TelnetServer recv error: Forcing an RST request to the telnet server can cause a crash

TelnetServer and the local terminal start simultaneously.

#!/usr/bin/env python3

import asyncio
import logging
import errno

from prompt_toolkit import PromptSession
from prompt_toolkit.patch_stdout import patch_stdout
from prompt_toolkit.contrib.telnet.server import TelnetServer

logging.basicConfig(
    level=logging.INFO,
    format="%(asctime)s %(levelname)s %(message)s"
)

logger = logging.getLogger(__name__)


def install_asyncio_exception_filter(loop):
    old_handler = loop.get_exception_handler()

    def exception_handler(loop, context):
        exc = context.get("exception")
        print(1231231)

        if isinstance(exc, ConnectionResetError):
            return

        if isinstance(exc, OSError):
            if getattr(exc, "errno", None) in (
                errno.ECONNRESET,
                errno.EPIPE,
            ):
                return

        if old_handler:
            old_handler(loop, context)
        else:
            loop.default_exception_handler(context)

    loop.set_exception_handler(exception_handler)


async def interact(connection):
    peer = connection.addr
    print("666 Client connected:"+ str(peer))
    logger.info("Client connected: %s", peer)

    try:
        session = PromptSession()

        await connection.send(
            "\r\n"
            "====================================\r\n"
            " prompt_toolkit Telnet Demo\r\n"
            "====================================\r\n"
            "Type 'exit' to quit.\r\n\r\n"
        )

        while True:
            try:
                with patch_stdout():
                    text = await session.prompt_async(
                        "telnet> "
                    )

            except (EOFError, KeyboardInterrupt):
                break

            if text.lower() in ("exit", "quit"):
                break

            await connection.send(
                f"You typed: {text}\r\n"
            )
        '''
        except ConnectionResetError:
            logger.info("Client reset connection: %s", peer)

        except OSError as e:
            if e.errno in (
                errno.ECONNRESET,
                errno.EPIPE,
            ):
                logger.info(
                    "Client disconnected unexpectedly: %s",
                    peer,
                )
            else:
                raise
        '''

    except Exception:
        logger.exception(
            "Unhandled exception from %s",
            peer,
        )

    finally:
        logger.info("Client disconnected: %s", peer)


#!/usr/bin/env python3
from prompt_toolkit import PromptSession
from prompt_toolkit.patch_stdout import patch_stdout
from prompt_toolkit.formatted_text import HTML
from prompt_toolkit.styles import Style
import asyncio

style = Style.from_dict({
    "prompt": "ansicyan bold",
    "command": "ansigreen",
    "error": "ansired bold",
})

async def main():
    loop = asyncio.get_running_loop()
    install_asyncio_exception_filter(loop)

    server = TelnetServer(
        host="0.0.0.0",
        port=8023,
        interact=interact,
    )

    asyncio.create_task( server.run() )

    print("Listening on 0.0.0.0:8023")




    session = PromptSession()

    print("Welcome to the demo interactive session!")
    print("Type 'exit' or 'quit' to leave.\n")

    while True:
        try:
            with patch_stdout():
                text = await session.prompt_async(HTML('<prompt>MyPrompt> </prompt>'), style=style)

        except (EOFError, KeyboardInterrupt):
            print("\n[Session terminated]")
            break

        text = text.strip()
        if not text:
            continue

        if text.lower() in ("exit", "quit"):
            print("[Goodbye]")
            break

        print(HTML(f"<command>You typed:</command> {text}"), style=style)


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

Exploit/Client request:

import socket
import time

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(("127.0.0.1", 8023))

s.send(b"hello")
time.sleep(1)


s.setsockopt(socket.SOL_SOCKET, socket.SO_LINGER,
             (1, 0))   # linger=0 -> RST

s.close()

Crash:

MyPrompt> 
MyPrompt> 2026-06-02 01:18:59,623 INFO New connection '127.0.0.1' 60252
2026-06-02 01:18:59,624 INFO Initializing telnet connection
2026-06-02 01:18:59,624 INFO Starting interaction '127.0.0.1' 60252
Press ENTER to continue...2026-06-02 01:19:00,642 INFO Connection closed by client. '127.0.0.1' 60252

Unhandled exception in event loop:
  File "/home/debian/.pyenv/versions/3.12.7/lib/python3.12/asyncio/events.py", line 88, in _run
    self._context.run(self._callback, *self._args)
  File "/home/debian/.pyenv/versions/3.12.7/lib/python3.12/site-packages/prompt_toolkit/contrib/telnet/server.py", line 185, in handle_incoming_data
    data = self.conn.recv(1024)
           ^^^^^^^^^^^^^^^^^^^^

Exception [Errno 104] Connection reset by peer
Press ENTER to continue...

Start in prompt_toolkit/contrib/telnet/server.py at handle_incoming_data, where the traceback shows conn.recv(1024) raising ConnectionResetError after a client sends an RST. Trace how incoming connection errors are handled and reproduce the report with the provided socket client. Done means an abrupt client reset no longer produces an unhandled event-loop exception.