Dear #VulpineClub :

This is an important message about a critical information disclosure bug in Mastodon.

tl;dr: Followers-only posts may be disclosed to unintended parties, and there is no fix yet.

Long version:
The "Relationships" page (vulpine.club/relationships) allows members to remove followers, and it does this by sending a Reject Follow message to the remote instance. The remote instance is supposed to remove the follower when it receives this message.

Since October 2019, Mastodon instances have not properly handled this Reject Follow message. The upshot of this is that if you used /relationships to remove a follower on a Mastodon instance, the unfollow didn't actually happen. This means that followers-only posts may be disclosed to unintended parties.

Furthermore, this fix has to be applied to every Mastodon instance on the Fediverse before this problem stops getting worse. Also, there is currently no known way to fix this problem, because no record is kept of Reject Follow messages.

The investigation into this is still in its early stages. We will be keeping an eye on the issue (github.com/tootsuite/mastodon/) to determine the best way to clean this up.

#VulpineClubBulletin

@rey But doesn't that also means that any mischeivous instance could just ignore those reject follow messages and keep following you anyway?

@penguin42 yes, absolutely

ultimately, i think the fix for this will be to directly address every recipient of a followers-only post (so that if you have 500 followers across 50 instances, it generates 500 direct posts, instead of just fanning out to 50 posts), but this is above my pay grade

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@rey I guess this fixes it where you trust the case where you trust admins but not individual users

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@penguin42 there is implicit trust of the admins, since there are much easier ways to see private messages when you have root :)

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