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Suppose use can ssh to the server and can execure FTP and connect to other sources, download a virus and execute it.

Is it possible to force user to execute only some known commands?

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It would be really helpful if you included more details about what users normally will be using this server for. What are some of the typical activities you wish to permit. –  Zoredache Apr 16 '10 at 21:18
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3 Answers

up vote 2 down vote accepted

from man sshd_config:

ForceCommand

Forces the execution of the command specified by ForceCommand ignoring any command supplied by the client and ~/.ssh/rc if present. The command is invoked by using the user's login shell with the -c option. This applies to shell, command, or subsystem execution. It is most useful inside a Match block. The command originally supplied by the client is available in the SSH_ORIGINAL_COMMAND environment variable. Specifying a command of “internal-sftp” will force the use of an in-process sftp server that requires no support files when used with ChrootDirectory

this lets you use a shell wrapper that lets only do specific things. one example is rssh.

If you only want this restriction for specific users, use the command=cmd option in the known_hosts file (documented in man sshd)

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If you want to restrict ForceCommand to specific users,groups or hosts, you can also use the Match directive in sshd_config. –  Marie Fischer Apr 16 '10 at 22:02
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Maybe setting up a chroot environment for your users could help. See http://serverfault.com/questions/39997/how-can-i-chroot-ssh-connections

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One method, while not perfect, would be to create a separate partition for the users home directories any locations which they have write access. The simply mount those partitions noexec.

Proper setup of file-system permissions will generally be very effective to limit the damage that can be done.

If the users are not at least somewhat trusted, then perhaps it is a bad idea to give them SSH access at all. Perhaps you need to setup VMs for them and confine each to their own environment.

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Perhaps you mean noexec? –  Novelocrat Apr 16 '10 at 22:52
    
Also, this general suggestion can giver a false sense of security. You'd need to lock down /tmp as well as any other globally writable directories, such as /var/tmp. This causes trouble for some package managers, which need someplace to unpack scripts that need to be run for proper configuration. –  Novelocrat Apr 16 '10 at 22:54
    
Yes, noexec. The typo has been corrected. –  Zoredache Apr 16 '10 at 23:17
    
RE: Sense of security the restricted shell alternative also has problems. People aren't careful and they permit applications which are able to launch sub-shells. If you are really paranoid, you really need to combine all of the above. A restricted shell and strong well-audited permissions setup. –  Zoredache Apr 16 '10 at 23:18
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