CVE-2026-55103 Discourse ActivityPub: Fix inbound actor validation and apply outbound request SSRF hardening
Remote attackers can edit, delete, or reply to federated posts without proper permissions due to missing identity and ownership checks in inbound ActivityPub processing.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-55103?
CVE-2026-55103 is a high-severity missing authorization vulnerability in discourse-activity-pub, published on 15 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
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.
Which versions are affected by CVE-2026-55103?
How do I fix CVE-2026-55103?
No fix has been published for CVE-2026-55103 yet. Limit exposure of the affected software until one is available.
Upgrade, and check permissions on every request on the server, not only in the interface. More about Missing authorization.
Is CVE-2026-55103 being exploited?
There is no known exploitation of CVE-2026-55103 in the wild and no public exploit code.
Sources
Also known as GHSA-jw39-rch6-mhwf.