CVE-2026-28759 — Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13,
CVE-2026-28759 — 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
What This Means For You
- If your environment is affected by CWE-863, review your exposure and prioritize patching based on your environment. Monitor vendor advisories for CVE-2026-28759 updates and patches.
Related ATT&CK Techniques
🛡️ Detection Rules
2 rules · 6 SIEM formats2 detection rules auto-generated for this incident, mapped to MITRE ATT&CK. Sigma YAML is free — export to any SIEM format via the Intel Bot.
Mattermost Unauthorized Channel Membership Removal Attempt (CVE-2026-28759)
title: Mattermost Unauthorized Channel Membership Removal Attempt (CVE-2026-28759)
id: scw-2026-05-18-ai-1
status: experimental
level: critical
description: |
Detects attempts to exploit CVE-2026-28759 by sending crafted membership sync messages to Mattermost. This rule specifically looks for POST requests to the channel API endpoint containing 'membership_sync' in the query, indicating a potential attempt to manipulate channel memberships without proper authorization.
author: SCW Feed Engine (AI-generated)
date: 2026-05-18
references:
- https://shimiscyberworld.com/posts/nvd-CVE-2026-28759/
tags:
- attack.initial_access
- attack.t1190
logsource:
category: webserver
detection:
selection:
cs-uri|contains:
- '/api/v4/channels/'
cs-method|exact:
- 'POST'
cs-uri-query|contains:
- 'membership_sync'
condition: selection
falsepositives:
- Legitimate administrative activity
Source: Shimi's Cyber World · License & reuse
Indicators of Compromise
| ID | Type | Indicator |
|---|---|---|
| CVE-2026-28759 | vulnerability | CVE-2026-28759 |
| CWE-863 | weakness | CWE-863 |
Written by SCW Vulnerability Desk
Source & Attribution
| Source Platform | NVD |
| Channel | National Vulnerability Database |
| Published | May 18, 2026 at 11:16 UTC |
This content was curated and summarized by Shimi's Cyber World for informational purposes. It is not copied or republished in full. All intellectual property rights remain with the original author and source.
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