CVE-2026-63115: Auth Bypass (CVSS 5.3)

Description

ActivityPub 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.

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CVSS v3.1 Score

5.3

MEDIUM

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

Weakness Type (CWE)

CWE-290 CWE-290

CWE-345 CWE-345

CWE-863 Incorrect Authorization

References

Other References

https://github.com/TryGhost/ActivityPub/commit/1526f540b983da97750b380868a210e6cdfb985e https://github.com/TryGhost/ActivityPub/pull/1929 https://github.com/TryGhost/ActivityPub/releases/tag/v1.2.6 https://github.com/TryGhost/ActivityPub/security/advisories/GHSA-mm4q-93gr-5mwc

Frequently Asked Questions

What is CVE-2026-63115? +

ActivityPub 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. It has a CVSS v3.1 base score of 5.3 (MEDIUM).

How severe is CVE-2026-63115? +

CVE-2026-63115 has a CVSS v3.1 score of 5.3 out of 10, rated MEDIUM. This is a medium-severity vulnerability that should be remediated as part of regular maintenance.

How do I check if I'm vulnerable to CVE-2026-63115? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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