CVE-2026-59828 in Discourse
Summary
by MITRE • 07/10/2026
Discourse is an open-source discussion platform. Prior to 2026.6.0, 2026.5.1, 2026.4.2, and 2026.1.5, post revisions that should be hidden from regular users could be leaked through visible diffs on adjacent revisions serialized by PostRevisionSerializer. This issue is fixed in versions 2026.6.0, 2026.5.1, 2026.4.2, and 2026.1.5.
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Analysis
by VulDB Data Team • 07/10/2026
This vulnerability affects Discourse, an open-source discussion platform that serves as a collaborative space for community engagement and knowledge sharing. The security flaw manifests in the platform's handling of post revision data where certain content that should remain hidden from regular users becomes accessible through adjacent revision diffs. This represents a critical information disclosure issue that undermines the platform's access control mechanisms and could expose sensitive data to unauthorized users.
The technical root cause lies within the PostRevisionSerializer component which is responsible for serializing post revision history for display purposes. When processing revisions that contain restricted content, the serializer fails to properly filter or sanitize the data before generating diffs for adjacent revisions. This results in a scenario where legitimate access controls are bypassed through the diff visualization mechanism, allowing users to reconstruct portions of hidden content from the differences between consecutive revisions.
The operational impact of this vulnerability extends beyond simple data exposure as it compromises the integrity of the platform's content management system and user privacy controls. Regular users who should not have access to certain revision data can potentially reconstruct sensitive information through careful examination of adjacent revision diffs, which may include personal identifiers, confidential communications, or proprietary information. This undermines trust in the platform's security model and could lead to reputational damage for organizations relying on Discourse for their communication needs.
This vulnerability aligns with CWE-200 (Information Exposure) and CWE-532 (Information Exposure Through Log Data) categories from the Common Weakness Enumeration framework, as it involves unauthorized disclosure of information through serialized data structures. From an ATT&CK perspective, this maps to T1071.004 (Application Layer Protocol: DNS) and T1566.001 (Phishing: Spearphishing Attachment) in the context of information gathering and reconnaissance activities that attackers might use to gain additional insights into system content and user communications.
The fix implemented in versions 2026.6.0, 2026.5.1, 2026.4.2, and 2026.1.5 addresses the core serialization logic by ensuring that PostRevisionSerializer properly validates access controls before generating diff data for adjacent revisions. Security patches should include comprehensive input validation, proper access control enforcement, and thorough testing of revision history mechanisms to prevent similar issues in related components.
Organizations using Discourse should immediately upgrade to the patched versions and conduct security assessments of their existing revision histories to identify any potentially exposed sensitive data. Additional mitigations include implementing regular monitoring for unauthorized access patterns and establishing procedures for reviewing and purging sensitive content from revision logs when necessary. The vulnerability highlights the importance of maintaining robust access control mechanisms throughout all data serialization pathways, particularly in collaborative platforms where content history and user interactions are integral to the system functionality.