Comment on Pixelfed leaks private posts from other Fediverse instances

• PhilipTheBucket@ponder.cat ⁨1⁩ ⁨week⁩ ago

  They’re really not.

  Here’s the relevant docs, and a little summary of how they relate to private posts and Mastodon’s specific implementation of them.

  Specs: www.w3.org/TR/activitypub/#delivery

  Mastodon code where you can verify that this is how they are handled: github.com/mastodon/mastodon/…/tag_manager.rb (look at the cc() and to() functions)

  Overall summary in detail:

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  ActivityPub Specification

  What ActivityPub Explicitly Mandates

  1. Audience Targeting (Section 5.1):

     • Activities must include audience targeting fields (to, cc, etc.)
     • Servers must deliver activities to the specified audiences

  2. Public Posts (Section 7.1.2 and 7.1.3):

     • Activities addressed to the Public collection must be delivered to all followers
     • Servers MAY deliver Public activities to all known sharedInbox endpoints

  3. Filtering (Sections 5.2 and 7.1.2):

     • Servers SHOULD filter inbox content according to the requester’s permissions
     • Servers MAY filter delivery targets according to implementation-specific rules

  4. Privacy of Non-Addressed Content (Section 7.1 Note on “Silent and private activities”):

     • For activities with no recipients, it’s “recommended” (not required) that objects remain private
     • An activity sent only to the “public” collection should be viewable in the actor’s outbox but not delivered to any actors

  What ActivityPub Leaves Undefined

  1. Visibility Levels:

     • No explicit definitions for “public,” “unlisted,” “private,” or “direct” as formal visibility modes
     • No mandatory behavioral requirements for different audience targeting patterns

  2. Privacy Enforcement:

     • No explicit requirements for how receiving servers should restrict visibility based on audience fields
     • No requirements that servers must hide content from non-addressed users

  3. Timeline Placement:

     • No specification about which posts should appear in public timelines vs. home timelines

  Mastodon’s Implementation

  Visibility Levels in Mastodon

  Mastodon implements four primary visibility levels:

  1. Public

     • UI Name: "Public"
     • ActivityPub Implementation:
       • to: [“www.w3.org/ns/activitystreams#Public”]
       • cc: [“example.com/users/username/followers”]
     • Behavior: Visible to everyone, appears in public timelines

  2. Unlisted

     • UI Name: “Unlisted” or “Quiet public” (newer versions)
     • ActivityPub Implementation:
       • to: [“example.com/users/username/followers”]
       • cc: [“www.w3.org/ns/activitystreams#Public”]
     • Behavior: Visible to everyone but doesn’t appear in public timelines

  3. Private

     • UI Name: "Followers-only"
     • ActivityPub Implementation:
       • to: [“example.com/users/username/followers”]
       • cc: [] (empty)
     • Behavior: Only visible to followers

  4. Direct

     • UI Name: "Mentioned people only"
     • ActivityPub Implementation:
       • to: [array of mentioned user URLs]
       • cc: [] (empty)
     • Behavior: Only visible to mentioned users

  Key Implementation Details

  1. Visibility Through Addressing:

     • Mastodon uses the to and cc fields to indicate intended visibility
     • The presence and position of the Public collection determines visibility level
     • No explicit “visibility” property is included in ActivityPub messages

  2. Convention-Based Visibility:

     • The difference between “public” and “unlisted” is determined by whether the Public collection is in to (public) or cc (unlisted)
     • This is a convention defined by Mastodon, not explicitly required by the spec

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  I also sent a user-targeted explanation of how Mastodon’s privacy settings work, that might be helpful for you to read. You can probably find it in my profile.

  source