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Buffer overflow ctf

Buffer overflow ctf

Recently while solving a challenge on Vulnhub. While many of the online blogposts explained how this method can be leveraged to gain shell from a buffer overflow vulnerability, but my google skills were not able to find a blogpost that explained each step, in detail. So, in this blogpost I aim to collect all the pieces of information that I collected and piece them together by means of this blogpost.

Also, buffer overflow itself is a vast topic so covering every aspect of the topic is not possible in one blogpost. Before proceeding, this blogpost expects you to know basics of what buffer overflow is, how it occurs and basic buffer overflow exploitation.

If you plan to do the CTF machine completely on your own and do not want it to be spoiled by my blogpost stop reading now. Without wasting any more words lets look at the exploitation. All the above-mentioned topics are covered after. To portray a background, we have a low privileged shell and an ELF executable binary file on the victim machine which is vulnerable to buffer overflow and our ticket to gaining root shell on the victim machine.

This file is named game3. Upon running game3 we see that it accepts input, and upon submitting a long input it threw a segmentation fault error.

At this point it was almost clear that this is a buffer overflow vulnerability and we would need to exploit this in order to possibly gain root access on the server. For the same we would require some debugging capability.

But unfortunately, any such tool was missing on the machine. Also, it was discovered that ASLR was enabled on the machine too, but the address changes were minimal and therefore a buffer overflow exploitation might be possible. The next task would be to transfer the game3 file to our local system. For the same after some hit and trial SFTP was utilized successfully to transfer the file to our local system. The game3 program was run using gdb and the letter input was provided to get our offset as 0x on the position So now we knew that the crash was occurring at position Next step would be to control EIP.

So, our first payload constructed was as follows. Next, our task would be to gain shell access using this buffer overflow. For acquiring the base address, we can use ldd command on our game3 binary.

For the system and exit addresses we can use readelf command to search the libc. These commands are as follows:. Once we have these addresses, we can now proceed and construct our payload for running on our local system and gain a shell. Before we do that a little background on why we need these addresses. User space in linux utilize system calls to interact with the kernel space.

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Libc are collection of C libraries which have system functions for interacting with kernel. Being in the user space we cannot directly call functions that can interact with the kernel, in such cases we can utilize the system calls present in the libc libraries to serve our purpose.

Thus, first we found the base address of the libc library that our binary game3 is utilizing for running itself.

Introduction

Next we found the offsets of other addresses so that we can find the main address by adding the base address to the offset. Any logical address has two parts namely segments and offsets. The offset displays distance from our base address.

buffer overflow ctf

Thus, when we add the base address to our offset, we will receive the actual address to where the function resides. Of Course, this is just a short summary of how things proceed to give us a shell using the below POC. Next, we would construct our final exploit code for gaining a shell on the CTF machine.

The only catch this time is that ASLR protection is enabled here and we cannot expect our addresses to remain the same every time we run the exploit code. Therefore, we would be using a loop to run our code until the required addresses are matched and we gain a root shell.

buffer overflow ctf

A segmentation fault is a run time error that is thrown when a program tries to access a memory location that it is not allowed to access.It still exists today partly because of programmers carelessness while writing a code. The least we can do is to avoid writing bad code that gives a chance to even script kiddies to attack your program and exploit it. In this buffer overflow tutorial, we will discuss the basics of the following :.

Similar to the first example, arr[0] refers to the left boundary while arr[9] refers to the right boundary. By now it should be clear what a buffer means. Moving on lets understand when a buffer overflows.

A buffer is said to be overflown when the data meant to be written into memory buffer gets written past the left or the right boundary of the buffer.

This way the data gets written to a portion of memory which does not belong to the program variable that references the buffer. In the above example, we declared an array of size 10 bytes. Please note that index 0 to index 9 can used to refer these 10 bytes of buffer. This is the point where buffer overrun happens because data gets written beyond the right boundary of the buffer. It is also important for you to understand how GCC compilation process works to create a C executable.

Some of us may think that though a buffer overflow is a bad programming practice but so is an unused variable on stack, then why there is so much hullabaloo around it?

What is the harm buffer overrun can cause to the application?

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Then this may lead to crash in most of the cases. The reason being, a pointer is not allowed to access heap memory that does not belong to it. Now, depending on the compiler you are using, chances are high that this will get unnoticed during compilation and would not crash during execution. The simple reason being that stack memory belongs to program so any buffer overflow in this memory could get unnoticed. So in these kind of scenarios, buffer over flow quietly corrupts the neighbouring memory and if the corrupted memory is being used by the program then it can cause unexpected results.

