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CVE-2026-31869— Discourse: Composer mentions endpoint leaks hidden group membership through PM `allowed_names` check

Quick assessment

Affected
discourse discourse
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

Discourse是Discourse开源的一套开源的社区讨论平台。该平台包括社区、电子邮件和聊天室等功能。 Discourse 2026.3.0-latest.1之前版本、2026.2.1之前版本和2026.1.2之前版本存在安全漏洞，该漏洞源于ComposerController#mentions端点处理不当，可能向任何可以给群组发消息的经过身份验证的用户泄露隐藏的群组成员身份。

AI Predicted 5.3 Difficulty: Easy EPSS 0.18% · P7
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I. Basic Information for CVE-2026-31869

Vulnerability Information
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Vulnerability Title
Discourse: Composer mentions endpoint leaks hidden group membership through PM `allowed_names` check
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N
Source: CVE Program / CVE List V5
Vulnerability Type
信息暴露
Source: CVE Program / CVE List V5
Vulnerability Title
Discourse 安全漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Discourse是Discourse开源的一套开源的社区讨论平台。该平台包括社区、电子邮件和聊天室等功能。 Discourse 2026.3.0-latest.1之前版本、2026.2.1之前版本和2026.1.2之前版本存在安全漏洞，该漏洞源于ComposerController#mentions端点处理不当，可能向任何可以给群组发消息的经过身份验证的用户泄露隐藏的群组成员身份。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
discourse discourse >= 2026.1.0-latest, < 2026.1.2 -

II. Public POCs for CVE-2026-31869

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III. Intelligence Information for CVE-2026-31869

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Vendor Advisories for CVE-2026-31869 (1)

Same Patch Batch · discourse · 2026-03-20 · 16 CVEs total

CVE-2026-33424 5.9 MEDIUM PM access granted through invites after access revocation
CVE-2026-33411 5.4 MEDIUM Discourse's solved topic stream has potential stored XSS in topic title
CVE-2026-33251 5.4 MEDIUM Discourse has a Hidden Solved topics permission bypass
CVE-2026-31805 5.3 MEDIUM Discourse has a poll authorization bypass via post_id array parameter
CVE-2026-33422 3.5 LOW Discourse exposes ip_address of flagged user
CVE-2026-33426 3.5 LOW Discourse users can edit or synonymize hidden tags they can't see
CVE-2026-30888 2.2 LOW Discourse has moderator privilege escalation via arbitrary post_id in suspend/silence endp
CVE-2026-30889 Discourse has Unauthorized Post Data Exposure in discourse-user-notes
CVE-2026-30891 Discourse hasUnauthorized Exposure of Private User Action Types
CVE-2026-32114 Discourse's unscoped status lookups leak restricted metadata
CVE-2026-33291 Discourse user can create Zendesk tickets even when it does not have access to topic
CVE-2026-33423 Discourse staff can modify any user's group notification level
CVE-2026-33425 Discourse has inferable private group membership or existence via exclude_groups parameter
CVE-2026-33427 Discourse Authorization Page Displays Unvalidated Redirect Domain
CVE-2026-33428 Discourse Allows Unauthorized Access to Deleted Posts Index via Group Membership

IV. Related Vulnerabilities

Same product: discourse
Same vendor: discourse
Same weakness: CWE-200

V. Comments for CVE-2026-31869

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