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

Severity: medium (CVSS 6.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.4% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-01-22 · updated 2026-06-17
Weakness: CWE-770

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.

Affected products

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

How to fix

Upgrade to a fixed version: mastodon 4.3.18, 4.4.12, 4.5.5. Check the vendor advisory for mitigations.

References

Get alerted when CVE-2026-23963 or the next vulnerability affects your stack