You also need to understand how you can prevent stack smashing attacks with GCC. Until now we discussed about what buffer overflows can do to your programs. We learned how a program could crash or give unexpected results due to buffer overflows. But, that it is not the worst part. Consider this example :. The program above simulates scenario where a program expects a password from user and if the password is correct then it grants root privileges to the user.

But do you know that there is a possibility of buffer overflow in this program. The gets function does not check the array bounds and can even write string of length greater than the size of the buffer to which the string is written. Now, can you even imagine what can an attacker do with this kind of a loophole? In the above example, even after entering a wrong password, the program worked as if you gave the correct password. There is a logic behind the output above.

To avoid buffer overflow attacks, the general advice that is given to programmers is to follow good programming practices. For example:. Thanks for the article. The example works because the memory location of the pass variable is just next to the location of the buffer, so it gets overwritten! My question: Is it always like this? Is is true? Defensive coding is important when it comes to accepting input, no matter if it is from a carbon unit or another machine.

Now we beginner know buffer overflow because of this article. How about buffer underrun? Thanks for the explanation with a good example.A Buffer Overflow is a vulnerability in which data can be written which exceeds the allocated space, allowing an attacker to overwrite other data.

The simplest and most common buffer overflow is one where the buffer is on the stack.

picoCTF 2018 [15] Buffer Overflow 1 & Hertz 2

Let's look at an example. There's a tiny mistake in this program which will allow us to see the secret. Let's see how we can use this to our advantage. The least significant byte of secret has been overwritten!

If we follow the next 3 bytes to be read in, we'll see the entirety of secret is "clobbered" with our 'A's. How can we use this to pass the seemingly impossible check in the original program? Well, if we carefully line up our input so that the bytes that overwrite secret happen to be the bytes that represent 0x in little-endian, we'll see the secret message. This will fill the name buffer with 'A's, then overwrite secret with the bit little-endian encoding of 0x As discussed on the stack page, the instruction that the current function should jump to when it is done is also saved on the stack denoted as "Saved EIP" in the above stack diagrams.

Buffer Overflow Attack Explained with a C Program Example

If we can overwrite this, we can control where the program jumps after main finishes running, giving us the ability to control what the program does entirely. Usually, the end objective in binary exploitation is to get a shell often called "popping a shell" on the remote computer. The shell provides us with an easy way to run anything we want on the target computer. Say there happens to be a nice function that does this defined somewhere else in the program that we normally can't get to:.

Well with our buffer overflow knowledge, now we can! We send 'A's to overwrite the bytes that is allocated for namethe 4 bytes for secretand the 4 bytes for the saved EBP. This idea is extended on in Return Oriented Programming. From here you can search these documents. Enter your search terms below. Toggle navigation. Buffer Overflow Stack buffer overflow Passing an impossible check Going one step further. Here's a secret.At the start of the article I discussed how I recently embarked on a mission to learn exploit development better and the purpose of this mini-series was too have reason to put pen to paper and finally learn all this shit : - Now in this article I want to move on a little bit from basic Stack Overflows and progress to SEH - Structured Exception Handling Overflows.

Exception Handler EH - Piece of code that will attempt to do something and have pre-defined courses to take depending on the outcome. For example, try do this if you fail do this. So basically Exception Handlers are pieces of codes written inside a program, with the sole purpose of dealing any exceptions or errors the application may throw.

For example:. So, How do they work? When a exception is triggered the OS will retrieve the head of the SEH-Chain and traverse the list and the handler will evaluate the most relevant course of action to either close the program down graceful or perform a specified action to recover from the exception.

More on the linking later. When we run an application its executed and each respective function that is ran from within the application there is a stack-frame created before finally being popped off after the function returns or finishes executing.

Now the same is actually true for Exception Handlers. So… How are they linked? Well for every Exception Handlerthere is an Exception Registration Record configured which are all chained together to form a linked list. But what if none of the pre-defined exception handler functions are applicable?

So to just recap, we have covered what exceptions are, the different types of handlers and we have also spoken about how Structured Exception Handlers really work, so now we should probably talk about this from an attackers point of view and how we can exploit these handlers to gain control over a programs execution flow similar to the EIP Overwrite in Part 1. Check out the script below.

We can now control the execution flow of the overall program. Now if we try to add a memory location of a JMP ESP instruction to the SE Handlerwindows will automatically zero-out all registers to prevent users from jumping to there shellcode but this is a really flawed protection mechanism.

Now I do not want to go into depth here with how we find applicable modules and instructions as I will cover it in the examples section but the long story short is mona. Similar to Part 1 where we used mona intensively it will also be of use when carrying out SEH Overflows - All we have to do is issue the below command.

