CVE-2026-31869 Discourse: Composer mentions endpoint leaks hidden group membership through PM `allowed_names` check
Authenticated attackers can infer hidden group membership in Discourse by probing the mentions endpoint, bypassing visibility controls.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-31869?
CVE-2026-31869 is a medium-severity vulnerability in Discourse Discourse, published on 20 Mar 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official description
Discourse is an open-source discussion platform. Prior to versions 2026.3.0-latest.1, 2026.2.1, and 2026.1.2, the ComposerController#mentions endpoint reveals hidden group membership to any authenticated user who can message the group. By supplying allowed_names referencing a hidden-membership group and probing arbitrary usernames, an attacker can infer membership based on whether user_reasons returns "private" for a given user. This bypasses group member-visibility controls. Versions 2026.3.0-latest.1, 2026.2.1, and 2026.1.2 contain a patch. To work around this issue, restrict the messageable policy of any hidden-membership group to staff or group members only, so untrusted users cannot reach the vulnerable code path.
Which versions are affected by CVE-2026-31869?
1 open source package ship an affected version, including discourse. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
discourse Bitnami | From 2026.1.0 | 2026.1.2, 2026.2.1, 2026.3.0 |
Products include Discourse Discourse.
How do I fix CVE-2026-31869?
Upgrade discourse to a fixed version for your release line: 2026.1.2, 2026.2.1 or 2026.3.0.
Is CVE-2026-31869 being exploited?
There is no known exploitation of CVE-2026-31869 in the wild and no public exploit code.
Other vulnerabilities in Discourse Discourse
- CVE-2021-41163 RCE via malicious SNS subscription payload
- CVE-2022-36066 Discourse vulnerable to RCE via admins uploading maliciously zipped file
- CVE-2022-39356 Discourse user account takeover via email and invite link
- CVE-2023-22468 Discourse vulnerable to Cross-site Scripting in local oneboxes
- CVE-2026-27934 Discourse leaks private topic title and post excerpt via user action API endpoint
Sources
Also known as BIT-discourse-2026-31869, GHSA-5f9h-vp7v-7vq5.