Mattermost vulnerabilities

Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21, and 11.8.x <= 11.8.3 fail to validate WebSocket command field types, allowing an authenticated user to crash the plugin process and deny service to all Boards users via a custom_focalboard_SUBSCRIBE_TEAM message with a non-string teamId.

Mattermost versions 11.8.x <= 11.8.2, 11.7.x <= 11.7.6, 10.11.x <= 10.11.21 fail to restrict channel member role assignment to channel-scoped roles, which allows a channel administrator to gain additional channel permissions via the channel member roles API.

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, or bulk-creating boards with an arbitrary channelId.

Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21 fail to enforce run-state validation on write operations for finished playbook runs, which allows a run participant to modify status, checklists, retrospective content, ownership, and participants on completed runs via REST and GraphQL API requests.

Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21 fail to restrict OAuth deauthorization and personal access token management endpoints to direct user sessions, which allowed an OAuth app with a delegated user token to revoke the user's authorizations or tokens for other integrations via account-management endpoints.

Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21 fail to prevent guest users from receiving Board Admin privileges during board archive import, which allows a board member to escalate a guest user to Board Admin via importing a crafted .boardarchive file.

Mattermost versions 11.7.x <= 11.7.6, 11.8.x <= 11.8.3 fail to re-validate that each target channel still belongs to the requesting admin's team when calling the policy unassign endpoint. This allows an authenticated team administrator to remove ABAC policy assignments from channels outside their team after a channel has been moved to another team.

Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21, 11.8.x <= 11.8.3 fail to reconcile SchemeAdmin flags with a user's current role, which allows a user demoted to System Guest to retain Board Admin privileges and perform admin-only operations via the Boards REST API or UI.

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 limit decompressed content size and enforce the configured maximum file size in the Boards archive import handler. An authenticated user can cause memory exhaustion or unbounded disk consumption via a crafted .boardarchive file uploaded to the import endpoint.

Mattermost fails to verify file deletion path, allowing an admin with SAML system-console write permissions to delete arbitrary files outside the config directory via the remove file endpoint.

Mattermost versions 11.6.x through 11.6.5, 10.11.x through 10.11.20, 11.8.x through 11.8.1, 11.7.x through 11.7.4 fail to limit the number of frames and enforce the file size cap on animated GIF uploads. An authenticated attacker can cause a denial of service via a crafted animated GIF uploaded as a custom emoji.

Mattermost versions 11.8.x through 11.8.0, 11.7.x through 11.7.3, 11.6.x through 11.6.5, 10.11.x through 10.11.20 fail to bound the time and resource consumption of server-side document content extraction. An authenticated user with file-upload permission can degrade file uploads for all users by repeatedly uploading small documents that are cheap to upload but expensive to extract, saturating the shared extraction worker pool.

A vulnerability in Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, and 10.11.x <= 10.11.19 fails to check the manage_shared_channels permission in the /share-channel autocomplete handler. This allows an authenticated user without that permission to enumerate configured remote cluster connection metadata via slash command autocomplete.

A vulnerability in Mattermost allows a user with the User Manager role to obtain invite links for private teams via the scheme teams endpoint. Lack of proper sanitization of team objects enables joining or sharing access to those teams.

A vulnerability in Mattermost allows an authenticated user with team access to modify metric settings in another user's playbook via a crafted import or update request with a foreign metric ID. Affected versions are 11.7.x <= 11.7.1, 11.6.x <= 11.6.4, and 10.11.x <= 10.11.19.

A vulnerability in Mattermost allows a user with webhook management permissions to create posts or direct messages attributed to another user. The issue stems from missing validation that the assigned incoming webhook user has access to the target team or channel.

A vulnerability in Mattermost allows a deactivated guest user to obtain an active session using a magic-link token issued before deactivation. The issue affects versions 11.7.x up to 11.7.2 and 11.6.x up to 11.6.4.

A vulnerability in Mattermost versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, and 10.11.x <= 10.11.19 fails to invalidate OAuth refresh tokens upon user account deactivation. This allows a deactivated user or an attacker with a valid refresh token to obtain new functional access tokens via the OAuth refresh token grant endpoint.

A vulnerability in Mattermost allows an authenticated attacker to perform a Denial of Service (DoS) attack on all users in a channel. The issue stems from missing validation of the length and content of message attachment field values, causing catastrophic backtracking in the client-side Markdown parser.

A vulnerability in Mattermost allows an authenticated user without access to a private channel to trigger interactive post actions on posts in that channel. The issue stems from a failure to verify that the channel referenced in an action cookie matches the channel of the target post.

A vulnerability in Mattermost allows an authenticated remote cluster to modify or delete posts authored by local users or other remote clusters in shared channels. The issue is due to missing post ownership verification in the shared channel inbound sync handler.

A vulnerability in Mattermost allows an ordinary group or direct message member to remove all participants from a conversation via the channel patch API. This affects versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, and 10.11.x <= 10.11.19, where the group_constrained flag is not properly restricted to public and private channels that support group synchronization.

A vulnerability in the Mattermost Agents plugin MCP server allows an attacker with access to the MCP server in stdio mode to perform server-side request forgery (SSRF) by supplying internal URLs as file attachments in post creation requests. Affected versions are 10.11.x <= 10.11.18, 11.6.x <= 11.6.3, and 11.5.x <= 11.5.6.

A vulnerability in Mattermost allows an authenticated attacker to exfiltrate data by injecting Markdown image syntax into AI bot tool result posts. Markdown image rendering restrictions are not properly applied to AI bot tool result posts, enabling data leakage to an attacker-controlled server when rendered by a victim's client.

