Buffer overflow x86

Buffer overflow x86


  • How to exploit Buffer Overflow
  • Stack Based Buffer Overflows on x86 (Windows) – Part I
  • ELF x86 – Stack buffer overflow basic 2
  • Linux Stack Based Buffer Overflow x86
  • How security flaws work: The buffer overflow
  • Linux Buffer Overflows x86 Part -2 ( Overwriting and manipulating the RETURN address)
  • Linux Buffer Overflows x86 – Part 3 (Shellcoding)
  • How to exploit Buffer Overflow

    This would be the stack just after the execution of the sub esp, 0x84, ie after finishing the prologue. Labeled with ellipses fields represent in the stack not only are local variables, but also as shown in the assembly code other program data itself does not interest us calculate how much take that data and we do not care, although we consider them to know how much padding we must introduce before touching EIP EIP remember that we want to put a specific address.

    Using the -l parameter will indicate the length we want, we must be guided by the size of the stack frame we see in the asm code.

    Breaks when accessing 0x, had a big endian 36 41 65 37 and converted into ASCII characters [asciitohex. Once you undoubtedly possess control program flow must tell you what to do, this is possibly the most exciting part, although we now remain in the fold of , somewhere must start, and it will be first without face protection. The prefer any exploitation start it first for Linux, Windows and expand into better stay sane as long as possible, and it turns out that Windows is exploiting many times the best way to the psychiatric.

    Quit anyway? This code is called shellcode, precisely because it is a code that usually get a shell. On the Internet there are numerous shellcodes that can be used yes, you should always verify that it does what it claims to do, before using it , but in the next episode we will see how to build a good shellcode for every occasion, no beauty want anyone programming in asm miss. Anyway, for now we better this shellcode xor eax, eax push eax push 0xf2f push 0x6ef mov ebx, esp push eax push ebx mov ecx, esp mov al, 0xb int 0x80 We can assemble with nasm nasm -o sc.

    We can get the assembly opcodes opcode is the value number usually in hex corresponding to an assembler instruction, ie machine language by xxd sc. Well, the shellcode should have no 0x00 because strcpy finishes copying to find that value, and made this shellcode is designed to avoid them.

    Observe the code of print. After executing strcp must be in the stack and data we provide. Hence we place a breakpoint a point where the program stops until we give the order to continue. So if we put the 0xffffd1f0 address in EIP, will proceed to execute our shellcode because there you will place. Do not forget to adjust the size of the filling we put depending on the size of the shellcode in our case Start it from the beginning?

    Error in re-setting breakpoint 1: No symbol table is loaded. I urge the curious to investigate containing the above stack of main addresses, can be illustrative. In any case one way to avoid problems is to run the program with a null environment. Anyway, I have not found a way to accurately calculate the exact address, sometimes is arranged by subtracting the difference between the lengths of names to the address on other occasions say that this is something hazardous.

    I really do not have much, just for practice to bring it to an environment where it does make sense, as a binary with the SUID bit, thus privilege elevation would be obtained.

    Many methods employ elevation of privilege vulnerabilities in binary as sudo or passwd. Certainly something much more interesting.

    Stack Based Buffer Overflows on x86 (Windows) – Part I

    Home » security » What a buffer overflow looks like What a buffer overflow looks like August 14, July 28, Once you know what to look for, a buffer overflow is almost as easy to spot as it is to understand.

    A buffer overflow is a long sequence of NOP operations followed by machine code. A common question on certification tests Buffer overflows tend to be much more repetitive than legitimate machine code.

    Buffer overflows consist of an ultra-repetitive NOP sled, followed by a payload. Buffer overflows happen when data mingles with code, making it possible to overwrite program code with your own data. If you find the concept confusing, my brief explanation of machine language may help clear things up for you. My heart sank the first time I heard about this kind of attack, because almost every program I wrote in C or machine language in the 90s probably had a buffer overflow in it.

    Storing data right next to the code that needed it made programs more efficient, so I usually did it. What a buffer overflow looks like A buffer overflow consists of two parts: a NOP sled, followed by a payload. Sending too few causes your payload to malfunction, so the attacker will probably pad the sled to be safe. Buffer overflows are very CPU-specific. On Intel x86, NOP is 0x Its most common legitimate use is for timing purposes.

    The and are very old designs, but still sometimes show up in embedded systems today. This instruction will always be more than a byte long, frequently four bytes. The most common payload will open a shell, probably listening on a weird port.

    But some pentesters will just run a harmless program, like the calculator. So the telltale sign is a long string of bytes containing 0x90, followed by a sequence of unpredictable, semirandom bytes. This makes a buffer overflow harder to spot. And unfortunately, the do-nothing sequence on some MIPS variants happens to be four zeroes. But it could also be perfectly innocent, because initializing memory to a known value is good practice, and everyone tends to choose zero.

    One some other MIPS variants, the do-nothing sequence is 60 00 00 If you see a sequence of a small number of bytes repeating over and over a lot, followed by something more complex, it may be a buffer overflow.

