EUVD-2026-81093Severity by source
AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Network-reachable, low complexity, availability-only whole-process freeze; PR:L reflects the authenticated-client server path (the malicious-server client path is PR:N/UI:R), scope unchanged, no C/I impact.
Primary rating from Vendor (GitHub_M).
CVSS VectorVendor: GitHub_M
Lifecycle Timeline
2DescriptionCVE.org
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.
AnalysisAI
A peer-supplied maximum packet size of zero is accepted during SSH channel-open negotiation in AsyncSSH versions prior to 2.24.0, and the resulting zero-byte send-window processing drives SSHChannel._flush_send_buf into a synchronous infinite loop with no await point, permanently freezing the asyncio event loop. Because the loop is synchronous, a single trigger halts every current and future connection handled by that process, producing a complete denial of service rather than a per-connection failure. …
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Vulnerability AssessmentAI
| Exploitation | Triggered when a peer supplies a maximum packet size of zero in an SSH channel-open exchange against AsyncSSH prior to 2.24.0. … Additional conditions and limiting factors are described in the full assessment. |
| Risk Assessment | This is a genuine but moderate-severity availability-only issue. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in. |
| Exploit Scenario | Full exploit scenario with step-by-step reproduction available after sign-in. |
| Remediation | Apply the vendor-released patch: upgrade AsyncSSH to 2.24.0 or later (release at https://github.com/ronf/asyncssh/releases/tag/v2.24.0, detailed in advisory GHSA-rw4j-r22c-9gc3; the enabling change is the re-enabled send_pktsize==0 validation from commits 756cbae5350789ce9735f15f704bae9b5a3608b8 and 9c354270c009285525e126721e8ed5fbed1f8a67). … Detailed patch versions, workarounds, and compensating controls in full report. |
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