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^[1] and its later counterpart bash,^[2] and in the Korn shell.^[3] 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.

Contents 1 Invocation 2 Limited operations 3 Weaknesses of a restricted shell 4 References

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 the Korn shell's restricted mode is produced by invoking it thus:

• rksh
• ksh -r

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:^[2]

• limitations on function definitions
• limitations on the use of slash-ed filenames in bash builtins

Restrictions in the restricted Korn shell are much the same as those in the restricted Bourne shell.^[4]

Weaknesses of a restricted shell

The restricted shell is not completely 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:

vi
:set shell=/bin/sh
:shell

References