An anonymous reader quotes a report from Ars Technica:
A security update will be applied to Drive," Google’s weird new email reads. If you visit drive.google.com, you’ll also see a message saying, “On September 13, 2021, a security update will be applied to some of your files.” You can even see a list of the affected files, which have all gotten an unspecified “security update.” So what is this all about? Google is changing the way content sharing works on Drive. Drive files have two sharing options: a single-person allow list (where you share a Google Doc with specific Google accounts) and a “get link” option (where anyone with the link can access the file). The “get link” option works the same way as unlisted YouTube videos – it’s not really private but, theoretically, not quite public, either, since the link needs to be publicized somewhere. The secret sharing links are really just security through obscurity, and it turns out the links are actually guessable.
Google knew about the problem of guessable secret links for a while and changed the way link generation works back in 2017 (presumably for Drive, too?). Of course, that doesn’t affect links you’ve shared in the past, and soon Google is going to require your old links to change, which can break them. Google’s new link scheme adds a “resourcekey” to the end of any shared Drive links, making them harder to guess. So a link that used to look like “https://drive.google.com/file/d/0BxI1YpjkbX0OZ0prTHYyQ1U2djQ/” will now look like "https://drive.google.com/file/d/0BxI1YpjkbX0OZ0prTHYyQ1U2djQ/view?resourcekey=0-OsOHHiQFk1QEw6vIyh8v_w." The resource key makes it harder to guess. If you head to drive.google.com/drive/update-drives in a browser, you should be able to see a list of your impacted files, and if you mouse over them you’ll see a button on the right to remove or apply the security update. “Applied” means the resourcekey will be required after September 13, 2021, and will (mostly) break the old link, while “removed” means the resourcekey isn’t required and any links out there should keep working. YouTube is also making similar changes. “In 2017, we rolled out an update to the system that generates new YouTube Unlisted links, which included security enhancements that make the links for your Unlisted videos even harder for someone to discover if you haven’t shared the link with them,” says YouTube in a support page.
YouTube creators can decide to opt out of this change. They also have the option of making Unlisted pre-2017 videos public or re-uploading as a new Unlisted video at the expense of stats.