ENPM809V

Getting Started

What we will be covering

  • Getting Familiar with Pwn.College Environment
  • Learning pwndbg
  • Introduction to Reverse Engineering
  • Introduction to pwntools

Preliminary Knowledge

ENPM691

  • We are going to build on the fundamentals of ENPM691
  • Need to understand the tools and build an environment for exploiting vulnerabilities
  • Combination of Reverse Engineering and Scripting

ENPM691

  • We will not be going over too much on basic buffer overflows
    • Review content of ENPM691

Classic Buffer Overflow

Local Variables

Parameters

Classic Buffer Overflow

What Now?

We can jump anywhere we want in our program!

 

What we will focus on today:

  • ret2win
  • ret2shellcode

Soon we will learn:

  • ROP Chain
  • Ret2libc

 

ret2win

ret2win is jumping to a function that does our desired behavior.

How do we do this?

  1. Choose function we want to jump to
  2. Get its address
  3. Buffer overflow
  4. Replace the return address with the function address (MAKE SURE IT'S IN LITTLE ENDIAN FOR X86_64)

 

ret2shellcode

Instead of jumping to a function, we write shellcode on the stack and jump to it

  1. Write shellcode
  2. Craft a buffer overflow payload with shellcode either at the very beginning or after return address.
    1. Make sure to overwrite return address to location of shellcode (or buffer)
    2. I recommend putting shellcode at beginning
  3. PROFIT

ret2shellcode

Shellcode

Padding/NOP Sled

Address to Shellcode

Shellcode

Padding/NOP Sled

Address to Shellcode

Introduction to Pwn.College

Why pwn.college

  • Consistent environment for exploitation
  • Comes with lots of tools
  • DON'T NEED ANOTHER COMPUTER
  • Browser or SSH connectable

What Pwn.College Offers

  • Many exploitation tools (pwntools, pwndbg, gef, etc.)
  • Emulation (helpful for kernel exploitation)
  • Reverse Engineering Tools (IDA, Ghidra, Binary Ninja, etc.)

Lets Get Started!

  • Go to https://pwn.college and create an account!
  • Next go to the course pwn.college link (on ELMS)
  • Select the classwork activity
    • What is the difference between Start and Practice?
  • Go to /challenge (play with the environment)

GDB and pwndbg

Why a debugger?

  • We can step through a program and see what is happening at each point
  • We can see values on the stack and registers as the program executes
  • We can test various payloads and see if they work
    • Or why they break.

Vanilla GDB Tricks

  • break <addr> <func> <*func+offset>
  • x/[length][format][address_expansion] - Display Bytes
    • example x/20xi - Display 20 instructions
    • eample x/20xg - Display
  • p <val> - print the value (register, memory, etc.)
  • Many many more

Reference - https://visualgdb.com/gdbreference/commands

Vanilla GDB Tricks

  • ~/.gdbinit - commands for GDB to run every time
  • hook-stop - Commands that are run after every step
  • layout src - if you have source, show the source
    • You can break out of this by doing Ctrl-x + a
  • layout registers - display the registers

Reference - https://visualgdb.com/gdbreference/commands

Vanilla GDB Tricks

# This can be .gdbinit or whenever you run gdb. 
define hook-stop
	x/20xi $rip
    info registers
end

Create your own hookstop!

Open pwn.college classwork and practice using the hook-stop.

pwndbg and GEF

  • pwndbg and GEF are GDB Extensions
  • They are designed for binary exploitation
  • Pattern matching
  • Better view of assembly/Registers
  • etc.

Set up pwndbg/GEF

  • pwn.college stores pwndebug and GEF into the /opt directory
    • Other utilities are there too
  • Modify your /home/hacker/.gdbinit file
    • GEF: Add the line: source /opt/gef/gef.py
    • pwndebug: Add the line source /opt/pwndebug/gdbinit.py

Set up pwndbg/GEF

  • pwn.college stores pwndebug and GEF into the /opt directory
    • Other utilities are there too
  • Modify your /home/hacker/.gdbinit file
    • GEF: Add the line: source /opt/gef/gef.py
    • pwndebug: Add the line source /opt/pwndebug/gdbinit.py

pwndebug

pwndbg

pwndbg

It tells you where it will return to!

pwndbg

  • Lots and lots of commands: pwndbg will show them
  • Important Commands:
    • retaddr
    • pwndebug heap
    • pwndebug kernel
    • cyclic

Demonstration

Reverse Engineering

Why Reverse Engineering?

