CVE-2026-91156 in Discourse
Summary
by MITRE • 10/14/2026
Discourse is an open-source discussion platform. Prior to 2026.9.0, 2026.8.1, 2026.7.3, and 2026.1.9, MessageBus audience calculation could deliver messages from a read-restricted category chat channel to an authenticated user who could view the category but was not allowed to use Chat under the site's group restrictions. The publication path bounded events by category visibility without also enforcing Chat eligibility, exposing live and recent message content available in the MessageBus backlog. The issue does not permit retrieval of the channel's full history through the Chat API and does not permit message creation or modification. This issue is fixed in versions 2026.9.0, 2026.8.1, 2026.7.3, and 2026.1.9.
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Analysis
by VulDB Data Team • 10/14/2026
Discourse is a widely deployed open-source discussion platform that facilitates community engagement through forums, chat channels, and various interactive features. A significant security vulnerability was identified in the MessageBus component prior to versions 2026.9.0, 2026.8.1, 2026.7.3, and 2026.1.9. This flaw pertains specifically to how audience calculations are performed for real-time message delivery within chat channels that have restricted access based on category visibility. The vulnerability arises from an inconsistency in permission enforcement where the system correctly restricts general category viewing but fails to adequately validate Chat-specific eligibility during the publication of events via MessageBus.
The technical root cause lies in the event publishing path, which bounds messages by category visibility without simultaneously enforcing group-based restrictions required for Chat usage. In a properly secured environment, an authenticated user must possess both permission to view the underlying category and explicit authorization to participate in or access chat features within that context. However, due to this logical flaw, if a user is granted read-only access to a category but lacks the necessary group permissions to use Chat, they can still receive real-time messages from restricted chat channels through the MessageBus backlog. This occurs because the audience calculation logic prioritizes category visibility over the more granular Chat eligibility checks, allowing live and recent message content to be delivered to unauthorized users who are technically excluded from using the chat functionality.
The operational impact of this vulnerability is primarily informational disclosure rather than full system compromise or data exfiltration. An authenticated attacker with limited privileges can intercept real-time communications intended for authorized participants in read-restricted category chats. This exposure allows the adversary to gain insight into ongoing discussions, potentially revealing sensitive organizational information, personal details, or strategic plans being discussed within those channels. It is important to note that the vulnerability does not permit retrieval of the channel's full historical message archive through the Chat API, nor does it allow for the creation or modification of messages. Therefore, while current and recent communications are exposed, past data remains protected by existing access controls, limiting the scope of potential damage to real-time interception only.
From a classification perspective, this vulnerability aligns with CWE-284 Improper Access Control, as the application fails to enforce appropriate restrictions on users attempting to access resources beyond their authorized level. It also relates to CWE-601 URL Redirection to Untrusted Site in some interpretations of audience targeting failures, though more accurately it represents a logic flaw in authorization checks often categorized under CWE-862 Missing Authorization. In the context of the MITRE ATT&CK framework, this behavior facilitates Information Discovery through techniques such as Collection or Data from Local System if viewed broadly, but specifically maps to unauthorized access patterns where an actor leverages misconfigured permissions to gather intelligence without direct interaction capabilities like posting or editing content.
Mitigation for this issue requires immediate upgrading of the Discourse instance to one of the fixed versions: 2026.9.0, 2026.8.1, 2026.7.3, or 2026.1.9. These releases correct the audience calculation logic in MessageBus to ensure that Chat eligibility is strictly enforced alongside category visibility when determining which users should receive real-time message updates. Administrators should verify their current version and apply patches promptly to prevent unauthorized interception of sensitive chat communications. Additionally, organizations should review their group permissions and role assignments to ensure that access controls are consistently applied across all features, including forums and chat modules, to maintain a defense-in-depth posture against similar logical flaws in future updates or custom plugins.