Mattermost fails to properly check the creator of an attached file when adding the file to a draft post, potentially exposing unauthorized file information.

Mattermost fails to delete the attachments when deleting a message in a thread allowing a simple user to still be able to access and download the attachment of a deleted message

Mattermost fails to properly authorize the requests fetching team associated AD/LDAP groups, allowing a user to fetch details of AD/LDAP groups of a team that they are not a member of. 

Mattermost versions 10.5.x <= 10.5.12, 10.11.x <= 10.11.2 fail to properly validate guest user permissions when accessing channel information which allows guest users to discover active public channels and their metadata via the `/api/v4/teams/{team_id}/channels/ids` endpoint

Mattermost fails to check channel membership when accessing message threads, allowing an attacker to access arbitrary posts by using the message threads API.

An issue was discovered in Mattermost Server before 5.7.1, 5.6.4, 5.5.3, and 4.10.6. It does not honor flags API permissions when deciding whether a user can receive intra-team posts.

Mattermost versions 11.1.x <= 11.1.2, 10.11.x <= 10.11.9, 11.2.x <= 11.2.1 fail to validate user permissions when creating Jira issues from Mattermost posts, which allows an authenticated attacker with access to the Jira plugin to read post content and attachments from channels they do not have access to via the /create-issue API endpoint by providing the post ID of an inaccessible post.. Mattermost Advisory ID: MMSA-2025-00550

An issue was discovered in Mattermost Server before 3.0.0. It potentially allows attackers to obtain sensitive information (credential fields within config.json) via the System Console UI.

Mattermost versions 10.5.x <= 10.5.8, 9.11.x <= 9.11.17 fail to properly validate access controls which allows any authenticated user to download sensitive files via board file download endpoint using UUID enumeration

Mattermost versions 9.10.x <= 9.10.1, 9.9.x <= 9.9.2, 9.5.x <= 9.5.8 fail to limit access to channels files that have not been linked to a post which allows an attacker to view them in channels that they are a member of.

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 properly validate cache keys for link metadata which allows authenticated users to access unauthorized posts and poison link previews via hash collision attacks on FNV-1 hashing

Mattermost versions 9.5.x <= 9.5.3, 9.6.x <= 9.6.1, 8.1.x <= 8.1.12 fail to restrict the audience of the "custom_playbooks_playbook_run_updated" webhook event, which allows a guest on a channel with a playbook run linked to see all the details of the playbook run when the run is marked by finished.

Mattermost versions 9.5.x <= 9.5.9 fail to properly filter the channel data when ElasticSearch is enabled which allows a user to get private channel names by using cmd+K/ctrl+K.

Mattermost versions 9.10.x <= 9.10.2, 9.11.x <= 9.11.1, 9.5.x <= 9.5.9 and 10.0.x <= 10.0.0 fail to properly authorize the requests to /api/v4/channels  which allows a User or System Manager, with "Read Groups" permission but with no access for channels to retrieve details about private channels that they were not a member of by sending a request to /api/v4/channels.

Mattermost versions 10.11.x <= 10.11.3, 10.5.x <= 10.5.11 fail to properly validate team membership permissions in the Add Channel Member API which allows users from one team to access user metadata and channel membership information from other teams via the API endpoint

Mattermost fails to perform correct authorization checks when creating a playbook action, allowing users without access to the playbook to create playbook actions. If the playbook action created is to post a message in a channel based on specific keywords in a post, some playbook information, like the name, can be leaked. 

An issue was discovered in Mattermost Server before 3.0.0. It offers superfluous APIs for a Team Administrator to view account details.

An issue was discovered in Mattermost Server before 2.2.0. It allows unintended access to information stored by a web browser.

Mattermost fails to perform proper authorization in the /plugins/focalboard/api/v2/users endpoint allowing an attacker who is a guest user and knows the ID of another user to get their information (e.g. name, surname, nickname) via Mattermost Boards.

Mattermost fails to check the Show Full Name option at the /api/v4/teams/TEAM_ID/top/team_members endpoint allowing a member to get the full name of another user even if the Show Full Name option was disabled

Mattermost versions <11 fail to enforce multi-factor authentication on WebSocket connections which allows unauthenticated users to access sensitive information via WebSocket events

Mattermost fails to properly verify the permissions needed for viewing archived public channels,  allowing a member of one team to get details about the archived public channels of another team via the GET /api/v4/teams/<team-id>/channels/deleted endpoint.

Mattermost fails to perform authorization checks in the /plugins/playbooks/api/v0/runs/add-to-timeline-dialog endpoint of the Playbooks plugin allowing an attacker to get limited information about a post if they know the post ID

Mattermost versions 9.5.x <= 9.5.3, 9.6.x <= 9.6.1 and 8.1.x <= 8.1.12 fail to enforce proper access controls which allows user to view arbitrary post contents via the /playbook add slash command

Mattermost versions 9.5.x <= 9.5.8 fail to properly authorize access to archived channels when viewing archived channels is disabled, which allows an attacker to view posts and files of archived channels via file links.

Mattermost fails to properly validate the "Show Full Name" option in a few endpoints in Mattermost Boards, allowing a member to get the full name of another user even if the Show Full Name option was disabled. 

