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CVE-2026-4339

SSRF via unvalidated attachment URLs in Mattermost Agents plugin MCP server

Mattermost versions 10.11.x <= 10.11.18, 11.6.x <= 11.6.3, 11.5.x <= 11.5.6 fail to validate attachment URLs against internal or private IP ranges in the Mattermost Agents plugin MCP server which allows an attacker with access to the MCP server in stdio mode to perform server-side request forgery (SSRF) and exfiltrate data from internal network services via supplying internal URLs as file attachments in post creation requests.. Mattermost Advisory ID: MMSA-2026-00635

CVSS 3.1 · 6.5 Medium EPSS < 1% KEV · Not listed
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Mattermost Mattermost Server
CVE-2026-8823

User Manager can demote bot accounts to guest without bot-management permission

Mattermost versions 11.7.x <= 11.7.0, 10.11.x <= 10.11.17 fail to validate bot targets when demoting users to guests which allows a lower-privileged administrator to degrade arbitrary bot accounts via the standard demote-user API.. Mattermost Advisory ID: MMSA-2026-00669

CVSS 3.1 · 3.8 Low EPSS < 1% KEV · Not listed
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Mattermost
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Mattermost Mattermost Server
CVE-2026-8074

Improper Permission Check Allows User Manager to Deactivate Bot Accounts

Mattermost versions 11.7.x <= 11.7.0, 10.11.x <= 10.11.17 fail to enforce bot-specific permission checks on the user active status endpoint, which allows a User Manager with user management write access but no Integrations access to deactivate bot accounts via the PUT /api/v4/users/{id}/active API endpoint.. Mattermost Advisory ID: MMSA-2026-00667

CVSS 3.1 · 3.8 Low EPSS < 1% KEV · Not listed
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Mattermost
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Mattermost Mattermost Server
CVE-2026-9162

Global session revocation does not invalidate active WebSocket connections

Mattermost versions 11.7.x <= 11.7.0, 11.6.x <= 11.6.2, 11.5.x <= 11.5.5, 10.11.x <= 10.11.17 fail to invalidate cached authentication state for active WebSocket connections during global session revocation, which allows a user with an existing WebSocket connection to remain authenticated and continue receiving real-time events until the cached session expires or the client reconnects.. Mattermost Advisory ID: MMSA-2026-00664

CVSS 3.1 · 4.3 Medium EPSS < 1% KEV · Not listed
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Mattermost
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Mattermost Mattermost Server
CVE-2026-5139

GitLab Plugin Allows Non-Admin Users to Modify Default Instance Configuration

Mattermost versions 11.7.x <= 11.7.0, 11.6.x <= 11.6.2, 11.5.x <= 11.5.5, 10.11.x <= 10.11.17 fail to enforce administrator authorization on the {{setDefaultInstance}} call within the {{/gitlab connect}} command handler, which allows any authenticated user to overwrite the global default GitLab instance configuration via the {{/gitlab connect <instance-name>}} slash command.. Mattermost Advisory ID: MMSA-2026-00644

CVSS 3.1 · 5.4 Medium EPSS < 1% KEV · Not listed
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Mattermost
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Mattermost Mattermost Server
CVE-2026-6673

Mattermost Jira plugin had unauthenticated {{/ac/installed}} lifecycle callback during pending Jira Cloud install

Mattermost versions 11.7.x <= 11.7.0, 11.6.x <= 11.6.2, 11.5.x <= 11.5.5, 10.11.x <= 10.11.17 fail to authenticate Atlassian Connect installed callbacks, allowing a remote unauthenticated attacker to inject a rogue sharedSecret and disrupt the Jira integration via POST to /ac/installed during the pending-install window.. Mattermost Advisory ID: MMSA-2026-00654

CVSS 3.1 · 6.4 Medium EPSS < 1% KEV · Not listed
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Mattermost
Products
Mattermost Mattermost Server
CVE-2026-6062

IDOR in Jira plugin subscription edit endpoint

Mattermost versions 11.7.x <= 11.7.0, 11.6.x <= 11.6.2, 11.5.x <= 11.5.5, 10.11.x <= 10.11.17 Fail to validate channel ownership of an existing subscription before applying edits which allows an authenticated attacker to hijack subscriptions from channels they have no access to via a crafted PUT request to the subscription edit endpoint.. Mattermost Advisory ID: MMSA-2026-00650

