CVE-2016-0777 doesn't interact with ProxyCommand in any special way. That said, after connecting to hostB with your example config above you will have 2 ssh ses

CVE-2016-0777 doesn't interact with ProxyCommand in any special way. That said, after connecting to hostB with your example config above you will have 2 ssh sessions: 1) from your client to hostA 2) from your client to hostB So to answer your question - no, vulnerable client on hostA is not a problem (or at least not in this particular use-case).