  • You are not necessarily going to have source code when determining vulnerabilities.
  • Need to have some tools under your tool belt to figure this out

Static Reverse Engineering Tools

  • Ghidra - Developed by NSA. Excellent decompiler, has scripting
  • Binary Ninja - Made by Vector35
  • radare2 - Open source - commandline. Super scriptable and extensible (r2ghidra)
  • IDA Pro - The "gold standard" for Windows reverse engineering. Decompiler requires license.

Ghidra

  • Ghidra - Developed by NSA. Excellent decompiler, has scripting
  • Has an awesome decompiler!
  • Lots of little things to make your life easier
  • Easier to work with right out of the box
  • DEMO!

Binary Ninja

  • Developed by Vector35
  • Is modern and sleek compared to its competitors
  • Has an awesome graph view and decompiler
  • Has the ability to support plugins in Python, C++, and rust
  • Has a good debugger
  • DEMO!!!!

Radare2

  • Open Source (just like Ghidra is open source)
  • The Vim of reverse engineering
    • Hard to learn, but can be used
  • Ability to script and add plugins
  • Has the ability to support Ghidra Decompiler
  • Has a GUI called Cutter
  • DEMO!

IDA Pro

  • The original decompiler
    • Definitely not open source (EXPENSIVE)
    • Has a free version without the decompiler
  • Can be very good at disassembling Windows and C++
  • Has a good debugger and graph view
  • Has scripting, but is very sensitive to the version

Dynamic Reverse Engineering Tools

strace and ltrace

  • Useful for seeing system calls and library calls
  • Executes the binary and lists any time it sees a system call or library call

valgrind

  • Memory tracker/leak detector
  • Useful for debugging memory errors/determining if there is a memory vulnerability.
  • Helpful for finding use-after-free, memory leaks, etc.
  • This is more of a debugging tool than a vulnerability finder, but still useful to detect if something exists.

strace /path/to/binary

Tracers

  • Tracers are great to determine whether certain code, system calls, or operating system behaviors are occurring. 
  • Can hook a function and execute log messaging before and/or after the function is executed
  • Modify input/output based on parameters/return values.

Tracers

  • uprobes/uretprobes - userspace tracer for functions
  • kprobes/kretprobes - kernel tracer 
    • uprobes and kprobes hook on a particular function (either in userspace or kernel space)
  • ftrace - kernel function tracer (similar to kprobes)
  • perf-events - hardware performance counters
  • eBPF - has all of the above plus more 
  • Many many others. Each with their own API.

pwntools

What is pwntools

  • A suite of python scripts to help make exploitation prototyping easier (for Linux)
    • Helps to create shellcode
    • Helps with debugging
    • Helps with remote exploitation
    • Automating exploit creation
    • Easier to parse ELFs
    • Much much more....

Installing pwntools

  • python3 -m pip install pwntools
    • This is already done for you on pwn.college

How do I start?

# NOTE: This whole thing can be automated with (pwn template /path/to/binary > solve.py)

from pwn import *

#define the binary we are going to work with
exe = context.binary = ELF("/path/to/binary")

"""
If we want to create a new test process without GDB
"""

io = process([exe.path, arg1, arg2, ...])

