ActivityPub Undo handler can unfollow or remove reposts on behalf of other accounts

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-63115
StatePUBLISHED
AssignerGitHub_M
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-10-30 22:16:57 UTC
Updated2026-11-02 19:18:09 UTC
DescriptionActivityPub is a multi-tenant ActivityPub server for Ghost, built with Fedify. From 1.0.0 until 1.2.6, the `Undo` handler allows a remote federated server to cause the service to act on a `follow` or `repost` as though the activity came from a different account. This permits removal of an existing `follow` relationship or `repost` belonging to another account without that account's involvement. No private data is exposed and content is not modified; the affected relationships can be re-established. Services proxying to Ghost's hosted ActivityPub server have already been updated, while self-hosted services require an updated image and redeployment. This issue is fixed in version 1.2.6.

Risk And Classification

Primary CVSS: v3.1 5.3 MEDIUM from [email protected]

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

EPSS: 0.001430000 probability, percentile 0.035960000 (date 2026-11-02)

Problem Types: CWE-290 | CWE-345 | CWE-863 | CWE-290 CWE-290: Authentication Bypass by Spoofing | CWE-345 CWE-345: Insufficient Verification of Data Authenticity | CWE-863 CWE-863: Incorrect Authorization

VersionSourceTypeScoreSeverityVector
3.1[email protected]Secondary5.3MEDIUMCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
3.1CNADECLARED5.3MEDIUMCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
Low
Availability
None

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

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA TryGhost ActivityPub affected >= 1.0.0, < 1.2.6 Not specified

References

ReferenceSourceLinkTags
github.com/TryGhost/ActivityPub/commit/1526f540b983da97750b380868a210e6c... [email protected] github.com
github.com/TryGhost/ActivityPub/releases/tag/v1.2.6 [email protected] github.com
github.com/TryGhost/ActivityPub/pull/1929 [email protected] github.com
github.com/TryGhost/ActivityPub/security/advisories/GHSA-mm4q-93gr-5mwc [email protected] github.com
CVE Program record CVE.ORG www.cve.org canonical
NVD vulnerability detail NVD nvd.nist.gov canonical, analysis

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