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CVE Record

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

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.

MediumCVSS 6.5Not KEV-listedUpdated
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Glexia's TakeAutomated analysismoderate

Security readout for executives and security teams

Plain-English summary

CVE-2026-23964 is a Mastodon access-control flaw affecting push notification settings. A logged-in user may change another user’s push notification preferences if they can guess or obtain a numeric subscription ID. Business impact is mainly notification disruption and limited exposure of a push endpoint, not account takeover.

Executive priority

Treat as a timely maintenance update for Mastodon services. The issue can disrupt user notifications and expose push endpoints, but available sources do not show account compromise or active exploitation. Patch during the next appropriate change window, faster for public or high-profile instances.

Technical view

Mastodon’s web push subscription update endpoint has an insecure direct object reference before 4.5.5, 4.4.12, and 4.3.18. The advisory says an authenticated user can modify another user’s notification policy and subscribed notification types, and the endpoint returns the subscription object including the push notification endpoint, but not keypair material.

Likely exposure

Exposure is limited to Mastodon instances running affected versions with users who have web push subscriptions. Affected ranges are below 4.3.18, 4.4.0 through below 4.4.12, and 4.5.0 through below 4.5.5.

Exploitation context

No source provided indicates active exploitation, and CISA KEV status is false in the supplied bundle. Exploitation requires a user context per the advisory description and depends on guessing or obtaining a numeric subscription ID.

Researcher notes

The core issue is CWE-863 insufficient authorization on a direct object reference for push subscriptions. Note a source inconsistency: the CVSS vector in the bundle lists PR:N, while the description says any authenticated user. Do not assume unauthenticated exploitability without vendor confirmation.

Mitigation direction

  • Upgrade Mastodon to 4.5.5, 4.4.12, or 4.3.18 as appropriate.
  • Prioritize instances with web push notifications enabled.
  • Review the Mastodon security advisory and release notes before deployment.
  • If immediate upgrade is not possible, check vendor guidance for temporary mitigations.

Validation and detection

  • Inventory Mastodon instance versions across production and staging.
  • Confirm no instance is in the listed affected version ranges.
  • Verify upgraded instances report 4.5.5, 4.4.12, or 4.3.18.
  • Review application logs for unusual push subscription update activity.
Prepared
Confidence
high
Sources
6

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.

ATT&CK lookup starting points

Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.

cwe · medium confidence lookup

CWE-863: Authorization and privilege behavior lookup

Authorization weaknesses can support privilege escalation and valid-account review, depending on exploit path. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.

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cve · low confidence lookup

CVE-2026-23964 mapping review

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Vulnerability profileCVE Program record
Severity
Medium
CVSS
6.5 (3.1)
Known Exploited
No
Published

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

1CVSS vectors
0Timeline events
0ADP providers
5Source links

CVSS vector scores

1 official score

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
6.5CVSS 3.1MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N3.92.5Primary CVE score

Vulnerability scoring details

Base CVSS 3.1 score

6.5Medium
CVSS 3.1 vector shape for CVE-2026-23964Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Privileges Required
NoneLowHigh
User Interaction
NoneRequired
Scope
ChangedUnchanged
Confidentiality Impact
HighLowNone
Integrity Impact
HighLowNone
Availability Impact
HighLowNone

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
mastodonmastodon< 4.3.18, >= 4.4.0, < 4.4.12, >= 4.5.0, < 4.5.5Listed
Weakness

CWE details

CWE-863

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.

CWE-863 · source CWE mapping

Incorrect Authorization

Incorrect Authorization represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

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