Date: Sun, 22 Feb 2004 21:21:10 -0800 From: David Schultz <das@FreeBSD.ORG> To: kientzle@acm.org Cc: Colin Percival <cperciva@FreeBSD.ORG> Subject: Re: cvs commit: src/sbin/nologin Makefile nologin.c Message-ID: <20040223052110.GA58255@VARK.homeunix.com> In-Reply-To: <40397824.3080607@kientzle.com> References: <200402221003.i1MA3PW0024791@repoman.freebsd.org> <403944D8.6050107@kientzle.com> <20040223025647.GA43467@VARK.homeunix.com> <40397824.3080607@kientzle.com>
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On Sun, Feb 22, 2004, Tim Kientzle wrote: > David Schultz wrote: > >On Sun, Feb 22, 2004, Tim Kientzle wrote: > >>Colin Percival wrote: > >> > >>>Report login attempts to syslog. Due to the statically-linked nature of > >>>nologin(8) ... > >> > >>Why is nologin statically linked? > > > >Because of environment-poisoning attacks such as the following: > > > > das@VARK:~> setenv LD_LIBRARY_PATH /home/das/exploit > > das@VARK:~> \login -p test > > > >This attack was executed with a dynamically-linked /sbin/nologin > >and a special libc.so.5 in the /home/das/exploit directory that > >replaces the _exit() stub with a routine that spawns a shell. > > Hmmmm.... Several other solutions come immediately to mind: > * Handle this in pam (or even in login) > (Just check if the user's shell is /sbin/nologin and > reject the login if it is.) > * Install /sbin/nologin setuid nobody or setgid nogroup > That would disable LD_LIBRARY_PATH processing for it. > * Have login -p not pass LD_LIBRARY_PATH > > Of these, the first is arguably the best, the second easiest > to implement. The third I'm unsure about; I can't > really picture a scenario where login -p should pass > LD_LIBRARY_PATH, but that's hardly conclusive. > > I agree, by the way, that there should be a way to > "mark" a program as ignoring LD_LIBRARY_PATH at > compile-time (other than making it setuid/setgid). Your first and third proposals would add special cases, and it's hard to guarantee that you've covered all the cases. LD_PRELOAD comes to mind. The second proposal is a hack, but it seems to be a much better hack than the static linking one we have right now. Please go ahead and implement it if you're so inclined, or I can take a look at it in a few days. It should be a two-line change. One unfortunate side-effect of dynamically linking root (and /bin/sh in particular) that is still unsolved is that custom versions of nologin that people have written as shell scripts are now insecure. Effectively, 'sh -p' is no longer useful. On one of the systems I use, for instance, there's a shell script that prints an informative message for users whose accounts were closed due to abuse or expiration. Thus, it would be useful to ultimately have a more comprehensive solution.
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