Twisted is an event-based framework for internet applications, supporting Python 3.6+. Prior to 26.10.0rc1, twisted.words.protocols.irc.IRC.dataReceived retains all bytes received without a line feed in its per-connection buffer and applies no maximum line length. The twisted.words.service.IRCUser server subclass inherits this behavior, and buffering occurs before PASS, NICK, or USER authentication commands. An unauthenticated connection can continuously grow the buffer until the shared server process runs out of memory and drops all users. This issue is fixed in version 26.10.0rc1.

Affected: PyPI: Twisted

## Summary

An unbounded-buffer vulnerability (CWE-770) in the Twisted words IRC server lets any unauthenticated remote client exhaust server memory. `twisted.words.protocols.irc.IRC.dataReceived` appends every received byte to a per-connection buffer and only removes data at a line feed, with no maximum length. Because `IRC` is a plain `Protocol` and not a `LineReceiver`, one connection that streams bytes without a line feed grows the buffer without bound and drives the whole single-threaded process to out-of-memory, dropping every user at once. The `twistd words` server inherits the method unchanged through `IRCUser`, and the data is buffered before any authentication.

## Details

`IRC.dataReceived` keeps everything up to the last line feed in `self.buffer`:

```python # twisted/words/protocols/irc.py class IRC(protocol.Protocol): # not a LineReceiver, so no MAX_LENGTH buffer = "" def dataReceived(self, data): if isinstance(data, bytes): data = data.decode("utf-8") lines = (self.buffer + data).split(LF) self.buffer = lines.pop() # no 3line feed -> whole input stays buffered ... ```

With no line feed in the data, `lines.pop()` returns the entire accumulated content to `self.buffer`, so it grows by every chunk and is never bounded; `IRC` defines no `MAX_LENGTH`, no `lineLengthExceeded`, and no timeout, so the connection is never closed. The `twistd words` server protocol `IRCUser` subclasses `IRC` without overri…