In Mastodon media can only be used by owners and only be associated with
a single post. We currently allow media to be associated with several
posts and until now did not limit their usage in posts to media owners.
However, media update and GET lookup was already limited to owners.
(In accordance with allowing media reuse, we also still allow GET
lookups of media already used in a post unlike Mastodon)
Allowing reuse isn’t problematic per se, but allowing use by non-owners
can be problematic if media ids of private-scoped posts can be guessed
since creating a new post with this media id will reveal the uploaded
file content and alt text.
Given media ids are currently just part of a sequentieal series shared
with some other objects, guessing media ids is with some persistence
indeed feasible.
E.g. sampline some public media ids from a real-world
instance with 112 total and 61 monthly-active users:
17.465.096 at t0
17.472.673 at t1 = t0 + 4h
17.473.248 at t2 = t1 + 20min
This gives about 30 new ids per minute of which most won't be
local media but remote and local posts, poll answers etc.
Assuming the default ratelimit of 15 post actions per 10s, scraping all
media for the 4h interval takes about 84 minutes and scraping the 20min
range mere 6.3 minutes. (Until the preceding commit, post updates were
not rate limited at all, allowing even faster scraping.)
If an attacker can infer (e.g. via reply to a follower-only post not
accessbile to the attacker) some sensitive information was uploaded
during a specific time interval and has some pointers regarding the
nature of the information, identifying the specific upload out of all
scraped media for this timerange is not impossible.
Thus restrict media usage to owners.
Checking ownership just in ActivitDraft would already be sufficient,
since when a scheduled status actually gets posted it goes through
ActivityDraft again, but would erroneously return a success status
when scheduling an illegal post.
Independently discovered and fixed by mint in Pleroma
1afde067b1
In MastoAPI media descriptions are updated via the
media update API not upon post creation or post update.
This functionality was originally added about 6 years ago in
ba93396649 which was part of
https://git.pleroma.social/pleroma/pleroma/-/merge_requests/626 and
https://git.pleroma.social/pleroma/pleroma-fe/-/merge_requests/450.
They introduced image descriptions to the front- and backend,
but predate adoption of Mastodon API.
For a while adding an `descriptions` array on post creation might have
continued to work as an undocumented Pleroma extension to Masto API, but
at latest when OpenAPI specs were added for those endpoints four years
ago in 7803a85d2c, these codepaths ceased
to be used. The API specs don’t list a `descriptions` parameter and
any unknown parameters are stripped out.
The attachments_from_ids function is only called from
ScheduledActivity and ActivityDraft.create with the latter
only being called by CommonAPI.{post,update} whihc in turn
are only called from ScheduledActivity again, MastoAPI controller
and without any attachment or description parameter WelcomeMessage.
Therefore no codepath can contain a descriptions parameter.
Direct users to add in the appropriate headers and update the listening
port instead of copy/pasting a config that's already outdated and
probably would otherwise have to be synced with the main example nginx
config.
Since the configuration options on the nginx side already exist in the
sample config, there's no need to tell users to copy-paste those
settings in again.
The /var/tmp directory is not mounted as tmpfs unlike /tmp which is
mounted as such on some distros like Fedora or Arch. Since there isn't
really a benefit to having the cache on tmpfs, this change should allow
for a larger cache if needed without worrying about running out of RAM.
Applying works fine with a 20220220135625 version, but it won’t be
rolled back in the right order. Fortunately this action is idempotent
so we can just rename and reapply it with a new id.
To also not break large-scale rollbacks past 2022 for anyone
who already applied it with the old id, keep a stub migration.
This promotes and expands our existing optional migration.
Based on usage statistics from several instances, see:
AkkomaGang/akkoma#764
activities_hosts is now retained after all since it’s essential
for the "instance" query parameter of *oma’s public timeline to
reliably work in a reasonable amount of time. (Although akkoma-fe has
no support for this feature and apparently barely anyone uses it.)
activities_actor_index was already dropped before in
20221211234352_remove_unused_indices; no need to drop it again.
Birthday indices were introduced in pleroma starting with
20220116183110_add_birthday_to_users which is past the
last common migration 20210416051708.
The current 10 GiB cache size is too large to fit into tmpfs for VMs and
other machines with smaller RAM sizes. Most non-Debian distros mount
/tmp on tmpfs.