Mattermost versions 9.9.x <= 9.9.1, 9.5.x <= 9.5.7, 9.10.x <= 9.10.0, 9.8.x <= 9.8.2, when shared channels are enabled, fail to redact remote users' original email addresses stored in user props when email addresses are otherwise configured not to be visible in the local server."

The Guest account feature in Mattermost version 6.7.0 and earlier fails to properly restrict the permissions, which allows a guest user to fetch a list of all public channels in the team, in spite of not being part of those channels.

Mattermost versions 9.6.x <= 9.6.0, 9.5.x <= 9.5.2, 9.4.x <= 9.4.4 and 8.1.x <= 8.1.11 fail to remove detailed error messages in API requests even if the developer mode is off which allows an attacker to get information about the server such as the full path were files are stored

Mattermost fails to properly restrict requests to localhost/intranet during the interactive dialog, which could allow an attacker to perform a limited blind SSRF.

Unrestricted information disclosure of all users in Mattermost version 6.7.0 and earlier allows team members to access some sensitive information by directly accessing the APIs.

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 fail to sanitize the team invite ID in the POST /api/v4/teams/:teamId/restore endpoint which allows an team admin with no member invite privileges to get the team’s invite id.

Mattermost versions 9.5.x <= 9.5.3, 9.7.x <= 9.7.1 and 8.1.x <= 8.1.12 fail to perform a proper authorization check in the /api/v4/groups/<group-id>/channels/<channel-id>/link endpoint which allows a user to learn the members of an AD/LDAP group that is linked to a team by adding the group to a channel, even if the user has no access to the team.

Mattermost versions 9.5.x <= 9.5.3, 9.6.x <= 9.6.1 and 8.1.x <= 8.1.12 fail to perform proper access control which allows a guest to get the metadata of a public playbook run that linked to the channel they are guest via sending an RHSRuns GraphQL query request to the server

Mattermost versions 10.5.x <= 10.5.4, 9.11.x <= 9.11.13 fail to properly restrict API access to team information, allowing guest users to bypass permissions and view information about public teams they are not members of via a direct API call to /api/v4/teams/{team_id}.

Mattermost versions 10.5.x <= 10.5.6, 10.8.x <= 10.8.1, 10.7.x <= 10.7.3, 9.11.x <= 9.11.16 fail to verify authorization when retrieving cached posts by PendingPostID which allows an authenticated user to read posts in private channels they don't have access to via guessing the PendingPostID of recently created posts.

Mattermost versions <11.0 fail to properly enforce the "Allow users to view archived channels" setting which allows regular users to access archived channel content and files via the "Open in Channel" functionality from followed threads

One of the API in Mattermost version 6.4.1 and earlier fails to properly protect the permissions, which allows the authenticated members with restricted custom admin role to bypass the restrictions and view the server logs and server config.json file contents.

Mattermost fails to check the required permissions in the POST /api/v4/channels/stats/member_count API resulting in channel member counts being leaked to a user without permissions.

Mattermost fails to properly restrict the access of files attached to posts in an archived channel, resulting in members being able to access files of archived channels even if the “Allow users to view archived channels” option is disabled.

Mattermost 6.3.0 and earlier fails to protect email addresses of the creator of the team via one of the APIs, which allows authenticated team members to access this information resulting in sensitive & private information disclosure.

Mattermost version 8.1.x before 8.1.9 fails to sanitize data associated with permalinks when a plugin updates an ephemeral post, allowing an authenticated attacker who can control the ephemeral post update to access individual posts' contents in channels they are not a member of.

Mattermost versions 8.1.x before 8.1.9, 9.2.x before 9.2.5, and 9.3.0 fail to sanitize the metadata on posts containing permalinks under specific conditions, which allows an authenticated attacker to access the contents of individual posts in channels they are not a member of.

Mattermost versions 10.7.x <= 10.7.0, 10.5.x <= 10.5.3, 9.11.x <= 9.11.12 fails to properly enforce access control restrictions for System Manager roles, allowing authenticated users with System Manager privileges to view team details they should not have access to via direct API requests to team endpoints, even when explicitly configured with 'No access' to Teams in the System Console.

When archiving a team, Mattermost fails to sanitize the related Websocket event sent to currently connected clients. This allows the clients to see the name, display name, description, and other data about the archived team.

Mattermost versions 10.5.x <= 10.5.5, 9.11.x <= 9.11.15, 10.8.x <= 10.8.0, 10.7.x <= 10.7.2, 10.6.x <= 10.6.5 fail to properly enforce channel member management permissions in playbook runs, allowing authenticated users without the 'Manage Channel Members' permission to add or remove users from public and private channels by manipulating playbook run participants when the run is linked to a channel.

Mattermost versions 10.5.x <= 10.5.5, 9.11.x <= 9.11.15, 10.8.x <= 10.8.0, 10.7.x <= 10.7.2, 10.6.x <= 10.6.5 fail to properly retrieve requestorInfo from playbooks handler for guest users which allows an attacker access to the playbook run.

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.

Mattermost fails to check the "Show Full Name" setting when rendering the result for the /plugins/focalboard/api/v2/users API call, allowing an attacker to learn the full name of a board owner.

Mattermost allows an attacker to request a preview of an existing message when creating a new message via the createPost API call, disclosing the contents of the linked message.