CVE-2026-8821

Mattermost versions 11.9.x = 11.9.0, 11.8.x = 11.8.4, 11.7.x = 11.7.7, 10.11.x = 10.11.22 fail to validate channel member-management permission during playbook run creation, allowing an authenticated channel member to add an arbitrary user to a restricted channel via the run owner field...

CVE-2026-8821 Playbooks run owner channel membership permission bypass

Mattermost versions 11.9.x = 11.9.0, 11.8.x = 11.8.4, 11.7.x = 11.7.7, 10.11.x = 10.11.22 fail to validate channel member-management permission during playbook run creation, allowing an authenticated channel member to add an arbitrary user to a restricted channel via the run owner field...

CVE-2026-8821

Mattermost versions 11.9.0, 11.8.4, 11.7.7, and 10.11.22 (and below within their respective branches) contain a permission bypass in the Playbooks feature. During playbook run creation, the system fails to validate the channel member-management permission, allowing an authenticated channel member...

CVE-2026-8821 Playbooks run owner channel membership permission bypass

Mattermost versions 11.9.x = 11.9.0, 11.8.x = 11.8.4, 11.7.x = 11.7.7, 10.11.x = 10.11.22 fail to validate channel member-management permission during playbook run creation, allowing an authenticated channel member to add an arbitrary user to a restricted channel via the run owner field...

EUVD-2026-75178

Open WebUI: Channel members can overwrite another member's message via the chat completions endpoint...

CVE-2026-87994 Open WebUI: Channel members can overwrite another member's message via the chat completions endpoint

Open WebUI is an extensible, feature-rich, and user-friendly self-hosted AI platform. From 0.9.5 until 0.11.1, the channel branch of chatcompletion in backend/openwebui/main.py checked channel write access and channel membership for a supplied messageid but did not verify that the calling user...

CVE-2026-87994 Open WebUI: Channel members can overwrite another member's message via the chat completions endpoint

Open WebUI is an extensible, feature-rich, and user-friendly self-hosted AI platform. From 0.9.5 until 0.11.1, the channel branch of chatcompletion in backend/openwebui/main.py checked channel write access and channel membership for a supplied messageid but did not verify that the calling user...

PT-2026-89171

Summary Any member of a channel who can post to it could also replace the text of a message written by a different member. The channel branch of the chat completions endpoint checked that the caller may write to the channel, and that the targeted message belongs to that channel, but never checked...

CVE-2026-16047

Mattermost versions 11.7.x = 11.7.6, 10.11.x = 10.11.21, 11.8.x = 11.8.3 fail to validate that users have read access to a channel before linking a board to it, which allows an authenticated attacker to discover the membership of private channels on the same team via creating, patching, importing...

CVE-2026-16047 Board channel linking without read channel permission validation

Mattermost versions 11.7.x = 11.7.6, 10.11.x = 10.11.21, 11.8.x = 11.8.3 fail to validate that users have read access to a channel before linking a board to it, which allows an authenticated attacker to discover the membership of private channels on the same team via creating, patching, importing...

CVE-2026-16047

Mattermost versions 11.7.x = 11.7.6, 10.11.x = 10.11.21, 11.8.x = 11.8.3 fail to validate that users have read access to a channel before linking a board to it, which allows an authenticated attacker to discover the membership of private channels on the same team via creating, patching, importing...

CVE-2026-16047

Mattermost versions 11.7.x ≤ 11.7.6, 10.11.x ≤ 10.11.21, and 11.8.x ≤ 11.8.3 contain an authorization flaw in the board-to-channel linking functionality. The server fails to verify that the requesting user has read access to a target channel before associating a board with it. An authenticated us...

EUVD-2026-60208

Mattermost versions 11.7.x = 11.7.6, 10.11.x = 10.11.21, 11.8.x = 11.8.3 fail to validate that users have read access to a channel before linking a board to it, which allows an authenticated attacker to discover the membership of private channels on the same team via creating, patching, importing...

CVE-2026-16047: Missing Authorization

Mattermost versions 11.7.x = 11.7.6, 10.11.x = 10.11.21, 11.8.x = 11.8.3 fail to validate that users have read access to a channel before linking a board to it, which allows an authenticated attacker to discover the membership of private channels on the same team via creating, patching, importing...

Open WebUI: Deactivated Channel Members Retain Full Access to Group/DM Channels

Deactivated Channel Members Retain Full Access to Group/DM Channels Affected ComponentChannel membership authorization check:- backend/openwebui/models/channels.py lines 663-673, isuserchannelmember- Used at 15 locations in backend/openwebui/routers/channels.py Affected VersionsCurrent main branc...

PT-2026-59374

Deactivated Channel Members Retain Full Access to Group/DM Channels Affected Component Channel membership authorization check: - backend/open webui/models/channels.py lines 663-673, is user channel member - Used at 15 locations in backend/open webui/routers/channels.py Affected Versions Current...

CVE-2026-2299

The Mattermost Google Drive plugin before version 1.1.0 fails to validate channel membership in the file creation endpoint, allowing authenticated users with a connected Google account to share Google Drive files to unauthorized private channels and disclose private channel membership...

EUVD-2026-39540

The Mattermost Google Drive plugin before version 1.1.0 fails to validate channel membership in the file creation endpoint, allowing authenticated users with a connected Google account to share Google Drive files to unauthorized private channels and disclose private channel membership...

CVE-2026-2299 Improper Access Control in Mattermost Google Drive Plugin File Creation Endpoint

The Mattermost Google Drive plugin before version 1.1.0 fails to validate channel membership in the file creation endpoint, allowing authenticated users with a connected Google account to share Google Drive files to unauthorized private channels and disclose private channel membership...

CVE-2026-2299 Improper Access Control in Mattermost Google Drive Plugin File Creation Endpoint

The Mattermost Google Drive plugin before version 1.1.0 fails to validate channel membership in the file creation endpoint, allowing authenticated users with a connected Google account to share Google Drive files to unauthorized private channels and disclose private channel membership...