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    ELF x86 – Stack buffer overflow basic 2

    This points to the location at the bottom of the memory stack Why is this bad EIP points to the next memory location to be executed.

    If a user can overflow the buffer and cause their own input to overwrite EIP then the user can also point EIP to any memory location they wish. After initial foothold of the user account and enumeration I found a binary that ran with superuser privileges. It turns out that this was exploitable via a buffer over flow. Running the HT program without any arguments with sudo: Running the program with arguments shows that it was indeed loading in the argument passed to it and trying to run the program.

    Testing for Buffer Overflow aka: Fuzzing Seeing as we can send arguments to the program, lets try and pass in an argument of characters.

    For this I will use python as python happens to be installed and the interpreter can be run directly from the command line to pass in arguments to the HT program. This caused a segmentation fault.

    IE: A break in memory causing the program to crash. This is what has actually happened: Confirming EIP was overwritten Now that we have confirmed that there is a buffer over flow occurring. We need to examine the program while it is running in memory so we can locate EIP and then point it to our malicious code. I will be using a command line debugger called GDB as it so happens to be installed on the machine I am exploiting. Shellcodes are machine codes.

    Basically, we write a set of instructions in a high-level language or assembly language then convert into hexadecimal values these values are just a bunch of opcodes and operands understood by the processor. We cannot run shellcode directly, instead it needs some sort of carrier buffer, file, process, environment variables, etc which will be read or executed by the vulnerable software. There are many shellcode execution strategies which are used depending on the exploitability and constraints of the vulnerable software.

    Shellcodes are written mainly to force the program to do actions on the behalf of the attacker. The attacker can write a specific function which will do the intended action once loaded into the same address space of the vulnerable software.

    Every action well almost in an operating system is done by calling a syscall or system calls. The shellcode will also contain a syscall for a specific action which the attacker needs to perform.

    Linux Stack Based Buffer Overflow x86

    Syscalls are the most important thing in the shellcode as you can force the program to perform action just by triggering a syscall. The userspace process runs within the userspace, every process running in the userspace has its own virtual address space and cannot access any other processes address space until explicitly allowed. When the process in the userspace wants to access the kernel space to execute some operations, it cannot directly access it.

    No processes in the userspace can access the kernel space. When an userspace process tries to access the kernel, an exception is raised and prevents the process from accessing the kernel space. The userspace processes uses the syscalls to communicate with the kernel processes to carry out the operations. Syscalls are a bunch of functions which gives an userland process direct access to the kernel space to carryout low level functions. In simpler words, Syscalls are an interface between user mode and kernel mode.

    In Linux, syscalls are generated using the software interrupt instruction, int 0x So, when a process in the user mode executes the int 0x80 instruction, the flow is transferred from the user mode to kernel mode by the CPU and the syscall is executed.

    Other than syscalls there are standard library functions like the libc which is extensively used in the linux OS. These standard library functions also calls the syscall in some way or another not everytime though. Also, using syscall is much nicer beacuse there is no linking involved with any library as needed in other programming languages.

    We will use the int 0x80 instruction in our shellcode. Now the system call handler is notified to call the routine which the userland process wanted. All the syscalls are stored in a syscall table, it is a big array which points to the memory address of the syscall routines. When the syscall is called, the syscall number is loaded into EAX register, this is how the system call handler comes to know which system call routine to execute.

    How security flaws work: The buffer overflow

    Keep in mind that the execution done here is carried out on the Kernel space stack and after the execution the control is given back to the userspace process and the remaining operation is carried out on userspace stack. Every syscall has arguments which are necessary to execute the related operations.

    All the linux syscalls are well documented with instructions of how to implement them in your program. Usually we can use the syscall in normal C program or we can go ahead and do the same in an assembly language.

    Linux Buffer Overflows x86 Part -2 ( Overwriting and manipulating the RETURN address)

    Since we are writing our own shellcode, we have to dip our hands in the world of assembly language. So what we will do is, write a C program with the syscall, disassemble the program, see the assembly instructions, write the assembly equivalent, make the necessary changes and extract the shellcode. Now we could have done this in assembly language directly but for getting the whole idea of how a program works from the high level to low level I am choosing this path.

    We will use execve syscall for this operation. They have the role to separate the function calls on the stack, so the EBP and ESP registers the base and the top of the stack will contain the stack memory area that is used by the currently called function. With other words, using these instructions, the EBP register will hold the address where the data local variables used by the current function begins and the ESP register will holds the address where the data used by the current function ends.

    In this case, this data represents the parameters of the function.

    Linux Buffer Overflows x86 – Part 3 (Shellcoding)

    It is required to do this because the EBP register contains an address of memory as a value and we need the value that is stored at that memory location. So what those two instructions do? In order to clean the stack, we just add 8 to the ESP value the two parameters size in order to restore it to the value before the function call.


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