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

Vulnerability summary

CVE-2026-62949 is a medium (CVSS 6.5) Loop with Unreachable Exit Condition vulnerability in ronf asyncssh. EPSS estimates a 0.39% chance of exploitation in the next 30 days, higher than 33% of scored vulnerabilities.

Timeline

1. 16-09-2026
   PublishedVulnerability first disclosed.
2. 17-09-2026
   Last modifiedVulnerability information updated.

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.

• CWE-835Loop with Unreachable Exit ConditionThe product contains an iteration or loop with an exit condition that cannot be reached, i.e., an infinite loop.