What’s Vulnerable › Vulnerabilities

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

• Severity: high (CVSS 8.6 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.2% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-10-15 · updated 2026-10-28
• Weakness: CWE-345

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.

Affected products

• discourse-activity-pub: before 1c8518b0f2abdf31cbea3e0b242f74d8994a9ba5

References

• https://github.com/discourse/discourse-activity-pub/commit/1c8518b0f2abdf31cbea3e0b242f74d8994a9ba5
• https://github.com/discourse/discourse-activity-pub/security/advisories/GHSA-jw39-rch6-mhwf

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