Information Disclosure in Discourse Post Revisions

Executive Summary

This vulnerability affects the Discourse open-source discussion platform. Before certain fixed versions (2026.6.0, 2026.5.1, 2026.4.2, and 2026.1.5), post revisions that were intended to be hidden from regular users could be exposed. This exposure happened through visible differences (diffs) on adjacent revisions that were serialized by the PostRevisionSerializer, allowing unauthorized users to see content that should have been concealed.

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Detection Guidance

This vulnerability involves hidden post revisions being leaked through visible diffs on adjacent revisions in the Discourse platform. Detection involves verifying whether your Discourse instance is running a vulnerable version prior to the patched releases (2026.6.0, 2026.5.1, 2026.4.2, and 2026.1.5).

To detect if your system is vulnerable, you can check the Discourse version by running the following command on your server:

• cd /var/discourse && ./launcher enter app
• bundle exec rails c
• puts Discourse::VERSION::STRING

If the version is older than the patched versions, your system is vulnerable.

To detect if hidden post revisions are leaking, you can attempt to view post revision diffs as a non-staff user and check if diffs reveal content or metadata from revisions that should be hidden. This can be done by reviewing the post revision history UI or by querying the API endpoints related to post revisions and inspecting the diff data for unauthorized exposure.

There are no specific network commands or automated detection scripts provided in the resources, but manual verification of Discourse version and testing revision diffs visibility as a non-staff user are recommended.

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Impact Analysis

The vulnerability can lead to unauthorized disclosure of information that was meant to be hidden from regular users. This means sensitive or private content in post revisions could be leaked, potentially compromising user privacy or exposing confidential discussions.

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Compliance Impact

This vulnerability allows leakage of post revisions that should be hidden from regular users through visible diffs on adjacent revisions. Such unintended data exposure could potentially lead to unauthorized disclosure of sensitive information.

Unauthorized data exposure may impact compliance with data protection regulations such as GDPR and HIPAA, which require protection of personal and sensitive information from unauthorized access.

However, the specific impact on compliance depends on the nature of the leaked data and the context in which the platform is used.

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Mitigation Strategies

To mitigate this vulnerability, upgrade your Discourse installation to one of the fixed versions: 2026.6.0, 2026.5.1, 2026.4.2, or 2026.1.5.

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