CVSS 3.1 · 6.4 Medium EPSS < 1% KEV · Not listed
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Mattermost
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Mattermost Mattermost Server
CVE-2026-4053

post edit time limit is not enforced on some post update operations

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13 fail to enforce the PostEditTimeLimit on non-message post fields which allows an authenticated user to modify post file attachments, props, and pin status after the edit window has expired via the post patch and update API endpoints.. Mattermost Advisory ID: MMSA-2026-00631

CVSS 3.1 · 3.1 Low EPSS < 1% KEV · Not listed
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Mattermost
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Mattermost Mattermost Server
CVE-2025-31363

Data exfiltration via AI plugin Jira tool

Mattermost versions 10.4.x <= 10.4.2, 10.5.x <= 10.5.0, 9.11.x <= 9.11.9 fail to restrict domains the LLM can request to contact upstream which allows an authenticated user to exfiltrate data from an arbitrary server accessible to the victim via performing a prompt injection in the AI plugin's Jira tool.

CVSS 3.1 · 3 Low EPSS < 1% KEV · Not listed
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Mattermost
Products
Mattermost Server
CVE-2026-8683

Overly long URLs crash the Mattermost Desktop App

Mattermost Desktop App versions <=6.1 5.5.13.0 fail to account for attempting to open extremely long URLs in the Mattermost Desktop App which allows a malicious server owner to crash the application via including a script to call window.open on a very large URL. Mattermost Advisory ID: MMSA-2026-00652

CVSS 3.1 · 6.5 Medium EPSS < 1% KEV · Not listed
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Mattermost Mattermost Desktop
CVE-2026-6517

Mattermost Desktop App fails to restrict the allow list of domains which NTLM credentials are passed

Mattermost Desktop App versions <=6.1 5.5.13.0 fail to restrict the allow list of domains to which NTLM credentials were forwarded to in the Mattermost Desktop App which allows any user on a server without the image proxy enabled to intercept other users credentials via embedding an image that routes to an external web server. Mattermost Advisory ID: MMSA-2026-00651

CVSS 3.1 · 6.3 Medium EPSS < 1% KEV · Not listed
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Mattermost
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Mattermost Mattermost Desktop
CVE-2026-6961

CVE-2026-6961: Path traversal via unsanitized FileInfo.Name in Mattermost federation sync

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 sanitize FileInfo.Name received from federated peers during shared channel file sync, which allows an attacker who controls a federated server to write files to arbitrary locations within the target server's filestore via path traversal sequences in the filename field.. Mattermost Advisory ID: MMSA-2026-00661

CVSS 3.1 · 7.6 High EPSS < 1% KEV · Not listed
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Mattermost
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Mattermost Mattermost Server
CVE-2026-7184

Mattermost Remote Cluster PATCH API Leaks Authentication Tokens

Mattermost versions 11.6.x <= 11.6.1, 11.5.x <= 11.5.4, 10.11.x <= 10.11.15 fail to sanitize the Remote Cluster API response on PATCH operations, which allows authenticated users with the {{manage_secure_connections}} permission to obtain remote cluster authentication tokens via a PATCH request to the remote cluster endpoint.. Mattermost Advisory ID: MMSA-2026-00662

CVSS 3.1 · 6.5 Medium EPSS < 1% KEV · Not listed
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Mattermost
Products
Mattermost Mattermost Server
CVE-2026-6739

Mattermost: Delegated admins could patch protected default system roles

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 require system-level permission when patching protected default system roles, which allows authenticated users with delegated user-management permissions to escalate privileges by altering built-in role permissions via the role patch API.. Mattermost Advisory ID: MMSA-2026-00656

CVSS 3.1 · 6.7 Medium EPSS < 1% KEV · Not listed
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Mattermost Mattermost Server
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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Mattermost
Products
Mattermost Mattermost Server
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
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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
Products
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
Products
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
Products
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
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Mattermost Mattermost Server