rssh is a shell for restricting SSH access to a machine to only scp,
sftp, or a small set of similar applications.  

  http://www.pizzashack.org/rssh/

Henrik Erkkonen has discovered that, through clever manipulation of
environment variables on the ssh command line, it is possible to
circumvent rssh.  As far as I can tell, there is no way to effect a
root compromise, except of course if the root account is the one
you're attempting to protect with rssh...

This project is old, and I have no interest in continuing to maintain
it.  I looked for easy solutions to the problem, but in discussing
them with Henrik, none which we found satisfactorily address the
problem.  Fixing this properly will require more work than I want to
put into it.

Note in particular that ensuring that the AcceptEnv sshd configuration
option need not be turned on for this exploit to work.

-- 
Derek D. Martin
http://www.pizzashack.org/
GPG Key ID: 0x81CFE75D