# 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.

- **Severity:** Medium 4.3 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 22 Jan 2026
- **Updated:** 22 Jan 2026
- **Also known as:** BIT-mastodon-2026-23963, GHSA-6x3w-9g92-gvf3

## 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 (Allocation of Resources Without Limits or Throttling)

## Which versions are affected by CVE-2026-23963?

1 open source package ship an affected version, including mastodon. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `mastodon` | Bitnami | All versions | 4.3.18, 4.4.12, 4.5.5 |

Products: 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.

## 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](https://insights.sbomify.com/CVE-2023-36460/): Mastodon vulnerable to arbitrary file creation through media attachments (critical 10.0)
- [CVE-2022-2166](https://insights.sbomify.com/CVE-2022-2166/): Improper Restriction of Excessive Authentication Attempts in mastodon/mastodon (critical 9.8)
- [CVE-2022-24307](https://insights.sbomify.com/CVE-2022-24307/): Missing authorization in n/a (critical 9.8)
- [CVE-2018-21018](https://insights.sbomify.com/CVE-2018-21018/): Auth bypass in n/a (critical 9.8)
- [CVE-2024-23832](https://insights.sbomify.com/CVE-2024-23832/): Mastodon Remote user impersonation and takeover (critical 9.4)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-23963 and fix it if it is affected.

What it is: 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.
Exploitation: There is no known exploitation of CVE-2026-23963 in the wild and no public exploit code.

Affected packages and fixed versions:
- mastodon (Bitnami): affected all versions; fixed in 4.3.18, 4.4.12, 4.5.5

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-23963/ (Markdown: https://insights.sbomify.com/CVE-2026-23963/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-23963
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-23963
- OSV: https://osv.dev/vulnerability/CVE-2026-23963

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-23963/
Summaries are generated and may contain mistakes; the official description above is the record.