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MastodonApplication · Joinmastodon

CVE-2026-23963

MEDIUM · 6.5 CVSS v3.1 Published 2026-01-22
Fix available
A fix is available. Upgrade to 4.3.18 / 4.4.12 or later.
See remediation →
71/100
Remediation priority · Elevated
Remotely reachable Zero-click

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 · unedited
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.

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 confidence

Mastodon versions prior to 4.5.5, 4.4.12, and 4.3.18 lack input validation limiting the maximum length of list names, filter names, and filter keywords. This allows authenticated users to store arbitrarily large strings, causing excessive storage consumption and computational overhead that can degrade server performance and render the web interface unusable for the affected user.

MitigationUpgrade to Mastodon v4.5.5, v4.4.12, or v4.3.18 or later to receive the patched version that enforces reasonable maximum length constraints on these input fields.

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
MastodonApplication
Affected:< 4.3.18>= 4.4.0, < 4.4.12>= 4.5.0, < 4.5.5

CVSS 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
Low
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

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 checks

Work through these to decide whether this CVE applies to you.

  1. Identify installed Mastodon version
    Check the version file in the Mastodon installation directory: cat /home/mastodon/live/VERSION or check the Package version if installed via package manager
    Affected if The installed version is prior to 4.3.18, between 4.4.0-4.4.11, or between 4.5.0-4.5.4
  2. Confirm user lists feature is accessible
    Verify that authenticated users can access the Lists feature via the web UI or API endpoint GET /api/v1/accounts/:id/lists
    Affected if The Lists feature is available to users (standard in Mastodon for authenticated accounts)
  3. Confirm user filters feature is accessible
    Verify that authenticated users can access the Filters feature via the web UI or API endpoint GET /api/v1/filters
    Affected if The Filters feature is available to users (standard in Mastodon for authenticated accounts)
  4. Check database for abnormally long list names
    Query the lists table for entries with unusually long name fields, for example: SELECT id, name, LENGTH(name) as name_length FROM lists WHERE LENGTH(name) > 255 ORDER BY name_length DESC;
    Affected if Any list entries exist with name length significantly exceeding 255 characters, indicating exploitation
  5. Check database for abnormally long filter entries
    Query the filters and filter_keywords tables for entries with unusually long phrase fields, for example: SELECT id, phrase, LENGTH(phrase) as phrase_length FROM filters WHERE LENGTH(phrase) > 255;
    Affected if Any filter or filter keyword entries exist with phrase length significantly exceeding 255 characters, indicating exploitation

A user is affected if their Mastodon installation runs a version prior to 4.3.18, 4.4.12, or 4.5.5, and any user account on the instance has created lists or filters that contain unexpectedly large text strings.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

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 · scoped
Upgrade available Upgrade to 4.3.18 / 4.4.12 / 4.5.5 or later
Fixed in 4.3.184.4.124.5.5
Interim mitigation

Upgrade to Mastodon v4.5.5, v4.4.12, or v4.3.18 or later to receive the patched version that enforces reasonable maximum length constraints on these input fields.

Recommended fix High confidence

Mastodon v4.5.5 (if on v4.5.x), v4.4.12 (if on v4.4.x), or v4.3.18 (if on v4.3.x)

  1. Identify your current Mastodon version by checking the running instance or your deployment configuration
  2. Determine which release branch you're on (v4.3.x, v4.4.x, or v4.5.x)
  3. Back up your Mastodon database and any custom configurations before proceeding
  4. Upgrade to Mastodon v4.3.18 if on the v4.3.x branch, v4.4.12 if on the v4.4.x branch, or v4.5.5 if on the v4.5.x branch
  5. After upgrade, verify the instance is running correctly and test creating lists/filters with long names to confirm the limit is now enforced

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Mastodon Scoped from the published advisory
  • Consultation2.0 h
  • Implementation3.0 h
  • Testing2.0 h
  • Review / QA2.0 h
9.0 hours of engineering $1,600
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-23963 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • 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
  • No spam, self-promotion, credentials, or personal data