Vulnerability · Published Oct 15, 2026 · Assigned by GitHub_M

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

High 8.6

In plain English

A high-severity insufficient verification of data authenticity in discourse-activity-pub from discourse. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 10% of all known vulnerabilities).

Severity (CVSS 3.1)8.6High

Exploit odds (30 days)0.2%top 90.1% of all CVEs

Exploited in the wildNo reportsCISA SSVC: none

Attack toolingNoneno public tooling known

Official 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 (as reported)

Vendor	Product	Versions
discourse	discourse-activity-pub	< 1c8518b0f2abdf31cbea3e0b242f74d8994a9ba5

Weakness: CWE-345, CWE-863 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:N/A:N

References

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