CVE-2026-62949

AsyncSSH is a Python package which provides an asynchronous client and server implementation of the SSHv2 protocol on top of the Python asyncio framework. Prior to 2.24.0, _process_channel_open and _process_channel_open_confirmation in asyncssh/connection.py accept a peer-supplied send_pktsize value of zero. When channel data reaches SSHChannel._flush_send_buf in asyncssh/channel.py, the zero value causes each loop iteration to slice and remove zero bytes without reducing the send window, leaving the synchronous loop permanently true with no await point. A malicious SSH server can trigger the client path through SSH_MSG_CHANNEL_OPEN_CONFIRMATION before the first channel write, while an authenticated client can trigger the server path through SSH_MSG_CHANNEL_OPEN and freeze every current and future connection handled by the process. This vulnerability is fixed in 2.24.0.

• https://github.com/ronf/asyncssh/commit/756cbae5350789ce9735f15f704bae9b5a3608b8
• https://github.com/ronf/asyncssh/commit/9c354270c009285525e126721e8ed5fbed1f8a67
• https://github.com/ronf/asyncssh/releases/tag/v2.24.0
• https://github.com/ronf/asyncssh/security/advisories/GHSA-rw4j-r22c-9gc3