Auth bypass in Joinmastodon Mastodon
CVE-2026-23961
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.
Vulnerability class: Broken Access Control
EPSS: 0.004 (32.9th percentile) — read the EPSS interpretation.
CVSS v3 metric
CVSS v3 base score 5.3 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N.
Affected products
- Joinmastodon Mastodon
- Mastodon — versions < 4.3.18, >= 4.4.0, < 4.4.12, >= 4.5.0, < 4.5.5
Weakness classification (CWE)
References
- security-advisories@github.com (x_refsource_CONFIRM, Vendor Advisory)
- security-advisories@github.com (x_refsource_MISC, Release Notes)
- security-advisories@github.com (x_refsource_MISC, Release Notes)
- security-advisories@github.com (x_refsource_MISC, Release Notes)
Frequently asked questions
- What is CVE-2026-23961?
- CVE-2026-23961 is a medium-severity vulnerability in Joinmastodon Mastodon, classified under Incorrect Authorization. CVSS score: 5.3/10. Published 2026-01-22.
- How severe is CVE-2026-23961?
- Medium severity. CVSS v3 base score is 5.3 out of 10.