Description
Mastodon is a free, open-source social network server based on ActivityPub. Mastodon 4.3 added notifications of severed relationships, allowing end-users to inspect the relationships they lost as the result of a moderation action. The code allowing users to download lists of severed relationships for a particular event fails to check the owner of the list before returning the lost relationships. Any registered local user can access the list of lost followers and followed users caused by any severance event, and go through all severance events this way. The leaked information does not include the name of the account which has lost follows and followers. This has been fixed in Mastodon v4.3.17, v4.4.11 and v4.5.4.
Scores
CVSS v3
6.5
EPSS
0.0004
EPSS Percentile
13.7%
Attack Vector
NETWORK
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N
CISA SSVC
Vulnrichment
Exploitation
none
Automatable
no
Technical Impact
partial
Details
CWE
CWE-201
Status
published
Products (1)
joinmastodon/mastodon
< 4.3.17
Published
Jan 08, 2026
Tracked Since
Feb 18, 2026