Buffer overflow ctf

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. 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. 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:.

Deepdive Containers - Kernel Sources and nsenter

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. 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.

In this article we will be using the 32 Byte Egghunter which makes use of the NTDisplayString system call which is displayed as.

Introduction

NTDisplayString is actually the same system-call used too display blue-screens within Windows, So how does this come into play with our Egghunter? The reason? Because I was trying to run the Egghunter on a 64bit arch of Windows, kind of stupid of me but I did not give it much thought due to the application being compiled as a 32bit application and not having much issues in the past.

Corelan did a great job explaining what each assembly instruction of an Egghunter does so please check out there article Here. In this example I am going to go over VulnServer which is an intentionally vulnerable server that listens on port for any incoming connections and supports numerous types of commands as previously saw in Part 1.

In this case I am only going to fuzz the GMON command to save time and to focus on the exploitation part itself. What we are doing here is very similar to the basic stack-overflow we covered in Part 1, in which we are doing the following. Once the application has crashed the script will seize running and we can check out Immunity. Now when we jump over to Immunity we may notice some interesting stuff, the first thing I notice is Access Violation when writing to [] along the footer of Immunity, this is telling us that the application is in a crashed state and really does not know what to do next - You may also notice that the EIP value is looking normal unlike Part 1 where it contained - This is due to the fact we have not over run the return address and gained control over the EIP Register but instead overrun the nSEH and SEH values on the stack.

Upon doing so we will notice something interesting the output we are used to seeing in the EIP Register is now showing in SE Handler. Right click and select Follow in Stack. Here we are again, finding the offset as I am sure you are aware this is a very common piece of exploit development and does not just apply to SEH Overlows - There are a couple different ways to do this such as manuallymetasploit and mona but I will stick to mona here due to preference.

As you can see we are looking at our never-repeating string and can not calculate the offset by simply using one of the below commands within mona. As you can see it took us bytes to overrun the value of nSEH and bytes to overrun the value of SE Handler - Before I jump into beginning to piece everything together I want to first take this time to find any bad chars.Learn how you can exploit BOF applications on Linux Operating system including the debugging tools and methods.

Experiment the art of exploiting windows based applications along with discovering handy tools and scripts that can help you with the exploitation process. Learn about the techniques and methods of exploiting applications with memory restrictions enabled and the bypassing techniques including ROP stuff.

Learn how you can create your own fuzzing scripts using python and some advanced topics in forensics and exploit development. Learn how you can create powerful shellcodes automatically and manually along with bypassing anti-viruses.

Buffer Overflows. Get started! Read More. Windows BOF Experiment the art of exploiting windows based applications along with discovering handy tools and scripts that can help you with the exploitation process. Advanced exploitations Learn about the techniques and methods of exploiting applications with memory restrictions enabled and the bypassing techniques including ROP stuff.

Coding ASM,PY Learn how you can create your own fuzzing scripts using python and some advanced topics in forensics and exploit development.

Shellcoding Learn how you can create powerful shellcodes automatically and manually along with bypassing anti-viruses.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:. To bypass this, we can add a nop slide in front of our shellcode payload which is basically a ton of nop instructions.

This allows the shellcode to execute as long as the calling address lands on one of the nop instructions.Notes for taking the OSCP in Read in book form on GitBook. Advanced buffer overflow and memory corruption security challenges.

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

buffer overflow ctf

Bruteforce commandline buffer overflows and automated exploit generation, linux, aggressive arguments. 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.

Curate this topic. To associate your repository with the buffer-overflow topic, visit your repo's landing page and select "manage topics. Learn more. Skip to content. Here are 91 public repositories matching this topic Language: All Filter by language. Sort options. Star 9.

buffer overflow ctf

Code Issues Pull requests. Updated Apr 4, Star Collection of things made during my OSCP journey. Updated Feb 22, Python. Updated Jun 13, Python. Updated Feb 18, A handy collection of my public exploits, all in one place.

Updated Apr 15, 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. 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.

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.

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.

Buffer Overflow Attack Explained with a C Program Example

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. Expand all Back to top Go to bottom.This blogpost contains a writeup of the second phase of the Hack. LU Wannabe challenge. During the first phase, we managed to get ourselves a limited shell www-data on a webserver.

In this phase, we had to exploit a custom C program compiled for Linux x64 which contained a couple of buffer overflow vulnerabilities. The whole process is described in more detail below. After briefly investigating the filesystem of the webserver, it became apparent what our next goal was. It was only readable by its owner, arthur. This binary had the suid bit set, meaning that it will be running under the privileges of its owner, arthur, when executed.

It was noticed that insp. This may explain why 7 teams managed to solve this challenge the morning shortly before the CTF deadline… or not. The x64 ELF control binary was examined for memory protection measures with the checksec script:. Our biggest concern will be the Non-eXecutable stack NX. This will require us to take a ROP-approach. Luckily, the binary was not compiled as a Position-Independent-Executable PIEso its code section executable by default is not randomized and we can leverage it to find suitable gadgets.

buffer overflow ctf

The application presents us with two different code paths that get triggered based on the first command line parameter: —clean or —sign. Looking more into the clean option, its functionality became clear: it parses all files in the.

This keeps the upload directory of the web application clean see also part one.

Introduction

However, the implementation is not perfect. The inspect function takes the name of an element of the. It also serves as an index in the found array see line The prefix is concatenated with the system call argument by means of snprintf see lineand copied into a local function variable buffer[].

Since found[i] contains maximum bytes and the prefix string is always smaller than 32 bytes, this should never go wrong, would it? When the argument for an identified system argument call is longer than bytes, the number of bytes that are written to the found[cnt][ ] variable is exactly linewhich leaves no room for a trailing null-byte static variables contain zeroes by default in C. In the beginning of the function, the value of the global cookie variable is assigned to a local variable overflow.

This can be verified easily by debugging with gdb. Inspect the variable contents:. Thus, in order to overwrite the stack, we must make sure the local overflow variable contains the same value as the global cookie variable after we overwrite its value with our own payload. However, there is another off-by-one error that can be reached in the inspect function. The global cnt variable is of type unsigned char, which can take values by design.

However, the global found[] array only contains elements When we construct a file containing valid system calls, the argument of the th line will be written to element found[], which is exactly the memory address of the cookie variable.

This can also be verified empirically with GDB by constructing a file that contains system calls, the last one containing a string of which the first four bytes will overwrite the global cookie variable contents:.

In order to successfully conduct step two, offsets to the overflow variable and the return address were calculated. Next, a suitable address to jump back to was located by examining the external functions that the control x64 ELF binary imports:. Especially the last import seemed interesting: system.


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