CVE-2026-23963 Mastodon missing length limits on list names, filter names, and filter keywords
Attackers can cause resource exhaustion and service disruption by submitting arbitrarily long strings in list and filter fields.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-23963?
CVE-2026-23963 is a medium-severity denial of service vulnerability in mastodon, published on 22 Jan 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official 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.
Weakness: CWE-770
Which versions are affected by CVE-2026-23963?
1 open source package ship an affected version, including mastodon. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
mastodon Bitnami | All versions | 4.3.18, 4.4.12, 4.5.5 |
Products include Joinmastodon Mastodon.
How do I fix CVE-2026-23963?
Upgrade mastodon to a fixed version for your release line: 4.3.18, 4.4.12 or 4.5.5.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-23963 being exploited?
There is no known exploitation of CVE-2026-23963 in the wild and no public exploit code.
Other vulnerabilities in mastodon
- CVE-2023-36460 Mastodon vulnerable to arbitrary file creation through media attachments
- CVE-2022-2166 Improper Restriction of Excessive Authentication Attempts in mastodon/mastodon
- CVE-2022-24307 Missing authorization in n/a
- CVE-2018-21018 Auth bypass in n/a
- CVE-2024-23832 Mastodon Remote user impersonation and takeover
Sources
Also known as BIT-mastodon-2026-23963, GHSA-6x3w-9g92-gvf3.