# CVE-2026-23964: Mastodon has insufficient access control to push notification settings

> Authenticated users can modify other users' push notification settings and leak subscription endpoints via insecure direct object reference.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **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-2026-23964, GHSA-f3q8-7vw3-69v4

## What is CVE-2026-23964?

CVE-2026-23964 is a medium-severity missing authorization vulnerability in mastodon, published on 22 Jan 2026. An attacker can exploit it over the internet without logging in, 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, an insecure direct object reference in the web push subscription update endpoint lets any authenticated user update another user's push subscription by guessing or obtaining the numeric subscription id. This can be used to disrupt push notifications for other users and also leaks the web push subscription endpoint. Any user with a web push subscription is impacted, because another authenticated user can tamper with their push subscription settings if they can guess or obtain the subscription id. This allows an attacker to disrupt push notifications by changing the policy (whether to filter notifications from non-followers or non-followed users) and subscribed notification types of their victims. Additionally, the endpoint returns the subscription object, which includes the push notification endpoint for this subscription, but not its keypair. Mastodon versions v4.5.5, v4.4.12, v4.3.18 are patched.

Weakness: CWE-639 (Authorization Bypass Through User-Controlled Key), CWE-863 (Incorrect Authorization)

## Which versions are affected by CVE-2026-23964?

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-2026-23964?

Upgrade mastodon to a fixed version for your release line: 4.3.18, 4.4.12 or 4.5.5.

## Is CVE-2026-23964 being exploited?

There is no known exploitation of CVE-2026-23964 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-2026-23964 and fix it if it is affected.

What it is: CVE-2026-23964 is a medium-severity missing authorization vulnerability in mastodon, published on 22 Jan 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-23964 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-2026-23964/ (Markdown: https://insights.sbomify.com/CVE-2026-23964/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-23964
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-23964
- OSV: https://osv.dev/vulnerability/CVE-2026-23964

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-23964/
Summaries are generated and may contain mistakes; the official description above is the record.