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CVE-2026-63115medium5.3

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

Impact

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…

Source excerpt · GitHub_M
Applies when

Services proxying to Ghost's hosted ActivityPub server have already been updated, while self-hosted services require an updated image and redeployment.

Affected packages

This source has no supported package-version data for matching. You can read the advisory here, but this record alone cannot power a dependency check.

Affected products

Vendor version evidence. Product records are separate from automatic dependency matching.

TryGhost · GitHub_M

ActivityPub

1 range
GitHub_M version evidence

Compare with this publisher’s ranges. This does not inspect your installation.

affected

>= 1.0.0, < 1.2.6

unaffected

unknown

Other versions: unknown

Original source 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.

Inside the record

Evidence available

Package identity · not supplied
Version evidence
Remediation guidance · not supplied
Weakness classification
Additional publishers · not supplied

Describes the evidence in this view, not whether the product is safe.

Original records

CVE-2026-63115

View the latest unmodified CVE JSON from the official source. It may be newer than this imported page.

cve.new JSON →

Underlying weaknesses

The source’s classification of what caused the vulnerability.

CWE-290 ↗

CWE-290: Authentication Bypass by Spoofing

GitHub_M

Related →
CWE-345 ↗

CWE-345: Insufficient Verification of Data Authenticity

GitHub_M

Related →
CWE-863 ↗

CWE-863: Incorrect Authorization

GitHub_M

Related →
Contributing organizations · 1
GitHub_M

Assigning organization

2026-10-30

What changed

Follow changes
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Large field values are marked as truncated previews. Original source snapshots remain retained.

References

4 sources
github.com/TryGhost/ActivityPub/security/advisories/GHSA-mm4q-93gr-5mwc
Security advisoryGitHub_M
github.com/TryGhost/ActivityPub/pull/1929
PatchGitHub_M
github.com/TryGhost/ActivityPub/commit/1526f540b983da97750b380868a210e6cdfb985e
PatchGitHub_M
github.com/TryGhost/ActivityPub/releases/tag/v1.2.6
Release notesGitHub_M