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CVE-2026-6689

*Missing* {{invite_user}} *permission check on team creation allows unprivileged users to set open-invite and allowed-domains team settings*

Mattermost versions 11.6.x <= 11.6.1, 11.5.x <= 11.5.4, 10.11.x <= 10.11.15, 10.11.x <= 10.11.16 Fail to enforce PermissionInviteUser when setting AllowOpenInvite or AllowedDomains during team creation (the check was only applied on update/patch), which allows an authenticated user holding PermissionCreateTeam but not PermissionInviteUser on the resulting team to configure invite-controlled team settings (make the team publicly joinable via open invite and/or constrain membership via allowed domains) that they are not permitted to set on an existing team via POST /api/v4/teams with allow_open_invite: true and/or a non-empty allowed_domains in the request body.. Mattermost Advisory ID: MMSA-2026-00655

CVSS 3.1 · 4.3 Medium EPSS < 1% KEV · Not listed
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CVE-2026-7387

Mattermost group syncable endpoints allow privilege escalation via scheme_admin

Mattermost versions 11.6.x <= 11.6.1, 11.5.x <= 11.5.4, 10.11.x <= 10.11.15, 10.11.x <= 10.11.16 Mattermost fails to require role-management authorization when setting the scheme_admin flag on group syncable link and patch endpoints, which allows a user with group-link permissions to escalate themselves and group members to team or channel admin via crafted API requests.. Mattermost Advisory ID: MMSA-2026-00665

CVSS 3.1 · 8.8 High EPSS < 1% KEV · Not listed
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Mattermost Mattermost Server
CVE-2026-3433

Mattermost fails to scope role_updated websocket events to authorized team and channel members

Mattermost versions 11.6.x <= 11.6.1, 11.5.x <= 11.5.4, 10.11.x <= 10.11.15, 10.11.x <= 10.11.16 fail to restrict role_updated websocket event broadcasts to members of the affected team or channel which allows an authenticated attacker with guest-level access to observe permission scheme change notifications for private teams they are not a member of via the websocket connection.. Mattermost Advisory ID: MMSA-2026-00616

CVSS 3.1 · 4.3 Medium EPSS < 1% KEV · Not listed
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Mattermost Mattermost Server
CVE-2026-6046

Plugin bot username conflict allows user account to be used as bot identity in Mattermost Server

Mattermost versions 11.6.x <= 11.6.1, 11.5.x <= 11.5.4, 10.11.x <= 10.11.15, 10.11.x <= 10.11.16 fail to validate that a username returned during bot registration belongs to a bot account, which allows an unprivileged attacker to intercept private messages sent by plugins via direct message channels by pre-registering a user account with a predictable plugin bot username.. Mattermost Advisory ID: MMSA-2026-00649

CVSS 3.1 · 5.3 Medium EPSS < 1% KEV · Not listed
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Mattermost
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Mattermost Mattermost Server
CVE-2026-3109

Missing timestamp validation in Zoom webhook handler

Mattermost Plugins versions <=11.4 10.11.11.0 fail to validate webhook request timestamps which allows an attacker to corrupt Zoom meeting state in Mattermost via replayed webhook requests. Mattermost Advisory ID: MMSA-2026-00584

CVSS 3.1 · 2.2 Low EPSS < 1% KEV · Not listed
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Mattermost
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Mattermost Mattermost Server
CVE-2026-3116

Improper Input Validation in Zoom Plugin Webhook Handler

Mattermost Plugins versions <=11.4 11.0.4 11.1.3 11.3.2 10.11.11.0 fail to validate incoming request size which allows an authenticated attacker to cause service disruption via the webhook endpoint. Mattermost Advisory ID: MMSA-2026-00589

CVSS 3.1 · 4.9 Medium EPSS < 1% KEV · Not listed
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Mattermost Mattermost Server
CVE-2026-4915

Server panic via outgoing webhook responses

Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to filter nil elements from outgoing webhook attachment payloads before processing, which allows an authenticated user to cause a denial of service (server process termination) via a crafted webhook callback response containing a null attachment entry.. Mattermost Advisory ID: MMSA-2026-00641

CVSS 3.1 · 6.5 Medium EPSS < 1% KEV · Not listed
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Mattermost Mattermost Server
CVE-2026-3117

Instance and webhook GitLab plugin commands were able to be run by non-admin users

Mattermost Plugins versions <=11.5 11.1.5 10.13.11 11.3.4.0 fail to properly check for permissions when processing commands in the Gitlab plugin which allows normal users to uninstall instances or setup webhook connections via the {{gitlab instance {option}}} or the {{/gitlab webhook {option}}} commands. Mattermost Advisory ID: MMSA-2026-00600

CVSS 3.1 · 6.5 Medium EPSS < 1% KEV · Not listed
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Mattermost Mattermost Server
CVE-2026-6342

Group prefix matching bypass for subscriptions

Mattermost Plugins versions <=11.5 11.1.5 10.13.11 11.3.4.0 fail to appropriately check for valid namespaces which allows plugin users to create subscriptions to groups that were not whitelisted via creating groups that share the same prefix as a whitelisted group. Mattermost Advisory ID: MMSA-2026-00601

