CVE-2026-23961

low-risk

Published 2026-01-22

Mastodon is a free, open-source social network server based on ActivityPub. Mastodon allows server administrators to suspend remote users to prevent interactions. However, some logic errors allow already-known posts from such suspended users to appear in timelines if boosted. Furthermore, under certain circumstances, previously-unknown posts from suspended users can be processed. This issue allows old posts from suspended users to occasionally end up on timelines on all Mastodon versions. Additionally, on Mastodon versions from v4.5.0 to v4.5.4, v4.4.5 to v4.4.11, v4.3.13 to v4.3.17, and v4.2.26 to v4.2.29, remote suspended users can partially bypass the suspension to get new posts in. Mastodon versions v4.5.5, v4.4.12, v4.3.18 are patched.

Do I need to act?

0.02% chance of exploitation

EPSS score — low exploit probability

Not on CISA KEV list

No confirmed active exploitation reported to CISA

Patch status unknown

Check vendor advisories for fix availability and mitigation guidance

CVSS 5.3/10 Medium

NETWORK / LOW complexity

Affected Products (1)

Mastodon

Affected Vendors

Joinmastodon

References (4)

Release Notes https://github.com/mastodon/mastodon/releases/tag/v4.3.18

Release Notes https://github.com/mastodon/mastodon/releases/tag/v4.4.12

Release Notes https://github.com/mastodon/mastodon/releases/tag/v4.5.5

Vendor Advisory https://github.com/mastodon/mastodon/security/advisories/GHSA-5h2f-wg8j-xqwp

/ 100

low-risk

Severity 21/34 · High

Exploitability 0/34 · Minimal

Exposure 5/34 · Minimal