CVE-2026-23961
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedMastodon 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.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceMastodon contains logic errors in its user suspension enforcement where boosted posts from suspended users can still appear in timelines, and on specific versions (v4.5.0-4.5.4, v4.4.5-4.4.11, v4.3.13-4.3.17, v4.2.26-4.2.29), remote suspended users can partially bypass suspension to introduce new posts. This is a server-side access control bypass in the ActivityPub federation logic.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data< 4.3.18>= 4.4.0, < 4.4.12>= 4.5.0, < 4.5.5CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- Low
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Identify installed Mastodon versionCheck the VERSION file in the Mastodon installation directory, or run `git describe --tags` if using git, or check the admin dashboard for version informationAffected if Version is < 4.3.18, >= 4.4.0 and < 4.4.12, or >= 4.5.0 and < 4.5.5
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Confirm ActivityPub federation is enabledCheck the Mastodon configuration file (typically config/defaults.yml or environment variables) for the `ActivityPub.enabled` setting, or verify the instance has remote followers/postsAffected if Federation is enabled and the instance interacts with remote ActivityPub servers
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Inspect database for boosted posts from suspended accountsQuery the database: look for records in the statuses table where `account_id` references a suspended account (status `account.suspended = true`) but the status appears in home/public timelines. Use SQL like: SELECT s.* FROM statuses s JOIN accounts a ON s.account_id = a.id WHERE a.suspended = true AND s.id IN (SELECT status_id FROM mentions OR SELECT status_id FROM reblogs)Affected if Any statuses from suspended accounts are found in timeline-related tables or have been delivered to local users
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Check for posts from remote suspended users in the databaseQuery the database for statuses where the remote server indicated the author was suspended at time of delivery. Check the `remote` column in accounts table and verify no unsuspended check is performed during ActivityPub inbound processing. Inspect application logs for ActivityPub::Worker::IngestJob processing of posts from known-suspended remote accountsAffected if Posts from remote suspended users exist in the statuses table or were processed without rejection
If running an affected version (4.5.0-4.5.4, 4.4.5-4.4.11, 4.3.13-4.3.17, or 4.2.26-4.2.29, or any version < 4.3.18, >= 4.4.0 < 4.4.12, >= 4.5.0 < 4.5.5) with ActivityPub federation enabled, the instance is likely affected by posts from suspended users appearing in timelines or remote suspended users bypassing suspension.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scoped4.3.184.4.124.5.5
Upgrade to Mastodon v4.5.5, v4.4.12, or v4.3.18 or later. For unsupported versions, review and patch the suspension enforcement logic in the federation code to ensure suspended users cannot have their posts processed or appear in timelines.
Mastodon v4.3.18 (for 4.3.x), v4.4.12 (for 4.4.x), or v4.5.5 (for 4.5.x)
- Identify your current Mastodon version by checking the version file or admin interface
- Backup your Mastodon database and any custom configurations before upgrading
- Upgrade to Mastodon v4.3.18 if running version 4.3.x
- Upgrade to Mastodon v4.4.12 if running version 4.4.x
- Upgrade to Mastodon v4.5.5 if running version 4.5.x
- If running v4.2.x, upgrade to v4.2.30 or later to address the partial bypass in v4.2.26-v4.2.29
- After upgrading, verify that suspended remote users no longer appear in timelines and that the suspension bypass is resolved
- Review the release notes for your target version to confirm all security patches are applied
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation2.0 h
- Implementation4.0 h
- Testing6.0 h
- Review / QA2.0 h
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Free · runs locallyCheck whether your project pulls in CVE-2026-23961 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-23961 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
- No weaponised exploit code, or anything meant to cause harm
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