/etc/passwd grungies

[Doug Gwyn]

> BEWARE: In /etc/passwd blank or otherwise badly formatted lines can
> cause *extremely* anomalous behaviour.

This is an understatement.  Any time a line of /etc/passwd
is edited so that it contains the wrong number of fields,
subsequent updating (e.g., by the "passwd" command) can
produce one or more lines in /etc/passwd of the form
	::0:0:::
which has the interesting consequence that one can "log in"
using a null username, not have to give a password, and end
up as superuser.

This problem was fixed in the /etc/passwd-reading library
routines in UNIX System V, but not in 4.2BSD.

I have seen this problem occur several times.