What we have here is a failure of security

Bruce Schneier contributes to the discussion on Hogwart's security.

This seems like an enormous hole in magical defenses

From Karl Lembke:

The moral of the story is, inadequate security can be worse than no security at all.

Veryard discusses networks of trust in the Potter series:

James Potter can nominate one friend to guard the secret of his whereabouts, and chooses his strongest and apparently most trustworthy friend – Sirius Black. But for Sirius, being the obvious choice makes him immediately vulnerable to attack: in systems engineering terms, he is a single point of failure. He decides that the Potters’ secret would be better guarded by a less obvious person, and delegates the responsibility to a weaker wizard – Peter Pettigrew – who immediately betrays the Potters.