Facebook suffered from an information disclosure vulnerability. If a user uploaded their contacts to Facebook and then proceeded to download their expanded dataset from the DYI (Download Your Information) section, they would receive a file called addressbook.html in their downloaded archive. The addressbook.html is supposed to house the contact information they uploaded. However, due to a flaw in how Facebook implemented this, it also housed contact information from other uploads other users have performed for the same person, provided they had one piece of matching data. This effectively built large dossiers on users and disclosed their information to anyone that knew at least one piece of matching data.
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