rssh is a restricted shell, allowing only a few commands like scp or sftp. It is often used as a complement to OpenSSH to provide limited access to users.
Florian Schilhabel from the Gentoo Linux Security Audit Team found a format string vulnerability in rssh syslogging of failed commands.
Using a malicious command, it may be possible for a remote authenticated user to execute arbitrary code on the target machine with user rights, effectively bypassing any restriction of rssh.
There is no known workaround at this time.
All rssh users should upgrade to the latest version:
# emerge --sync
# emerge --ask --oneshot --verbose ">=app-shells/rssh-2.2.2"