• Source: Restricted shell

The restricted shell is a Unix shell that restricts some of the capabilities available to an interactive user session, or to a shell script, running within it. It is intended to provide an additional layer of security, but is insufficient to allow execution of entirely untrusted software. A restricted mode operation is found in the original Bourne shell and its later counterpart Bash, and in the KornShell. In some cases a restricted shell is used in conjunction with a chroot jail, in a further attempt to limit access to the system as a whole.




Invocation


The restricted mode of the Bourne shell sh, and its POSIX workalikes, is used when the interpreter is invoked in one of the following ways:

sh -r note that this conflicts with the "read" option in some sh variants
rsh note that this may conflict with the remote shell command, which is also called rsh on some systems
The restricted mode of Bash is used when Bash is invoked in one of the following ways:

rbash
bash -r
bash --restricted
Similarly KornShell's restricted mode is produced by invoking it thus:

rksh
ksh -r




= Setting up rbash

=
For some systems (e.g., CentOS), the invocation through rbash is not enabled by default, and the user obtains a command not found error if invoked directly, or a login failure if the /etc/passwd file indicates /bin/rbash as the user's shell.
It suffices to create a link named rbash pointing directly to bash. Though this invokes Bash directly, without the -r or --restricted options, Bash does recognize that it was invoked through rbash and it does come up as a restricted shell.
This can be accomplished with the following simple commands (executed as root, either logged in as user root, or using sudo):




Limited operations


The following operations are not permitted in a restricted shell:

changing directory
specifying absolute pathnames or names containing a slash
setting the PATH or SHELL variable
redirection of output
Bash adds further restrictions, including:

limitations on function definitions
limitations on the use of slash-ed filenames in Bash builtins
Restrictions in the restricted KornShell are much the same as those in the restricted Bourne shell.




Weaknesses of a restricted shell


The restricted shell is not secure. A user can break out of the restricted environment by running a program that features a shell function. The following is an example of the shell function in vi being used to escape from the restricted shell:

Or by simply starting a new unrestricted shell, if it is in the PATH, as demonstrated here:




List of programs


Beyond the restricted modes of usual shells, specialized restricted shell programs include:

rssh – used with OpenSSH, permitting only certain file copying programs, namely scp, sftp, rsync, cvs, and rdist
smrsh, which limits the commands sendmail can invoke




See also


Remote Shell




References

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