# 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.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 16 Sep 2026
- **Updated:** 17 Sep 2026
- **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, MINI-pf68-wpvf-px7f, MINI-wgrv-j93h-q2xg, PYSEC-2026-4026, UBUNTU-CVE-2026-62949, openSUSE-SU-2026:21953-1

## 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 (Loop with Unreachable Exit Condition)

## Which versions are affected by CVE-2026-62949?

3 open source packages ship an affected version, including asyncssh.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `asyncssh` | PyPI | All versions | 2.24.0 |
| `asyncssh` | PyPI | All versions | 2.24.0 |
| `asyncssh` | PyPI | All versions | 2.24.0 |
| `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.

## Is CVE-2026-62949 being exploited?

There is no known exploitation of CVE-2026-62949 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-62949 and fix it if it is affected.

What it is: 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.
Exploitation: There is no known exploitation of CVE-2026-62949 in the wild and no public exploit code.

Affected packages and fixed versions:
- asyncssh (PyPI): affected all versions; fixed in 2.24.0
- asyncssh (PyPI): affected all versions; fixed in 2.24.0
- asyncssh (PyPI): affected all versions; fixed in 2.24.0

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-62949/ (Markdown: https://insights.sbomify.com/CVE-2026-62949/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-62949
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-62949
- OSV: https://osv.dev/vulnerability/CVE-2026-62949

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-62949/
Summaries are generated and may contain mistakes; the official description above is the record.