The remote host is missing an update to tmux announced via advisory DSA 2212-1.

https://secure1.securityspace.com/smysecure/catid.html?in=DSA%202212-1

Daniel Danner discovered that tmux, a terminal multiplexer, is not properly dropping group privileges. Due to a patch introduced by Debian, when invoked with the -S option, tmux is not dropping permissions obtained through its setgid installation. The oldstable distribution (lenny) is not affected by this problem, it does not include tmux. For the stable distribution (squeeze), this problem has been fixed in version 1.3-2+squeeze1. For the testing distribution (wheezy), this problem has been fixed in version 1.4-6. For the testing distribution (sid), this problem has been fixed in version 1.4-6. We recommend that you upgrade your tmux packages.