CVE-2026-31869

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.

CVSS v3 4.3 MEDIUM

4.3^/10

CVSS v3 : MEDIUM

Vector :

Exploitability : 2.8 / Impact : 1.4

Attack Vector NETWORK

Attack Complexity LOW

Privileges Required LOW

User Interaction NONE

Confidentiality Impact LOW

Integrity Impact NONE

Availability Impact NONE

Scope UNCHANGED

References

Link	Resource
https://github.com/discourse/discourse/security/advisories/GHSA-5f9h-vp7v-7vq5	Mitigation Vendor Advisory

Configurations

Configuration 1 (hide)

OR	cpe:2.3:a:discourse:discourse::::::::
	cpe:2.3:a:discourse:discourse::::::::
	cpe:2.3:a:discourse:discourse:2026.3.0::::latest:::

History

No history.

Information

Published : 2026-03-20 03:15

Updated : 2026-03-24 20:22

NVD link : CVE-2026-31869

Mitre link : CVE-2026-31869

CVE.ORG link : CVE-2026-31869

JSON object : View

Products Affected

discourse

discourse

CWE

CWE-200

Exposure of Sensitive Information to an Unauthorized Actor

CWE-285

Improper Authorization

CWE-639

Authorization Bypass Through User-Controlled Key

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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."}, {"lang": "es", "value": "Discourse es una plataforma de discusi\u00f3n de c\u00f3digo abierto. Antes de las versiones 2026.3.0-latest.1, 2026.2.1 y 2026.1.2, el endpoint ComposerController#mentions revela la membres\u00eda oculta de un grupo a cualquier usuario autenticado que puede enviar mensajes al grupo. Al proporcionar allowed_names que hacen referencia a un grupo de membres\u00eda oculta y al sondear nombres de usuario arbitrarios, un atacante puede inferir la membres\u00eda bas\u00e1ndose en si user_reasons devuelve 'private' para un usuario dado. Esto elude los controles de visibilidad de miembros del grupo. Las versiones 2026.3.0-latest.1, 2026.2.1 y 2026.1.2 contienen un parche. Para solucionar este problema, restrinja la pol\u00edtica de mensajer\u00eda de cualquier grupo de membres\u00eda oculta solo al personal o a los miembros del grupo, para que los usuarios no confiables no puedan alcanzar la ruta de c\u00f3digo vulnerable."}], "lastModified": "2026-03-24T20:22:46.050", "configurations": [{"nodes": [{"negate": false, "cpeMatch": [{"criteria": "cpe:2.3:a:discourse:discourse:*:*:*:*:*:*:*:*", "vulnerable": true, "matchCriteriaId": "4BE96625-3609-410C-B41E-4A824C1A57C0", "versionEndExcluding": "2026.1.2", "versionStartIncluding": "2026.1.0"}, {"criteria": "cpe:2.3:a:discourse:discourse:*:*:*:*:*:*:*:*", "vulnerable": true, "matchCriteriaId": "FD31CF04-CF2F-4FB9-8880-9243BC7671A7", "versionEndExcluding": "2026.2.1", "versionStartIncluding": "2026.2.0"}, {"criteria": "cpe:2.3:a:discourse:discourse:2026.3.0:*:*:*:latest:*:*:*", "vulnerable": true, "matchCriteriaId": "E3FE9277-4F6B-4FD0-991F-F0FB8D226E1C"}], "operator": "OR"}]}], "sourceIdentifier": "security-advisories@github.com"}