Twisted: Unauthenticated Remote Denial of Service in the Twisted conch Telnet Transport via an Unbounded Subnegotiation Buffer

Summary

An allocation-without-limits vulnerability (CWE-770) in the Twisted conch telnet transport lets any unauthenticated remote peer exhaust server memory over a single connection. When the peer opens a telnet subnegotiation with IAC SB and then streams bytes without ever sending the IAC SE terminator, every byte is appended to a per-connection buffer that has no maximum length. The buffer persists across reads and is released only on the terminator the attacker withholds, so it grows without bound until the process is killed.

Details

The option-negotiation state machine in Telnet.dataReceived enters the subnegotiation state on IAC SB and sets an empty command list. In that state every non-IAC byte is appended to the list with no length check:

# src/twisted/conch/telnet.py  (Telnet.dataReceived)
elif self.state == "subnegotiation":
    if b == IAC:
        self.state = "subnegotiation-escaped"
    else:
        self.commands.append(b)      # unbounded: no len(self.commands) check

self.commands is an instance attribute that persists across dataReceived calls and is deleted only when IAC SE completes the subnegotiation. Negotiation runs at the transport layer before authentication, and the bytes are consumed inside dataReceived before any application callback, so an application cannot impose a bound. Twisted caps other parser buffers, for example NetstringReceiver.MAX_LENGTH, but not this one.

Affected product: Twisted, `twiste…