Pipecat is an open-source Python framework for building real-time voice and multimodal conversational agents. Prior to 1.4.0, the pipecat development runner registers a /ws WebSocket endpoint for telephony testing that accepts connections without authentication, reads an attacker-supplied callSid from a Twilio stream-start handshake in src/pipecat/runner/utils.py, and passes it to TwilioFrameSerializer so the server can issue an authenticated Twilio REST API hang-up request with the server operator's credentials; equivalent unauthenticated call-control sinks exist for Telnyx and Plivo. This issue is fixed in version 1.4.0.