Mattermost versions 11.7.x <= 11.7.0, 11.6.x <= 11.6.2, 11.5.x <= 11.5.5, and 10.11.x <= 10.11.17 fail to invalidate cached authentication state for active WebSocket connections during global session revocation. This 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 versions 11.7.x up to 11.7.0 and 10.11.x up to 10.11.17 fail to enforce bot-specific permission checks on the user active status endpoint. This 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 versions 11.7.x <= 11.7.0, 11.6.x <= 11.6.2, 11.5.x <= 11.5.5, and 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.

Mattermost versions 11.7.x <= 11.7.0, 11.6.x <= 11.6.2, 11.5.x <= 11.5.5, and 10.11.x <= 10.11.17 fail to validate channel ownership of an existing subscription before applying edits. This 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 versions 11.7.x <= 11.7.0, 11.6.x <= 11.6.2, 11.5.x <= 11.5.5, and 10.11.x <= 10.11.17 fail to enforce administrator authorization on the {{setDefaultInstance}} call within the {{/gitlab connect}} command handler, allowing any authenticated user to overwrite the global default GitLab instance configuration via the {{/gitlab connect <instance-name>}} slash command.

Mattermost versions 11.6.x <= 11.6.1, 11.5.x <= 11.5.4, and 10.11.x <= 10.11.16 fail to require role-management authorization when setting the scheme_admin flag on group syncable link and patch endpoints, allowing a user with group-link permissions to escalate themselves and group members to team or channel admin via crafted API requests.

Mattermost versions 11.6.x <= 11.6.1, 11.5.x <= 11.5.4, and 10.11.x <= 10.11.15 fail to sanitize the Remote Cluster API response on PATCH operations. This 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 versions 11.6.x <= 11.6.1, 11.5.x <= 11.5.4, and 10.11.x <= 10.11.16 have a vulnerability that allows improper processing of the FileInfo.Name received from federated peers. An attacker controlling a federated server can exploit this vulnerability to write files to arbitrary locations within the target server's filestore via path traversal sequences in the filename field.

Mattermost versions 11.6.x <= 11.6.1, 11.5.x <= 11.5.4, and 10.11.x <= 10.11.16 fail to require system-level permission when patching protected default system roles. This allows authenticated users with delegated user-management permissions to escalate privileges by altering built-in role permissions via the role patch API.

Mattermost versions 11.6.x <= 11.6.1, 11.5.x <= 11.5.4, and 10.11.x <= 10.11.16 fail to enforce proper permissions when creating teams, allowing authenticated users with PermissionCreateTeam but without PermissionInviteUser to modify team invite settings.

Mattermost versions 11.6.x <= 11.6.1, 11.5.x <= 11.5.4, and 10.11.x <= 10.11.16 fail to validate that a username returned during bot registration belongs to a bot account. This 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 versions 11.6.x <= 11.6.1, 11.5.x <= 11.5.4, and 10.11.x <= 10.11.15 and 10.11.x <= 10.11.16 fail to restrict 'role_updated' websocket event broadcasts to members of the affected team or channel. This allows an authenticated attacker with guest-level access to observe permission scheme change notifications for private teams they are not a member of.

In Mattermost versions 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14, nil elements are not filtered from outgoing webhook attachment payloads before processing. An authenticated user can cause a denial of service (server process termination) via a crafted webhook callback response containing a null attachment entry.

Mattermost versions 11.6.0, 11.5.x up to 11.5.3, 11.4.x up to 11.4.4, and 10.11.x up to 10.11.14 fail to validate the OAuth token scope on the callback, allowing an authenticated user to gain access to private repositories by modifying the scope parameter in the GitHub authorization URL.

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, allowing 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 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, allowing an unauthenticated remote attacker to crash the server process and cause a full service outage via a crafted binary WebSocket message.

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, allowing an attacker to cause a denial of service via crafted oversized HTTP requests.

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, allowing an authenticated attacker to crash the plugin process via a crafted HTTP request to the PR details endpoint.

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, allowing an authenticated user to crash the server by timing the creation of a persistent notification message between the server deleting existing persistent notifications and archiving the channel.

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 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, allowing an authenticated user to access and download files belonging to other users or teams via crafted Boards API requests using valid file IDs.

Mattermost versions 11.6.x through 11.6.0, 11.5.x through 11.5.3, 11.4.x through 11.4.4, and 10.11.x through 10.11.14 fail to properly validate integration URLs for path traversal. This allows a malicious authenticated user to call arbitrary APIs using the system admin's Mattermost auth token via path traversal in the integration action URL.

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, allowing an authenticated team member to create runs in teams where they lack permission by specifying a different team ID in the API request.

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 fail to sanitize sensitive configuration fields before including them in support packet generation, which allows a Mattermost System Admin or any party with access to a support packet to obtain sensitive credentials in plaintext via downloading a support packet from the System Console. Mattermost Advisory ID: MMSA-2026-00607.

Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, and 11.4.x <= 11.4.3 fail to check public/private permissions, allowing members without these permissions to access public playbooks via /get.

Mattermost versions 11.5.x <= 11.5.1 and 11.4.x <= 11.4.3 fail to validate the X-Requested-With header on the burn-on-read reveal endpoint, allowing an authenticated channel member to force the reveal of a burn-on-read message without recipient consent via a crafted Markdown image tag. Mattermost Advisory ID: MMSA-2026-00636.