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TI-2026-74866 Aliases CVE-2026-55103

Discourse ActivityPub lets remote actors edit or delete federated posts

Priority MEDIUM 43CWE-345Published · Updated

Key facts

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

Attack profile from CVSS 3.1 · CNA. Fix status from CNA. Impact type from CWE-345.

Summary

Discourse ActivityPub before commit 1c8518b is affected. 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, which is an insufficient verification of data authenticity (CWE-345). A remote attacker without privileges can edit, delete, or reply to federated posts without the required permissions.

Written with Claude Haiku 5.5 from the sources below · 2026-10-29

Description from the CNA

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.

Why it matters now

Exploitation None reported CISA SSVC · not in CISA KEV
EPSS 0.002 9th percentileScored 2026-11-07
Fix Fixed discourse-activity-pub (discourse): 1c8518b

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
discoursediscourse-activity-pubbefore 1c8518b1c8518bPatch availableCNA
  • discourse-activity-pub Vendordiscourse Affected before 1c8518b Fixed in 1c8518b Patch available CNA

Remediation

The sources list no fixed version for discourse-activity-pub. The fix is commit 1c8518b.

Written with Claude Haiku 5.5 from the sources below · 2026-10-29

  • Fix · CNA discourse-activity-pub (discourse) Fixed in 1c8518b

Timeline

  1. 2026-10-16 Threatist record created from CVE-2026-55103
  2. 2026-10-16 Priority changed: MEDIUM → LOW
  3. 2026-10-28 CISA SSVC decision points changed
  4. 2026-10-28 Priority changed: LOW → MEDIUM

References

Other 2

  • TI-2022-22806 Discourse BBCode plugin vulnerable to arbitrary CSS injection CVSS 3.1CRITICAL 9.8
  • TI-2021-19656 Denial of Service in discourse CVSS 3.1MEDIUM 6.8
  • TI-2024-36150 Potential Backup file leaked via Nginx in Discourse CVSS 3.1HIGH 7.5
  • TI-2021-18002 RCE via malicious SNS subscription payload CVSS 3.1CRITICAL 10.0
  • TI-2026-24651 Discourse: Open redirect via `sso_destination_url` cookie in `enter` CVSS 4.0MEDIUM 5.1
  • TI-2022-18730 Discourse Patreon vulnerable to improper validation of email during Patreon authentication CVSS 3.1CRITICAL 9.8
See all 328 →

Sources

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