CVE-2026-55103Network attack vectorHinoki check available

Post tampering via missing actor and ownership checks

Published Oct 15, 2026 · Updated Oct 15, 2026

Authorization bypass in Discourse ActivityPub Plugin allows remote attackers to alter federated posts via signed ActivityPub requests. The inbox processing pipeline fails to bind the request signer to the Activity actor, while post handlers omit actor-ownership checks. Exploitation requires reachable, enabled federation processing and permits unauthorized edits, deletions, or replies to federated posts rather than general administrative access.

CVSS severity8.6
High
EPSS probability0.20%
Next 30 days · Oct 19, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkAvailable
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Summary

What happened

Authorization bypass in Discourse ActivityPub Plugin allows remote attackers to alter federated posts via signed ActivityPub requests. The inbox processing pipeline fails to bind the request signer to the Activity actor, while post handlers omit actor-ownership checks. Exploitation requires reachable, enabled federation processing and permits unauthorized edits, deletions, or replies to federated posts rather than general administrative access.

The record

CVE
CVE-2026-55103
Published
Oct 15, 2026
Updated
Oct 15, 2026
Vendor
Discourse
Product
Discourse ActivityPub Plugin
Classifications
CWE-863, CWE-345, T1565.001
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Oct 15, 2026CVE publishedPublication date reported by the CVE source.
  2. Oct 15, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 1c8518b0f2abdf31cbea3e0b242f74d8994a9ba5

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Discourse · Discourse ActivityPub Pluginversion=< 1c8518b0f2abdf31cbea3e0b242f74d8994a9ba5

Published CVSS scores

8.6GitHub CNACVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:N/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Activity data is delayed. The latest report is Sep 28, 2026.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 22, 2026Sep 28, 2026
Latest reporting daySep 28, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-863Incorrect Authorization
CWE-345Insufficient Verification of Data Authenticity
T1565.001Stored Data Manipulation

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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