Vulnerability · Published Sep 16, 2026 · Assigned by GitHub_M

CVE-2026-62949AsyncSSH: asyncio event-loop freeze via SSH maximum packet size = 0 in SSH_MSG_CHANNEL_OPEN / OPEN_CONFIRMATION

Medium 6.5

In plain English

A medium-severity infinite loop in asyncssh from ronf. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.4% (higher than 33% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)6.5Medium

Exploit odds (30 days)0.4%top 66.8% of all CVEs

Exploited in the wildNo reportsCISA SSVC: none

Attack toolingNoneno public tooling known

Official 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.

Affected packages & fixed versions

Ecosystem	Package	Fixed in	Advisory
Debian:12	python-asyncssh	no fix listed	DEBIAN-CVE-2026-62949
Debian:13	python-asyncssh	no fix listed	DEBIAN-CVE-2026-62949
Debian:14	python-asyncssh	no fix listed	DEBIAN-CVE-2026-62949
PyPI	asyncssh	2.24.0	PYSEC-2026-4026
PyPI	asyncssh	2.24.0	GHSA-rw4j-r22c-9gc3
Ubuntu:26.04:LTS	python-asyncssh	no fix listed	UBUNTU-CVE-2026-62949
Ubuntu:26.10	python-asyncssh	no fix listed	UBUNTU-CVE-2026-62949

Affected products (as reported)

Vendor	Product	Versions
ronf	asyncssh	< 2.24.0

Weakness: CWE-835 · Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

References

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