Buffer Overflow Attack Explained with a C Program Example

Now just like exploit we have to ensure that we choose a module with 0 bad chars in the memory address as well as avoid and SEH Safeguards such as SafeSEHwhich I will talk about a later.

An Egghunter is a small piece of shellcode, typically 32 Bytes that can be used to search all memory space for our final-stage shellcode. I would like to provide a high level overview of how Egghunters work here without going crazy in depth, as I have already said above.

This is nowhere near enough space to place some shell code but it is enough to place a 32 Byte Egghunter.This program executes any shellcode that you give it. Can you spawn a shell and use that to read the flag. The solution is basically the same as the shellcode challenge from last year click the link for my writeup on that. This should be easy. Overflow the correct buffer in this program and get a flag. Same as buffer-overflow-0 from last year. You beat the first overflow challenge.

Now overflow the buffer and change the return address to the flag function in this program? Same as buffer-overflow-1 from last year. One thing to clarify is how I found the offset of the return address. This is obtained by using a tool like radare2 and looking at the stack layout of the function:. As you can see, the buffer is located at ebp-0x48 and we know there are another 4 bytes for the saved ebp register.

Another approach would be to use something like this and deduce the offset by looking at the segfault address.

This is a simple buffer overflow challenge like OverFlow 1 read this to see how I found the return address offsetbut instead of 32 bit, it is now 64 bit. Our payload would look something like this:. The problem with calling the win function directly is not because of buffering issues. Instead, it is triggered by a stack misalignment. When we send a payload without calling the main function:.

We see in gdb with gef that it crashed on movaps :. To fix this, we really just need to shift the stack by 8 bytes through calling any other function before the win function:. This program is a little bit more tricky. This is similar to handy-shellcode but a random offset is added to the address that it is calling:.Notes for taking the OSCP in Read in book form on GitBook. Advanced buffer overflow and memory corruption security challenges. Bruteforce commandline buffer overflows and automated exploit generation, linux, aggressive arguments.

Red-team tool to hook libc read syscall with a buffer overflow vulnerability.

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Customizable TCP fuzzing tool to test for remote buffer overflows. Tool for finding Buffer Overflows in simple binaries. CTF use mainly. Add a description, image, and links to the buffer-overflow topic page so that developers can more easily learn about it.

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Updated Jul 11, C.Simply a buffer is a memory place or location which is used by a running program. This memory location is used to store some temporary data that is being used by the program.

Break Down : int main This defines the main function char username[20] This is where we specify the variable name but the most important thing about this line is char Now before we talk about the buffer overflow we need to understand how the application memory works. A stack is a memory buffer that is used to store the functions of the program and local variables.

To demonstrate this, we will take a look at this image. First We have the code and this is the source code of the program. This has the main instructions of the program. After that we have the buffer where the global variables are stored, The difference between a local variable and a global variable is that a local variable is limited to a certain function.

Then we have the Stack and this is the important part of the memory for us because this is where the buffer overflow happens. This is the place where local variable and function calls are stored. Last thing is Heap and this is a dynamic memory allocation.

buffer overflow ctf

Now we know what does the application memory look like and what is the stack but what are memory addresses? Basically when a program is compiled and executedAll the instructions of the program take place in the application memory and an address is assigned to themThis address is usually in the format of hexadecimal bytes.

Buffer Overflow Attack Explained with a C Program Example

Now we know what is a buffer and we took a deeper look on the memory construction. Now you might already figured out why and when does a buffer overflow happen.

A buffer overflow happens when the length of the data entered exceeds the buffer limit and this causes the program to write data outside the allocated buffer area and may overwrite some parts of the memory that were used to hold data used by the program which makes it unavailable and causes the program to crash.

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To demonstrate this we will go back to our first example. But if the entered data is more than 20 chars length the program will crash because some data will be overwritten outside the buffer causing some parts of the program to be corrupted. That happened because we entered 10 extra chars, The program only expected 20 or less. And we will assign the buffer for that variable to be We see that most of the memory addresses are overwritten with Now you might ask yourselfHow will that be harmful?

Since we can pass the buffer and overwrite the program then we can overwrite it with a payload that executes a system call and spawns a root shell. I will do more write ups about buffer overflows and other binary exploitation techniques, for now I will start with protostar.

In the meantime, you can read my other Hack The Box write-ups! You can download protostar from here I will solve the first level which is stack0 for this article then I will solve the rest of the levels in other write-ups. We already know that the buffer is 64 chars so we just need to input 65 chars or more and the variable value will change.

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