CVE-2026-62949 AsyncSSH: asyncio event-loop freeze via SSH maximum packet size = 0 in SSH_MSG_CHANNEL_OPEN / OPEN_CONFIRMATION
A malicious SSH server or authenticated client can freeze the AsyncSSH event loop by sending a zero packet size, causing a denial of service.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-62949?
CVE-2026-62949 is a medium-severity denial of service vulnerability in ronf asyncssh, published on 16 Sep 2026. Any signed-in user can exploit it over the network, with no action from the victim.
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.
Weakness: CWE-835
Which versions are affected by CVE-2026-62949?
3 open source packages ship an affected version, including asyncssh.
| Package | Affected | Fixed in |
|---|---|---|
asyncssh PyPI | All versions | 2.24.0 |
asyncssh PyPI | All versions | 2.24.0 |
asyncssh PyPI | All versions | 2.24.0 |
Linux distribution packages (1)
python-asyncssh openSUSE:Leap 16.0 | All versions | 2.21.0-bp160.2.1 |
How do I fix CVE-2026-62949?
Upgrade asyncssh to 2.24.0 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-62949 being exploited?
There is no known exploitation of CVE-2026-62949 in the wild and no public exploit code.
Sources
Also known as CLEANSTART-2026-KI90686, CLEANSTART-2026-LF94693, DEBIAN-CVE-2026-62949, ECHO-6459-65b4-6f8c, GHSA-rw4j-r22c-9gc3, MINI-m56g-9332-2x7g and 5 more.