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

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

6.5 · CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

EPSS

• CVE-2026-62949: 0.39% estimated exploitation activity in the next 30 days; percentile 33.05 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary A malicious SSH server can wedge an AsyncSSH client, and an authenticated client can wedge an AsyncSSH server, by sending a channel maximum packet size of 0 in SSHMSGCHANNELOPENCONFIRMATION (server→client) or SSHMSGCHANNELOPEN (client→server). AsyncSSH stores the peer-supplied value verbatim with no lower-bound check; the first time channel data is written, SSHChannel.flushsendbuf enters a synchronous infinite loop that cannot be interrupted by asyncio.waitfor or any timeout. The loop body has no await, so it blocks the entire asyncio event loop — for a server, one malicious authenticated channel freezes all current and future connections. RFC 4254 §5.1 leaves receiver behavior for a peer-reported "maximum packet size = 0" undefined, so the value must be rejected rather than stored. Root cause asyncssh/channel.py: With sendpktsize == 0, pktsize is 0, so buf[:0] is empty, del buf[:0] is a no-op, and sendwindow is never decremented — the while condition is permanently true, and…

Published by the source

17 Sep 2026, 14:53 UTC

Last modified by the source

1 Oct 2026, 17:55 UTC

Source last checked

26 Oct 2026, 13:52 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-rw4j-r22c-9gc3

2. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GHSA-rw4j-r22c-9gc3

3. Revision 1 · created · 23 Oct 2026

   Published from ghsa record GHSA-rw4j-r22c-9gc3

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# Patch Next advisory brief: CVE-2026-62949

Advisory: AsyncSSH: asyncio event-loop freeze via SSH maximum packet size = 0 in SSH_MSG_CHANNEL_OPEN / OPEN_CONFIRMATION
IDs: CVE-2026-62949, GHSA-rw4j-r22c-9gc3
Record: https://patchnext.com/advisories/cve-2026-62949 (revision 3, source checked 26 Oct 2026, 13:52 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: asyncssh 2.24.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI asyncssh: affected <= 2.23.1; fixed in 2.24.0

## Suggested inspection (read-only)
- `python -m pip show asyncssh`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on asyncssh.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-rw4j-r22c-9gc3 (GitHub-reviewed advisory GHSA-rw4j-r22c-9gc3 lists affected ranges: PyPI asyncssh >=0 <=2.23.1)
- Verified: https://osv.dev/vulnerability/GHSA-rw4j-r22c-9gc3 (OSV record GHSA-rw4j-r22c-9gc3 lists affected ranges: PyPI asyncssh >=0 <2.24.0 (fixed))

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