Discourse EUVD-2026-36582

Exploitation requires that the target Discourse instance is running an affected version (2026.1.0-2026.1.3, 2026.3.0, or 2026.4.0) with the chat feature enabled and at least one public category chat channel in active use; the attacker must be able to reach the MessageBus endpoint over the network and subscribe to the unscoped chat event channel. Limiting factors are that only public-category chat events are exposed (private-category and DM chat are not implicated by the description), the issue only leaks chat traffic generated while the attacker is subscribed (no historical retrieval), and the impact is confidentiality-only with no write or denial-of-service capability.

CVSS 3.1 base score is 7.5 (AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N) reflecting network-reachable, unauthenticated read access to chat content with no integrity or availability impact, which is consistent with the description's information-disclosure nature. No EPSS score, KEV listing, or POC was provided, so there is no signal of active exploitation; risk should be treated as opportunistic rather than confirmed in-the-wild abuse. The realistic priority is moderate-to-high for Discourse instances that host sensitive discussions in public-category chat (where 'public' here means category visibility, not necessarily the message content), and lower for forums that do not use the chat plugin. Note that C:H may be generous given that only public-category chat (not DMs or private categories per the description) is exposed - but it remains appropriate because chat content can still be sensitive and is leaked to users explicitly not entitled to receive it.

An attacker registers or uses any account on a Discourse forum that has chat enabled site-wide but disabled for their user, opens a MessageBus long-poll subscription to the public chat event channels, and passively receives chat messages from public category channels in real time as legitimate users send them. Because PR:N and UI:N, an unauthenticated client able to reach MessageBus could likewise subscribe and harvest payloads without any victim interaction.