CVSS 3.1 · 4.3 Medium EPSS < 1% KEV · Not listed
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Mattermost Mattermost Server
CVE-2026-6341

Incomplete group locking implementation

Mattermost Plugins versions <=11.5 11.1.5 10.13.11 11.3.4.0 fail to have API-level checks on which groups the user can create issues or attach comments to which allows a user that is member of multiple groups to create issues to a locked group via direct API requests. Mattermost Advisory ID: MMSA-2026-00602

CVSS 3.1 · 4.3 Medium EPSS < 1% KEV · Not listed
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Mattermost
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Mattermost Mattermost Server
CVE-2026-6334

OAuth authorization code client binding not enforced during token redemption in Mattermost

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13 fail to enforce client identity binding during the OAuth authorization code redemption flow which allows an authenticated OAuth client to redeem authorization codes issued to a different client via a crafted token exchange request.. Mattermost Advisory ID: MMSA-2026-00570

CVSS 3.1 · 3.1 Low EPSS < 1% KEV · Not listed
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Mattermost
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Mattermost Mattermost Server
CVE-2026-28735

GitHub OAuth Scope Validation

Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to validate the OAuth token scope on the callback which allows an authenticated Mattermost user to gain access to private repositories via modifying the scope parameter in the GitHub authorization URL.. Mattermost Advisory ID: MMSA-2026-00628

CVSS 3.1 · 5.4 Medium EPSS < 1% KEV · Not listed
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Mattermost Mattermost Server
CVE-2026-3473

Improper file ownership validation in the Boards API allows unauthorised file access

Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to validate file ownership and access control, which allows an authenticated user to access and download files belonging to other users or teams via crafted Boards API requests using valid file IDs.. Mattermost Advisory ID: MMSA-2026-00620

CVSS 3.1 · 5.9 Medium EPSS < 1% KEV · Not listed
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Mattermost Mattermost Server
CVE-2026-3636

Sanitize team member data returned by API

Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to sanitize team member data when returned via API to users without elevated permissions which allows a user without permissions to get data about team members roles via invoking various team API endpoints.. Mattermost Advisory ID: MMSA-2026-00626

CVSS 3.1 · 4.3 Medium EPSS < 1% KEV · Not listed
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Mattermost
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Mattermost Mattermost Server
CVE-2026-4635

Persistent notification timing attack causing server denial of service

Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to archive the channel before removing persistent notifications which allows authenticated user to crash the server via timing the creation of persistent notification message between the server deleting existing persistent notifications and archiving the channel.. Mattermost Advisory ID: MMSA-2026-00637

CVSS 3.1 · 6.5 Medium EPSS < 1% KEV · Not listed
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Mattermost
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Mattermost Mattermost Server
CVE-2026-4646

Insufficient input validation in GitHub plugin API causes denial of service

Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to validate user-supplied input in API request handlers which allows an authenticated attacker to crash the plugin process via a crafted HTTP request to the PR details endpoint.. Mattermost Advisory ID: MMSA-2026-00638

CVSS 3.1 · 4.3 Medium EPSS < 1% KEV · Not listed
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Mattermost
Products
Mattermost Mattermost Server
CVE-2026-5308

Missing request body size limits on Zoom plugin HTTP endpoints

Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to enforce request body size limits on plugin HTTP endpoints which allows an attacker to cause a denial of service via crafted oversized HTTP requests.. Mattermost Advisory ID: MMSA-2026-00646

CVSS 3.1 · 4.9 Medium EPSS < 1% KEV · Not listed
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Mattermost
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Mattermost Mattermost Server
CVE-2026-5740

Unauthenticated WebSocket binary frame causes denial of service in Mattermost Server

Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to properly validate msgpack-encoded WebSocket frames before memory allocation which allows an unauthenticated remote attacker to crash the server process and cause a full service outage for all users via a crafted binary WebSocket message sent to the public WebSocket endpoint.. Mattermost Advisory ID: MMSA-2026-00647

CVSS 3.1 · 7.5 High EPSS < 1% KEV · Not listed
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Mattermost Mattermost Server
CVE-2026-5755

Denial of service via crafted TIFF file upload

Mattermost versions 11.6.x <= 11.6.0, 11.5.x <= 11.5.2, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to validate the TIFF IFD offset in the image header before allocating memory, which allows authenticated users with file upload or posting permissions to cause a denial of service (server OOM) via uploading a crafted TIFF file or posting a URL that serves one.. Mattermost Advisory ID: MMSA-2026-00648

CVSS 3.1 · 6.5 Medium EPSS < 1% KEV · Not listed
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Mattermost Mattermost Server
CVE-2026-4055

Insufficient permission validation on cross-team playbook run creation

Mattermost versions 11.5.x <= 11.5.1 fail to validate team-level run_create permission against the target team when creating a playbook run which allows an authenticated team member to create runs in teams where they lack permission via specifying a different team ID in the run creation API request. Mattermost Advisory ID: MMSA-2026-00629

CVSS 3.1 · 4.3 Medium EPSS < 1% KEV · Not listed
Vendors
Mattermost
Products
Mattermost Mattermost Server