postinst is in libpam-cracklib 1.1.8-1ubuntu2.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 | #!/bin/sh
set -e
if dpkg --compare-versions "$2" lt 1.0.1-6; then
pam-auth-update --package
fi
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