{"id":"CVE-2026-62949","aliases":["GHSA-rw4j-r22c-9gc3"],"url":"https://o3.security/vulnerability/CVE-2026-62949","summary":"AsyncSSH: asyncio event-loop freeze via SSH maximum packet size = 0 in SSH_MSG_CHANNEL_OPEN / OPEN_CONFIRMATION","details":"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.","published":"2026-09-16T20:00:46.657Z","modified":"2026-09-18T03:48:28.206744247Z","cvss":{"score":6.5,"severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H"},"epss":{"score":0.00388,"percentile":0.32661,"asOf":"2026-09-17"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"PyPI","name":"asyncssh","fixedVersion":"2.24.0"}],"fix":{"url":"https://github.com/ronf/asyncssh/commit/756cbae5350789ce9735f15f704bae9b5a3608b8","label":"ronf/asyncssh@756cbae"},"references":[{"type":"WEB","url":"https://github.com/ronf/asyncssh/releases/tag/v2.24.0"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/62xxx/CVE-2026-62949.json"},{"type":"ADVISORY","url":"https://github.com/ronf/asyncssh/security/advisories/GHSA-rw4j-r22c-9gc3"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-62949"},{"type":"FIX","url":"https://github.com/ronf/asyncssh/commit/756cbae5350789ce9735f15f704bae9b5a3608b8"},{"type":"FIX","url":"https://github.com/ronf/asyncssh/commit/9c354270c009285525e126721e8ed5fbed1f8a67"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-09-18T03:48:28.206744247Z"}}