CVE-2026-23964
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedMastodon 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.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceMastodon versions prior to 4.5.5, 4.4.12, and 4.3.18 contain an Insecure Direct Object Reference (IDOR) in the web push subscription update endpoint. The endpoint does not verify that the authenticated user owns the subscription being modified, allowing any authenticated user to update another user's push subscription settings by guessing or obtaining the numeric subscription ID. This enables attackers to modify notification filters and subscribed types, disrupt push notifications for victims, and leak the victim's push subscription endpoint URL.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data< 4.3.18>= 4.4.0, < 4.4.12>= 4.5.0, < 4.5.5CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- Low
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- Low
- Integrity
- Low
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Check Mastodon versionLocate the installed Mastodon version by checking the version file, Git tags, package.json, or the admin dashboard under Site Settings. Compare against the affected ranges: < 4.3.18, >= 4.4.0 and < 4.4.12, or >= 4.5.0 and < 4.5.5.Affected if The installed version falls within any of the affected version ranges.
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Confirm web push is enabledVerify that web push notifications are enabled on the instance by checking the admin settings or the presence of push subscription configuration in the database. This feature is required for the vulnerable endpoint to be exploitable.Affected if Web push subscriptions are enabled on the instance.
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Identify the push subscription update endpointLocate the API endpoint responsible for updating push subscriptions. This is typically under /api/v1/push/subscription or similar path in the Mastodon API. Review the controller code to see if it validates that the authenticated user owns the subscription being modified.Affected if The endpoint does not verify subscription ownership before allowing updates.
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Test ownership validationUsing an authenticated account, attempt to update a push subscription with a numeric ID that belongs to a different user. If the update succeeds without ownership verification, the vulnerability is present.Affected if The update request succeeds for a subscription not owned by the authenticated user.
If the installed Mastodon version is within the affected ranges and web push is enabled, the instance is likely vulnerable to IDOR on the push subscription update endpoint.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scoped4.3.184.4.124.5.5
Upgrade to Mastodon versions v4.5.5, v4.4.12, or v4.3.18 which contain the patch for this vulnerability.
Mastodon v4.5.5 (or v4.4.12 or v4.3.18 depending on your version branch)
- Backup your Mastodon instance database and files before upgrading
- Check your current Mastodon version (typically in the admin interface or via CLI tools)
- Upgrade to Mastodon v4.5.5, v4.4.12, or v4.3.18 (depending on your current major version branch)
- After upgrading, verify the web push subscription endpoint now properly authorizes users to only modify their own subscriptions
- Test that push notifications continue to work correctly for your users
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation2.0 h
- Implementation4.0 h
- Testing2.0 h
- Review / QA2.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-23964 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-23964 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
- No weaponised exploit code, or anything meant to cause harm
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