Mattermost Plugins versions <=1.1.5 fail to sanitize filenames received from federated peers before using them to construct export destination paths, which allows an administrator of a remote federated Mattermost server to write files to arbitrary locations within the target server's filestore via a malicious filename delivered through the shared-channel attachment sync protocol. Mattermost Advisory ID: MMSA-2026-00659

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 check integration URL for path traversal which allows an malicious authenticated user to call an arbitrary API via system admin Mattermost auth token using via path traversal in integration action URL.. Mattermost Advisory ID: MMSA-2026-00640

Mattermost versions 10.4.x <= 10.4.1, 9.11.x <= 9.11.7, 10.3.x <= 10.3.2, 10.2.x <= 10.2.2 fail to properly validate input when patching and duplicating a board, which allows a user to read any arbitrary file on the system via duplicating a specially crafted block in Boards.

Mattermost versions 11.4.x <= 11.4.0, 11.3.x <= 11.3.1, 11.2.x <= 11.2.3, 10.11.x <= 10.11.11 fail to validate Advanced Logging file target paths which allows system administrators to read arbitrary host files via malicious AdvancedLoggingJSON configuration in support packet generation. Mattermost Advisory ID: MMSA-2025-00562

Mattermost versions 10.8.x <= 10.8.3, 10.5.x <= 10.5.8, 9.11.x <= 9.11.17, 10.10.x <= 10.10.1, 10.9.x <= 10.9.3 fail to validate import directory path configuration which allows admin users to execute arbitrary code via malicious plugin upload to prepackaged plugins directory

Mattermost versions 10.8.x <= 10.8.3, 10.5.x <= 10.5.8, 9.11.x <= 9.11.17, 10.9.x <= 10.9.2 fails to sanitize path traversal sequences in template file destination paths, which allows a system admin to perform path traversal attacks via malicious path components, potentially enabling malicious file placement outside intended directories.

Mattermost versions 10.8.x <= 10.8.3, 10.5.x <= 10.5.8, 10.10.x <= 10.10.0, 10.9.x <= 10.9.3 fail to sanitize file names which allows users with file upload permission to overwrite file attachment thumbnails via path traversal in file streaming APIs.

Mattermost versions 10.8.x <= 10.8.1, 10.7.x <= 10.7.3, 10.5.x <= 10.5.7, 9.11.x <= 9.11.16 fail to sanitize input paths of file attachments in the bulk import JSONL file, which allows a system admin to read arbitrary system files via path traversal.

Mattermost version 2.10.0 and earlier fails to sanitize deeplink paths, which allows an attacker to perform CSRF attacks against the server.

Mattermost webapp fails to validate route parameters in/<TEAM_NAME>/channels/<CHANNEL_NAME> allowing an attacker to perform a client-side path traversal.

Mattermost versions 10.9.x <= 10.9.1, 10.8.x <= 10.8.3, 10.5.x <= 10.5.8, 9.11.x <= 9.11.17 fail to properly validate file paths during plugin import operations which allows restricted admin users to install unauthorized custom plugins via path traversal in the import functionality, bypassing plugin signature enforcement and marketplace restrictions.

Mattermost versions 10.4.x <= 10.4.1, 9.11.x <= 9.11.7, 10.3.x <= 10.3.2, 10.2.x <= 10.2.2 fail to properly validate board blocks when importing boards which allows an attacker could read any arbitrary file on the system via importing and exporting a specially crafted import archive in Boards.

Mattermost fails to validate if a relative path is passed in /plugins/playbooks/api/v0/telemetry/run/<telem_run_id> as a telemetry run ID, allowing an attacker to use a path traversal payload that points to a different endpoint leading to a CSRF attack.