A flaw was found in rssh. If rsync is built with popt (as it is in Debian), the popt library will attempt to open and parse ~/.popt on the server and interpret it as configuration for command line option parsing. If the client can rsync a .popt file to the home directory of the user protected with rssh, this allows arbitrary code execution on the server via several paths. The most obvious is via the popt exec feature which execs an arbitrary program as an external option filter, but a more sneaky approach is through the popt alias feature which allows one to alias a benign option (such as --server) to one that will run arbitrary code (such as --rsh).
Created rssh tracking bugs for this issue:
Affects: epel-all [bug 1672384]
Affects: fedora-all [bug 1672383]