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Re: Mail delivery privileges
From: Peter W <peterw () usa net>
Date: Fri, 18 May 2001 21:04:37 -0400

On Fri, May 18, 2001 at 04:35:08PM -0400, Greg A. Woods wrote:

[ On Friday, May 18, 2001 at 11:18:51 (-0400), Wietse Venema wrote: ]

3 - User-specified shell commands. Traditionally, a user can specify
any shell command in ~user/.forward, and that command will execute
with the privileges of that user.

Personally I'm loathe to allow ordinary users to specify delivery to
programs in the first place, and forcing them at minimum to arrange for
their mail filters to run unprivileged seems like a very small price

That's certainly the way it works on Plan 9:

       If  the file /mail/box/username/pipeto exists and is read-
       able and executable by everyone, it will be run  for  each
       incoming  message for the user.  The message will be piped
       to it rather than appended to his/her mail box.  The  file
       is run as user `none'.

So users with "pipeto" scripts are vulnerable to other users' "pipeto"
scripts, since they all run as the same user. "Mutual Assured Corruption" 
you might say. I think that sounds like a *large* price to pay!

Note that there are solutions to the filtering issue which do not
require the final destination of filtered messages to be an inbox that's
writable by the unprivileged user (eg. just pass them back to the mail
system for re-delivery to a new mailbox).

Your earlier post assumed that users didn't want to use ~/.forward to
specify custom actions. Now you're assuming all the user wants to do
is "filter" the mail, i.e., decide which mailbox to put it in. But
users want to do more with their mail than simply "filter" it.

To protect users from each others' ~/.forward instructions, it is necessary,
as Wietse said, for the delivery agent to start with superuser privileges.
There are ways to make things a little bit safer, e.g. have the delivery
agent drop privileges to nobody:bobpipe (where only bob is a member of 
bobpipe) instead of bob:users when running the ~/.forward command, but that
only protects bob from his own mistakes in ~/.forward and still leaves
the delivery agent starting out with superuser privs...

-Peter


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