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Re: Linux NIS security problem hole and fix
From: joerg.czeranski () informatik tu-clausthal de (Joerg Czeranski)
Date: Fri, 8 Sep 1995 00:57:04 +0200


Ken Weaverling <weave () hopi dtcc edu> wrote:

[...]

OK, here it goes... Ya know how you put +, -, and @ entries in /etc/passwd
to incorporate stuff from an NIS map?  Well, you can login with that
entry too.   + is a damn easy login to try, since most /etc/passwd files
using NIS use an entry like...

+:::::

... as the last line.

This is why just disabling NIS is not enough. If you forget to remove these
entries from /etc/passwd, you are screwed.

The fix is to put a * in the password field of the NIS entries. This prevents
login from the local /etc/passwd but doesn't lock the incorporated NIS
entries (a bit inconsistent, but oh well)  example:

+:*::::

But beware: on other implementations of NIS (notably SunOS, Solaris,
Ultrix and Dec Unix (OSF/1)) this entry has a different meaning:
it indeed means to include the NIS passwd map and replace the password field
with "*", i.e. lock all the passwords.

On those implementations the only correct entry is "+::::::" (or
"+::0:0:::", as the UID and login-GID field can't be overridden).
It is also often valid to drop the trailing colons and simply
use "+".

Anyway it seems to be rather non-trivial to add NIS to a libc, as
the correct behaviour seems to be documented only by "the way SunOS does
it is right".

CERT advised me of the above fix. They couldn't test the fix since they
don't have a LINUX machine anywhere. Pretty incredible that no one at
CERT runs a free Unix that can run on a 386 with 4 megs...

Not that much incredible if you take into account that Linux is a kernel,
not an OS, and that a very high percentage of security-relevant bugs
are discovered in libraries, tools and configuration files.

CERT would have to run at least all of the major distributions,
and each in a variety of configurations (with NIS added and without),
to be in a position to really support Linux.

It wouldn't hurt if they ran the current Slackware (or whatever is the
most often used distribution) in some standard configuration though.

joerch

--
Joerg Czeranski                 EMail czeranski () informatik tu-clausthal de
Osteroeder Strasse 55                 czeranski () rz tu-clausthal de
D 38678 Clausthal-Zellerfeld    WWW   http://www.in.tu-clausthal.de/~injc/



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