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TI-2026-83421 Aliases CVE-2026-63115

Undo handler in Ghost ActivityPub removes other accounts' follows

Priority LOW 29CWE-290Published · Updated

Key facts

Attack vector
Network
Privileges required
None
User interaction
None
Attack complexity
Low
Impact type
Security bypass
Confidentiality · integrity · availability
None · Low · None
Fix status
Fixed

Attack profile from CVSS 3.1 · CNA. Fix status from CNA. Impact type from CWE-290.

Summary

TryGhost ActivityPub, a multi-tenant ActivityPub server for Ghost, is affected from 1.0.0 before 1.2.6. A flaw in the Undo handler lets a remote federated server make the service act on a follow or repost as if it came from a different account (CWE-863). An unprivileged remote attacker can remove another account's follow or repost without that account's involvement, but no private data is exposed and content is not modified.

Written with Claude Haiku 5.5 from the sources below · 2026-11-02

Description from the CNA

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.

Why it matters now

Exploitation None reported CISA SSVC · not in CISA KEV
EPSS 0.00143 3rd percentileScored 2026-11-07
Fix Fixed ActivityPub (TryGhost): 1.2.6

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation
None
Automatable
Yes
Technical impact
Partial

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

VendorProductAffected versionsFixed inStatusSource
TryGhostActivityPub1.0.0 to before 1.2.61.2.6Patch availableCNA
  • ActivityPub VendorTryGhost Affected 1.0.0 to before 1.2.6 Fixed in 1.2.6 Patch available CNA

Remediation

Update TryGhost ActivityPub to version 1.2.6. Services proxying to Ghost's hosted ActivityPub server are already updated, while self-hosted services need an updated image and redeployment.

Written with Claude Haiku 5.5 from the sources below · 2026-11-02

  • Fix · CNA ActivityPub (TryGhost) Fixed in 1.2.6

Timeline

  1. 2026-10-30 Threatist record created from CVE-2026-63115
  2. 2026-11-02 CISA SSVC decision points changed

References

Other 4

  • TI-2026-21459 Ghost has a SQL Injection in its Content API CVSS 3.1CRITICAL 9.4
  • TI-2022-10277 An arbitrary file upload vulnerability in the file upload module of Ghost v4.39.0 allows… CVSS 3.1CRITICAL 9.8
  • TI-2024-22785 Ghost through 5.85.1 allows remote attackers to bypass an authentication rate-limit… CVSS 3.1CRITICAL 9.1
  • TI-2023-14843 Ghost vulnerable to disclosure of private API fields CVSS 3.1HIGH 7.5
  • TI-2025-09049 Ghost 6.0.6 - SSRF via oEmbed Bookmark CVSS 4.0MEDIUM 6.1
  • TI-2023-20918 Arbitrary file read via symlinks in Ghost CVSS 3.1MEDIUM 6.5
See all 109 →

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences