Resource exhaustion in Joinmastodon Mastodon
CVE-2026-23963
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.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 4.3 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L.
EPSS exploit prediction
EPSS: 0.003 (25.4th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 25th percentile — 25.4% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.003 · newest: 0.003 · change: +0.000
Affected products
- Joinmastodon Mastodon
- Mastodon — versions < 4.3.18, >= 4.4.0, < 4.4.12, >= 4.5.0, < 4.5.5
Weakness classification (CWE)
References
- github.com/mastodon/mastodon/security/advisories/GHSA-6x3w-9g92-gvf3 (Vendor Advisory)
- github.com/mastodon/mastodon/releases/tag/v4.3.18 (Release Notes)
- github.com/mastodon/mastodon/releases/tag/v4.4.12 (Release Notes)
- github.com/mastodon/mastodon/releases/tag/v4.5.5 (Release Notes)
Frequently asked questions
- What is CVE-2026-23963?
- CVE-2026-23963 is a medium-severity vulnerability in Joinmastodon Mastodon, classified under Allocation of Resources Without Limits or Throttling. CVSS score: 4.3/10. Published 2026-01-22.
- How severe is CVE-2026-23963?
- Medium severity. CVSS v3 base score is 4.3 out of 10.