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CVE-2026-28759— Insufficient authorization in shared channel membership sync allows remote cluster to remove users from arbitrary channels

Quick assessment

Affected
Mattermost Mattermost
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.

Mattermost是美国Mattermost公司的一个开源协作平台。 Mattermost 11.5.1及之前的11.5.x版本、10.11.13及之前的10.11.x版本和11.4.3及之前的11.4.x版本存在安全漏洞，该漏洞源于共享频道成员同步时未验证远程集群是否有频道访问权限，可能导致恶意远程集群移除任意用户。

CVSS 4.3 · Medium EPSS 0.15% · P5

Affected Version Matrix 7

VendorProduct Version RangeStatus
Mattermost Mattermost 11.5.0≤ 11.5.1 affected
10.11.0≤ 10.11.13 affected
11.4.0≤ 11.4.3 affected
11.6.0 unaffected
11.5.2 unaffected
10.11.14 unaffected
11.4.4 unaffected
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I. Basic Information for CVE-2026-28759

Vulnerability Information
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Vulnerability Title
Insufficient authorization in shared channel membership sync allows remote cluster to remove users from arbitrary channels
Source: CVE Program / CVE List V5
Vulnerability Description
Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 fail to validate that a remote cluster has access to a channel before processing membership removal requests during shared channel membership sync, which allows a malicious remote cluster to remove any user from any channel, including private channels, via crafted membership sync messages targeting channels the remote cluster is not authorized to access. Mattermost Advisory ID: MMSA-2026-00576
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N
Source: CVE Program / CVE List V5
Vulnerability Type
授权机制不正确
Source: CVE Program / CVE List V5
Vulnerability Title
Mattermost 安全漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Mattermost是美国Mattermost公司的一个开源协作平台。 Mattermost 11.5.1及之前的11.5.x版本、10.11.13及之前的10.11.x版本和11.4.3及之前的11.4.x版本存在安全漏洞，该漏洞源于共享频道成员同步时未验证远程集群是否有频道访问权限，可能导致恶意远程集群移除任意用户。
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
Mattermost Mattermost 11.5.0 ~ 11.5.1 -

II. Public POCs for CVE-2026-28759

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III. Intelligence Information for CVE-2026-28759

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Same Patch Batch · Mattermost · 2026-05-18 · 21 CVEs total

CVE-2026-6346 8.7 HIGH Sensitive credentials exposed in plaintext in Mattermost support packets
CVE-2026-6347 7.6 HIGH Mattermost Calls plugin exposes TURN server credentials in plaintext in support packets
CVE-2026-6345 6.5 MEDIUM Prevent password disclosure and force reset during Slack import
CVE-2026-5163 6.5 MEDIUM Missing authorization check in AI message rewrite endpoint allows access to private thread
CVE-2026-3117 6.5 MEDIUM Instance and webhook GitLab plugin commands were able to be run by non-admin users
CVE-2026-3471 6.5 MEDIUM Opening a window with {{javascript:alert()}} as URL causes crash in the Mattermost Desktop
CVE-2026-2325 4.3 MEDIUM Improper Input Validation in MS Teams Meetings API Handler
CVE-2026-3637 4.3 MEDIUM Mattermost fails to enforce create_post permission when editing posts
CVE-2026-6342 4.3 MEDIUM Group prefix matching bypass for subscriptions
CVE-2026-6341 4.3 MEDIUM Incomplete group locking implementation
CVE-2026-6340 4.3 MEDIUM Memory Exhaustion via Malicious 7zip File Upload
CVE-2026-6339 4.3 MEDIUM Missing request origin validation on burn-on-read reveal endpoint
CVE-2026-6343 4.3 MEDIUM Mattermost Playbooks Plugin fails to enforce view permissions in list endpoints, allowing
CVE-2026-28732 4.3 MEDIUM Slash command trigger-word update allowed command hijacking
CVE-2026-3495 3.8 LOW Unescaped variables during error page composition
CVE-2026-4273 3.7 LOW Insufficient token rotation validation in remote cluster invite confirmation
CVE-2026-6333 3.5 LOW SSRF via Host Header Spoofing in Custom Slash Commands
CVE-2026-4643 3.5 LOW Calling window.close() from server-side content causes crash in the Mattermost Desktop App
CVE-2026-6334 3.1 LOW OAuth authorization code client binding not enforced during token redemption in Mattermost
CVE-2026-4286 3.1 LOW Playbooks Plugin fails to validate team transfers, allowing unauthorized removal of member

IV. Related Vulnerabilities

Same product: Mattermost
Same vendor: Mattermost
Same weakness: CWE-863

V. Comments for CVE-2026-28759

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