"""
If we are going to debug 
"""
# Optional gdb script if we are going to debug

gdbscript = '''
break main
break func1
continue
'''

io = gdb.debug([exe.path, arg1, arg2, ...], gdbscript=gdbscript)

Interacting with the Binary

# Receive until it sees the data specified
io.recvuntil(b"line from stdio")

#Send as a new line
io.sendline(b"line to send") 

# Sends without the new line
io.send(b"data to send") 

# Receive until seeing a new character
io.recvline()

# combination of recvuntil and send
io.sendafter(b"data to recv", b"data to send") 

# Combination of recvuntil and sendline
io.sendlineafter(b"data to recv", b"data to send in newline")

Shellcraft

  • pwntool's shellcode writing utility (for various architectures)
  • Many different python methods to generate specific shellcode.
  • Assemble it via asm function
  • Example:
    • shellcraft.cat("/flag")
    • shellcraft.sh()
    • shellcraft.mov('r9', 0x1234)
    • More here: https://docs.pwntools.com/en/dev/shellcraft.html

Logging

  • Generate messages for various purposes (better than just printing)
    • log.debug(msg)
    • log.info(msg)
    • log.warning(msg)
    • log.error(msg)
    • etc.

Others

  • Pwntools ROP Library (will be covered later)
  • fmtstr - for format string based exploitation
  • p32, p64, u32, u64 - Packing and unpacking bytes of certain bits (good for addresses)
  • Fit and flat - functions for packing whole payloads
  • iters - extension of itertools
  • cyclic - for creating repeatable patterns
  • Net - for working with network interfaces

Shellcoding

What is shellcoding

  • Small piece of code used to gain further access into a vulnerability (generally written in assembly).
    • Often called shellcode because it is used to get a shell
  • Often chaining together various system calls
    • Don't have access to library calls as often in shellcode.

What is shellcoding

# binsh shellcode
   0:    6a 68                    push   0x68
   2:    48                       dec    eax
   3:    b8 2f 62 69 6e           mov    eax,  0x6e69622f
   8:    2f                       das
   9:    2f                       das
   a:    2f                       das
   b:    73 50                    jae    0x5d
   d:    48                       dec    eax
   e:    89 e7                    mov    edi,  esp
  10:    68 72 69 01 01           push   0x1016972
  15:    81 34 24 01 01 01 01     xor    DWORD PTR [esp],  0x1010101
  1c:    31 f6                    xor    esi,  esi
  1e:    56                       push   esi
  1f:    6a 08                    push   0x8
  21:    5e                       pop    esi
  22:    48                       dec    eax
  23:    01 e6                    add    esi,  esp
  25:    56                       push   esi
  26:    48                       dec    eax
  27:    89 e6                    mov    esi,  esp
  29:    31 d2                    xor    edx,  edx
  2b:    6a 3b                    push   0x3b
  2d:    58                       pop    eax
  2e:    0f 05                    syscall

Properties of Shellcode

  1. Cannot have null characters.
  2. Position Independent
  3. Smaller = Better
  4. Self-Contained (as much as possible)

Why?

How do I write Shellcode

  1. Use pwn.tools
  2. Use C/Assembly and compile/assemble it

How do I write Shellcode

  • pwn.tools has the shellcraft library (as mentioned earlier) which can generate shellcode
    • Can be used via command line or in python script
#asm uses the assembler to write it
#shellcraft generates the assembly
shellcode = asm(shellcraft.sh())

How do I write Shellcode

  • To use GCC/Assembly - you first write your shellcode (this example uses assembly)
  • Assemble it:
    • gcc -nostdlib -static shellcode.s -o shellcode-elf
  • Extract the text section using objcopy
    • objecopy --dump-section .text=shellcode-raw shellcode.elf

How do I Run Shellcode

  • You need a mechanism for executing it in a binary program.
    • Exploit that executes shellcode
    • Allocate memory
    • Create a test program to execute it

Creating a Test Program

/* Create a program with the following lines */

//Allocate memory
page = mmap(0x1337000, 0x1000, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_PRIVATE|MAP_ANON, 0, 0); 
//Read shellcode into the mmaped memory
read(0, page, 0x1000);
//Execute the mmapped memory
((void(*)())page)();

Commands Reference

Source: pwn.college

Classwork

Classwork

  • You have some 32 bit vulnerable binaries to exploit.
  • Practice some of the tools that you are learning on these binaries.
  • Get comfortable with debugging
    • Practice Vanilla GDB
    • Practice GDB with an extension (recommend pwndbg)
  • Start using reverse engineering tools!

Getting Started with ENPM809V

By Ragnar Security

Getting Started with ENPM809V

  • 229

More from Ragnar Security