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CVE-2026-63115— ActivityPub Undo handler can unfollow or remove reposts on behalf of other accounts

Quick assessment

Affected
TryGhost ActivityPub
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

ActivityPub 是 Ghost 的一个多租户 ActivityPub 服务器，基于 Fedify 构建。在版本 1.0.0 至 1.2.6 期间， 处理器允许远程联邦服务器诱导服务对 （关注）或 （转载）活动执行操作，仿佛该活动来自另一个账户。这导致攻击者能够在未经目标账户参与的情况下，删除属于其他账户的现有关注关系或转载内容。该漏洞不会泄露私有数据，也不会修改内容；受影响的关系可以重新建立。已更新指向 Ghost 托管 ActivityPub 服务器的代理服务，而自托管服务需要更新镜像并重新部署。此问题已

CVSS 5.3 · Medium
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I. Basic Information for CVE-2026-63115

Vulnerability Information
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Vulnerability Title
ActivityPub Undo handler can unfollow or remove reposts on behalf of other accounts
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
Source: CVE Program / CVE List V5
Vulnerability Type
使用欺骗进行的认证绕过
Source: CVE Program / CVE List V5

Affected Products

Vendor Product Affected Versions CPE Subscribe
TryGhost ActivityPub >= 1.0.0, < 1.2.6 -

II. Public POCs for CVE-2026-63115

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III. Intelligence Information for CVE-2026-63115

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Patches & Fixes for CVE-2026-63115 (2)

Vendor Advisories for CVE-2026-63115 (1)

Vendor Pages for CVE-2026-63115 (1)

IV. Related Vulnerabilities

Same product: ActivityPub
Same vendor: TryGhost
Same weakness: CWE-290