, we can step into the
function to see what the function actually does. On the picture below, we can
see the function in question:
We can see that the function is pretty simple: we’re loading the
address of the currently active thread (TIB) in the register eax and then accessing
the structure member that’s located at the 0×30 offset; the PEB data structures
lies at that offset. After that, we’re loading the address of PEB in eax and
then accessing its data member at 0×2 offset, which holds the data member
named BeingDebugged. Thus, we’ve successfully taken a look at what
the IsDebuggerPresent function actually does and how it does it. We can see
that it’s very simple and not really hard to bypass.
We can determine that IsDebuggerPresent is being used when we
try to reverse engineer an executable and the program terminates prematurely, a
different execution path is taken, or something else unexpected happens. In
such cases, we must first check the Imports table if the IsDebuggerPresent
function is being called anywhere in the executable. If that is the case, we
can simply delete the instructions that call the IsDebuggerPresent function
call, so it won’t bother us when reversing the executable.
On the other hand, if we’re developing a program and we would
like to use the IsDebuggerPresent function call, we can copy the above
instructions directly into our code, so that we’re not actually calling the
IsDebuggerPresent function directly, but using its function body instructions
to figure out whether the debugger is being used to run the executable. This is
just another trick so that reverse engineers won’t immediately notice the use
of IsDebuggerPresent function call and will make the debugging slightly more
complicated.
Conclusion
For a deeper understanding of reverse engineering, check out the
reverse engineeringtraining course offered by the InfoSec Institute. In this article we’ve
seen a few techniques to harden the reverse engineering process. The technique
easiest to bypass is symbol elimination where we have to delete all the symbols
presented in the executable. This effectively makes the names of the functions
unavailable when debugging, which leaves it up to the debugger to properly name
the functions. Another technique is program obfuscation, which can be a pretty
simple operation like xoring the whole executable then running it, but it can
also be pretty complicated. Things get further complicated if we’re using
obfuscation with the anti-reversing techniques, which detects if the program is
being reversed and terminates the program prematurely if so, greatly hardening
the reverse engineering of the executable.
References:
[1]: Reversing: Secrets of Reverse Engineering, Eldad Eilam.
compiled, garbage collected concurrent 
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