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CVE-2026-55103— Discourse ActivityPub: Fix inbound actor validation and apply outbound request SSRF hardening

Quick assessment

Affected
discourse discourse-activity-pub
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.

Discourse ActivityPub 插件为 Discourse 平台添加了 ActivityPub 协议支持。在提交 1c8518b0f2abdf31cbea3e0b242f74d8994a9ba5 之前，对传入的 ActivityPub 请求在处理时，未能充分验证签名请求中身份认证的参与者（actor）是否与 Activity 中声明的参与者一致，以及该参与者是否拥有所引用的联邦式帖子（federated post）。远程攻击者可利用这些缺失的身份和所有权检查机制，在未经适当权限的情况下，编辑、删除或回复

CVSS 8.6 · High EPSS 0.20% · P9

Affected Version Matrix 1

VendorProduct Version RangeStatus
discourse discourse-activity-pub < 1c8518b0f2abdf31cbea3e0b242f74d8994a9ba5 affected
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I. Basic Information for CVE-2026-55103

Vulnerability Information
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Vulnerability Title
Discourse ActivityPub: Fix inbound actor validation and apply outbound request SSRF hardening
Source: CVE Program / CVE List V5
Vulnerability Description
Discourse ActivityPub adds ActivityPub support to Discourse. Prior to commit 1c8518b0f2abdf31cbea3e0b242f74d8994a9ba5, inbound ActivityPub processing does not sufficiently verify that the actor authenticated by the signed request matches the Activity actor and owns the referenced federated post. A remote attacker can exploit these missing identity and ownership checks to edit, delete, or reply to federated posts without the required permissions. This vulnerability is fixed with commit 1c8518b0f2abdf31cbea3e0b242f74d8994a9ba5.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:N/A:N
Source: CVE Program / CVE List V5
Vulnerability Type
对数据真实性的验证不充分
Source: CVE Program / CVE List V5

Affected Products

Vendor Product Affected Versions CPE Subscribe
discourse discourse-activity-pub < 1c8518b0f2abdf31cbea3e0b242f74d8994a9ba5 -

II. Public POCs for CVE-2026-55103

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III. Intelligence Information for CVE-2026-55103

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Patches & Fixes for CVE-2026-55103 (1)

Vendor Advisories for CVE-2026-55103 (1)

IV. Related Vulnerabilities

Same vendor: discourse
Same weakness: CWE-345

V. Comments for CVE-2026-55103

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