Cleaning up the users and locking down the shells in /etc/passwd

[Bear Giles]

Matt, why not create a hardening package? Just write a script that scrubs
/etc/passwd and /etc/group and then create a small package that runs it once
(in postinst). I would also install it in, e.g., /etc/cron.daily so it's
rerun periodically. I haven't kept track of existing hardening packages but
I wouldn't be surprised if somebody else has already done this.

P.S., if you want to have fun with package installation failing try mounting
/tmp as noexec. It's easy to fix - if you know how to do it. If not you'll
bang your head against the wall for hours while you're trying to figure out
why installations are failing.

bear


On Mon, Sep 26, 2011 at 4:05 PM, Matt Alexander <
ubuntu.com at mattalexander.com> wrote:

> On Sat, Sep 24, 2011 at 9:48 AM, Colin Watson <cjwatson at ubuntu.com> wrote:
>
>> On Thu, Sep 22, 2011 at 04:33:00PM -0700, Matt Alexander wrote:
>> > Would it be possible to remove the vast majority of users from
>> /etc/passwd
>> > and instead rely on the application being installed to create the
>> specific
>> > user if needed?  Most of the users appear to be historical remnants that
>> > have been carried over from release to release.
>>
>> For almost everything, and certainly for the overwhelming majority of
>> new entries, we do exactly as you say.  However, I (as base-passwd
>> maintainer) will not remove entries from the global static list unless
>> there is a very compelling reason to do so beyond cleaning up cruft;
>> packages are entitled to assume that they are present without declaring
>> any particular dependency and there's no reasonable way to know what
>> removing such entries would break.
>>
>
> I end up modifying the passwd/group files on my computers for auditing
> purposes and to ensure that the only accounts on the system are required
> accounts.  Removing cruft seems like a perfectly valid reason.  In 10 years
> will Ubuntu still have a uucp user and a news user and an irc user?  Seems
> silly.  Let's clean things up and keep it to just the accounts that must be
> there.  We can then easily fix packages that wrongly assumed that their
> particular user would be always be there.
>
>
>
>>
>> In any case, there are only 18 entries in the global static list
>> (/usr/share/base-passwd/passwd.master), and even without thinking about
>> it too hard I know that at least four or five are still in use and
>> probably more, so there's not that much to be gained.  All other system
>> entries in the passwd file are created dynamically by applications.
>>
>> Since I took over base-passwd in 2002, I have added no new global static
>> users and only two new global static groups, the last of which was in
>> 2004.
>>
>> > In addition, for users in the passwd file that must be there, could you
>> > please set their shell to /usr/sbin/nologin?
>>
>> Yes, I would like to do this eventually
>> (http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=274229).  However,
>> http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=184979 has to be fixed
>> first, otherwise everyone will have their upgrades interrupted by a
>> non-debconf prompt.  I haven't had time to work on #184979 in quite a
>> number of years, and to the best of my knowledge nobody has ever
>> contributed a patch for it; I'd be happy to review one if somebody did
>> so.
>>
>> The one wart here is that using /usr/sbin/nologin will break anything
>> that runs commands as one of those users using the 'su' command.  This
>> isn't theoretical; one of my packages used to do so some years ago,
>> although it now uses start-stop-daemon instead.  The breakage is
>> probably worthwhile, I'll admit, but I can't say that there would be no
>> problems with changing those users' shell since there's been such a long
>> time for packages to get used to it being /bin/sh.
>>
>> Cheers,
>>
>> --
>> Colin Watson                                       [cjwatson at ubuntu.com]
>>
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