Ssh – Disabling command substitution in shell

bashmercurialssh

I have an ssh box that runs Subversion and Mercurial services. On connection to this box, a script verifies that the user is only running mercurial or svn related commands:

    #!/bin/bash
    #
    # Verify that the requested command is an svnserve call.
    #
    # Note that sshd stores the command requested by the client in
    # the variable "SSH_ORIGINAL_COMMAND".

    echo $SSH_ORIGINAL_COMMAND | grep -E '^hg -R'
    ISHG=$?

    if [[ $SSH_ORIGINAL_COMMAND = "svnserve -t" || $ISHG -eq 0 ]]
    then
            exec $SSH_ORIGINAL_COMMAND
    else
            echo "You are only allowed svn access to this server."
    fi

The problem is, hg verification is not very clean or secure. If I include backticks in my remote ssh command, the "echo $SSH_ORIGINAL_COMMAND" line will happily execute it. Does anyone have any suggestions for cleaning this up a little bit?

Thanks!

Best Answer

Yeah. Don't use shell for that. Write a program in a language that would allow you to make sure that the only binaries it's going to execute are white-listed.

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