An issue dealing with password changes, with respect to the MINDAYS field in /etc/shadow was reported on the Debian BTS (http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=514437) that affects all versions of PAM 1.x. Because of this, if an administrative user sets the password minimum days via chage or passwd, /etc/shadow is updated correctly, but PAM allows the user to change their password with no regard for the MINDAYS setting, effectively allowing them to re-use old passwords immediately and disregard any established password policies that should be enforced.

This is due to the fact that no minimum age password checks are done by PAM in 1.x; in the old versions it was done in unixverifyshadow() by checking the value of of spmin. In newer PAM this check is no longer there.