Over 55% of the world's websites run on Content Management Systems (CMS). Unfortunately, this huge user population has made CMS-based websites a high-profile target for hackers. Worse still, the vast majority of the website hosting industry has shifted to a "backup and restore" model of security, which relies on error-prone AV scanners to prompt users to roll back to a pre-infection nightly snapshot. This research had the opportunity to study these nightly backups for over 300,000 unique production websites. In doing so, we measured the attack landscape of CMS-based websites and assessed the effectiveness of the backup and restore protection scheme. To our surprise, we found that the evolution of tens of thousands of attacks exhibited clear long-lived multi-stage attack patterns. We now propose TARDIS, an automated provenance inference technique, which enables the investigation and remediation of CMS-targeting attacks based on only the nightly backups already being collected by website hosting companies. With the help of our industry collaborator, we applied TARDIS to the nightly backups of those 300K websites and found 20,591 attacks which lasted from 6 to 1,694 days, some of which were still yet to be detected.