2/1/2024 0 Comments Buffer overflow attackNow, let’s write the output of this file into a file called payload1. Let us also ensure that the file has executable permissions. We want to produce 300 characters using this perl program so we can use these three hundred “A”s in our attempt to crash the application. Let’s create a file called and simply create a variable. So we can use it as a template for the rest of the exploit. We’re going to create a simple perl program. Now let’s see how we can crash this application. This is intentional: it doesn’t do anything apart from taking input and then copying it into another variable using the strcpy function. Vulnerable: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, BuildID=9e7fbfc60186b8adfb5cab10496506bb13ae7b0a, for GNU/Linux 3.2.0, not strippedĪs we can see, it’s an ELF and 64-bit binary. Let’s run the file command against the binary and observe the details. We should have a new binary in the current directory. To do this, run the command make and it should create a new binary for us. Let’s compile it and produce the executable binary. Now we are fully ready to exploit this vulnerable program. This looks like the following: sudo bash -c “echo 0 > /proc/sys/kernel/randomize_va_space” Let’s disable ASLR by writing the value 0 into the file /proc/sys/kernel/randomize_va_space. We are producing the binary vulnerable as output. We are simply using gcc and passing the program vulnerable.c as input. Gcc -fno-stack-protector vulnerable.c -o vulnerable -z execstack -D_FORTIFY_SOURCE=0 The following makefile can be used to compile this program with all the exploit mitigation techniques disabled in the binary. In this section, let’s explore how one can crash the vulnerable program to be able to write an exploit later. To keep it simple, let’s proceed with disabling all these protections.įor the purposes of understanding buffer overflow basics, let’s look at a stack-based buffer overflow. To be able to exploit a buffer overflow vulnerability on a modern operating system, we often need to deal with various exploit mitigation techniques such as stack canaries, data execution prevention, address space layout randomization and more. GNU Debugger (GDB) is the most commonly used debugger in the Linux environment. In the Windows environment, OllyDBG and Immunity Debugger are freely available debuggers. A debugger can help with dissecting these details for us during the debugging process. When writing buffer overflow exploits, we often need to understand the stack layout, memory maps, instruction mnemonics, CPU registers and so on. Understanding how to use debuggers is a crucial part of exploiting buffer overflows. The user-supplied buffer often overwrites data on the heap to manipulate the program data in an unexpected manner. The successful exploitation of heap-based buffer overflow vulnerabilities relies on various factors, as there is no return address to overwrite as with the stack-based buffer overflow technique. Heap overflows are relatively harder to exploit when compared to stack overflows. When a user-supplied buffer is stored on the heap data area, it is referred to as a heap-based buffer overflow. As mentioned earlier, a stack-based buffer overflow vulnerability can be exploited by overwriting the return address of a function on the stack. When a user-supplied buffer is stored on the stack, it is referred to as a stack-based buffer overflow. The following are some of the common buffer overflow types. However, a buffer overflow is not limited to the stack. We have just discussed an example of stack-based buffer overflow. That’s the reason why this is called a stack-based buffer overflow. If this overflowing buffer is written onto the stack and if we can somehow overwrite the saved return address of this function, we will be able to control the flow of the entire program. This function doesn’t perform any bounds checking implicitly thus, we will be able to write more than 256 characters into the variable buffer and buffer overflow occurs. However, we are performing this copy using the strcpy function. This argument is being passed into a variable called input, which in turn is being copied into another variable called buffer, which is a character array with a length of 256. If you look closely, we have a function named vuln_func, which is taking a command-line argument. This is a simple C program which is vulnerable to buffer overflow. So let’s take the following program as an example. In simple words, it occurs when more data is put into a fixed-length buffer than the buffer can handle. Buffer overflow is a class of vulnerability that occurs due to the use of functions that do not perform bounds checking.
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