David Wagner wrote:
>Peter W wrote:
>>To protect users from each others' ~/.forward instructions, it is necessary,
>>as Wietse said, for the delivery agent to start with superuser privileges.
>
>[...] Imagine: ~/.forward-program could be a
>setuid executable, owned by the user, and a non-root delivery
>agent could exec() the relevant ~/.forward-program. [...] What am I missing?

Many folks responsed privately (thanks, all!). Here's a summary.

 - It doesn't work if home directories are on a partition mounted nosetuid.
   (Fix: Put the .forward-program elsewhere.)
 - You have to be very careful when writing a setuid ~/.forward-program.
   (Agreed.)
 - Writeable home directories are very dangerous. (Agreed.)
 - A more problematic issue is that a malicious user can gain access to
   the uid that the delivery agent runs under by calling setuid(2)
   or by simply stripping the setuid bit off of their ~/.forward-program.
   (Uh oh...)

I must admit that I had not anticipated this last attack, which is
a very serious threat. It seems to me that it might be possible to
defend against this attack, if the appropriate mail delivery agent
runs under a 'nobody'-like uid with absolutely no privileges. However,
this may be tricky to configure safely, and the fact that I overlooked
one subtle attack leaves some reason to be concerned that there may be
other even more subtle attacks lurking in the shadows. I think we can
say that it might be possible to make my proposal work, but the details
appear to be non-trivial.

Thanks to Peter W., Greg Woods, Chris Siebenmann, Emmanuel Galanos,
Steve Bellovin, Henrik Nordstrom, and Roman Werpachowski for comments.

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