Vulnerability in Joinmastodon Mastodon
CVE-2026-22246
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.
EPSS: 0.002 (14.6th percentile) — read the EPSS interpretation.
CVSS v3 metric
CVSS v3 base score 6.5 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N.
Affected products
- Joinmastodon Mastodon
- Mastodon — versions < 4.3.17, >= 4.4.0-beta.1, < 4.4.11, >= 4.5.0-beta.1, < 4.5.4
Weakness classification (CWE)
References
- security-advisories@github.com (x_refsource_CONFIRM, Third Party Advisory)
- security-advisories@github.com (Patch, x_refsource_MISC)
- security-advisories@github.com (Patch, x_refsource_MISC)
- security-advisories@github.com (Patch, x_refsource_MISC)
Frequently asked questions
- What is CVE-2026-22246?
- CVE-2026-22246 is a medium-severity vulnerability in Joinmastodon Mastodon, classified under Insertion of Sensitive Information into Sent Data. CVSS score: 6.5/10. Published 2026-01-08.
- How severe is CVE-2026-22246?
- Medium severity. CVSS v3 base score is 6.5 out of 10.