CVE-2026-22246: Local Mastodon users can enumerate and access severed relationships of every other local user

Severity: medium (CVSS 4.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-01-08 · updated 2026-06-17
Weakness: CWE-201

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.

Affected products

mastodon: before 4.3.17; from 4.4.0 before 4.4.11; from 4.5.0 before 4.5.4 — fixed in 4.3.17, 4.4.11, 4.5.4

How to fix

Upgrade to a fixed version: mastodon 4.3.17, 4.4.11, 4.5.4. Check the vendor advisory for mitigations.

References

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