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RE: Restricted Bash - Not so restrictive (in 4.2 as well)


From: Sarnath K - ERS, HCLTech
Subject: RE: Restricted Bash - Not so restrictive (in 4.2 as well)
Date: Thu, 12 Jan 2012 15:56:19 +0530

Hello Jonathan,

Thanks for your inputs. I was able to created a super-restricted login.
Here are a few things that I learnt during this process:

1. "vim" has a restricted mode called "rvim (or) vim -Z". This way, I can 
restrict the user from running shell commands from vim and peep into the 
Filesystem
    a) CAVEAT: "vim" allows the user to "read" and "write" files in the 
file-system provided the user _knows_ the path (or guesses some file path)
    b) So, to make it foolproof, I had to go with "nano" editor
        - which supports a restricted mode that does not allow the user to edit 
any other file than the one specified in the command line
    c) As per your advice, I created pre-processing shell scripts which screens 
the user-commands and disallows anything that contains a "/" in the arguments.
         (actually, they do not allow more than 1 argument)
    d) Similarly, I got pre-processing scripts for compiling and running the 
test application.

I still have to block "FTP/SFTP" access. Otherwise, it will still not be 
foolproof - I think.
I will look around,

Thanks for all your help!

Best Regards,
Sarnath

-----Original Message-----
From: Jonathan Nieder [mailto:address@hidden
Sent: Wednesday, January 11, 2012 5:13 PM
To: Sarnath K - ERS, HCLTech
Cc: address@hidden; address@hidden
Subject: Re: Restricted Bash - Not so restrictive (in 4.2 as well)

Sarnath K - ERS, HCLTech wrote:

> Apart from this, BASH Documentation says "When a command that is
> found to be a shell script is executed (see COMMAND EXECUTION
> above), rbash turns off  any  restrictions in the shell spawned to
> execute the script.".
>
> Does this mean that a guy with restricted shell can just write a
> shell script to do a privileged operation and get away with it?

"./my-custom-script" contains a "/".

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