Bug 2535643 (CVE-2026-62949)

Summary: CVE-2026-62949 asyncssh: AsyncSSH: Denial of Service via zero-sized SSH packets
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: amctagga, aoconnor, bniver, flucifre, gmeno, groman, mbenjamin, mhackett, sostapov, vereddy
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in AsyncSSH, a Python package for SSHv2 protocol implementation. A malicious SSH server or an authenticated client can exploit this vulnerability by sending a specially crafted SSH packet with a zero-sized maximum packet size. This can cause the AsyncSSH event loop to freeze, leading to a Denial of Service (DoS) for all current and future connections handled by the process.
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Description OSIDB Bzimport 2026-09-16 20:16:01 UTC 
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.