{"id":"CVE-2026-62949","state":"PUBLISHED","title":"AsyncSSH: asyncio event-loop freeze via SSH maximum packet size = 0 in SSH_MSG_CHANNEL_OPEN / OPEN_CONFIRMATION","description":"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.","publishedAt":"2026-09-16T20:00:46.657Z","updatedAt":"2026-09-17T15:01:48.863Z","severity":{"score":6.5,"label":"Medium","version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H","source":"GitHub_M"},"assigner":"GitHub_M","products":["ronf asyncssh"],"affected":[{"vendor":"ronf","product":"asyncssh","versions":[{"version":"< 2.24.0","status":"affected"}]}],"references":["https://github.com/ronf/asyncssh/security/advisories/GHSA-rw4j-r22c-9gc3","https://github.com/ronf/asyncssh/commit/756cbae5350789ce9735f15f704bae9b5a3608b8","https://github.com/ronf/asyncssh/commit/9c354270c009285525e126721e8ed5fbed1f8a67","https://github.com/ronf/asyncssh/releases/tag/v2.24.0"],"weaknesses":["CWE-835"],"ingestedAt":"2026-10-29T20:12:32.778Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2026/62xxx/CVE-2026-62949.json","commit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-62949","state":"PUBLISHED","datePublished":"2026-09-16T20:00:46.657Z","dateUpdated":"2026-09-17T15:01:48.863Z","assignerShortName":"GitHub_M","assignerOrgId":"a0819718-46f1-4df5-94e2-005712e83aaa","dateReserved":"2026-07-14T22:32:17.731Z"},"containers":{"cna":{"title":"AsyncSSH: asyncio event-loop freeze via SSH maximum packet size = 0 in SSH_MSG_CHANNEL_OPEN / OPEN_CONFIRMATION","descriptions":[{"lang":"en","value":"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."}],"affected":[{"vendor":"ronf","product":"asyncssh","versions":[{"version":"< 2.24.0","status":"affected"}]}],"references":[{"url":"https://github.com/ronf/asyncssh/security/advisories/GHSA-rw4j-r22c-9gc3","name":"https://github.com/ronf/asyncssh/security/advisories/GHSA-rw4j-r22c-9gc3","tags":["x_refsource_CONFIRM"]},{"url":"https://github.com/ronf/asyncssh/commit/756cbae5350789ce9735f15f704bae9b5a3608b8","name":"https://github.com/ronf/asyncssh/commit/756cbae5350789ce9735f15f704bae9b5a3608b8","tags":["x_refsource_MISC"]},{"url":"https://github.com/ronf/asyncssh/commit/9c354270c009285525e126721e8ed5fbed1f8a67","name":"https://github.com/ronf/asyncssh/commit/9c354270c009285525e126721e8ed5fbed1f8a67","tags":["x_refsource_MISC"]},{"url":"https://github.com/ronf/asyncssh/releases/tag/v2.24.0","name":"https://github.com/ronf/asyncssh/releases/tag/v2.24.0","tags":["x_refsource_MISC"]}],"metrics":[{"cvssV3_1":{"attackComplexity":"LOW","attackVector":"NETWORK","availabilityImpact":"HIGH","baseScore":6.5,"baseSeverity":"MEDIUM","confidentialityImpact":"NONE","integrityImpact":"NONE","privilegesRequired":"LOW","scope":"UNCHANGED","userInteraction":"NONE","vectorString":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H","version":"3.1"}}],"problemTypes":[{"descriptions":[{"cweId":"CWE-835","lang":"en","description":"CWE-835: Loop with Unreachable Exit Condition ('Infinite Loop')","type":"CWE"}]}],"providerMetadata":{"shortName":"GitHub_M","orgId":"a0819718-46f1-4df5-94e2-005712e83aaa","dateUpdated":"2026-09-16T20:00:46.657Z"},"source":{"advisory":"GHSA-rw4j-r22c-9gc3","discovery":"UNKNOWN"}},"adp":[{"title":"CISA ADP Vulnrichment","metrics":[{"other":{"type":"ssvc","content":{"timestamp":"2026-09-17T15:01:41.382412Z","id":"CVE-2026-62949","options":[{"Exploitation":"none"},{"Automatable":"no"},{"Technical Impact":"partial"}],"role":"CISA Coordinator","version":"2.0.3"}}}],"providerMetadata":{"shortName":"CISA-ADP","orgId":"134c704f-9b21-4f2e-91b3-4a467353bcc0","dateUpdated":"2026-09-17T15:01:48.863Z"}}]}}}