CVE-2026-10085

Mattermost is affected in versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, and 10.11.x

CVE-2026-10085 Ordinary group/direct message member can enable group_constrained and remove all channel participants

Mattermost versions 11.7.x = 11.7.2, 11.6.x = 11.6.4, 10.11.x = 10.11.19 fail to restrict the groupconstrained channel flag to public and private channels that support group synchronization, which allows an ordinary group or direct message member to remove all participants from the conversation v...

CVE-2026-10085 Ordinary group/direct message member can enable group_constrained and remove all channel participants

Mattermost versions 11.7.x = 11.7.2, 11.6.x = 11.6.4, 10.11.x = 10.11.19 fail to restrict the groupconstrained channel flag to public and private channels that support group synchronization, which allows an ordinary group or direct message member to remove all participants from the conversation v...

CVE-2026-9708 Incoming webhook user attribution via unvalidated webhook owner

Mattermost versions 11.7.x = 11.7.2, 11.6.x = 11.6.4, 10.11.x = 10.11.19 fail to validate that an assigned incoming webhook user has access to the target team or channel, which allows a requester with webhook management permissions to create posts or direct messages attributed to another user via...

CVE-2026-9708

Mattermost vulnerable versions (11.7.x <= 11.7.2, 11.6.x <= 11.6.4, 10.11.x

CVE-2026-9708 Incoming webhook user attribution via unvalidated webhook owner

Mattermost versions 11.7.x = 11.7.2, 11.6.x = 11.6.4, 10.11.x = 10.11.19 fail to validate that an assigned incoming webhook user has access to the target team or channel, which allows a requester with webhook management permissions to create posts or direct messages attributed to another user via...

CVE-2026-10103 Authenticated remote cluster can modify or delete posts it does not own in Mattermost Connected Workspaces shared channels

Mattermost versions 11.7.x = 11.7.2, 11.6.x = 11.6.4, 10.11.x = 10.11.19 fail to verify post ownership in the shared channel inbound sync handler, which allows an authenticated remote cluster to modify or delete posts authored by local users or other remotes via crafted sync messages referencing...

CVE-2026-10103

Mattermost versions affected: 11.7.x (up to 11.7.2), 11.6.x (up to 11.6.4), and 10.11.x (up to 10.11.19) have a flaw in the shared channel inbound sync handler that fails to verify post ownership. This allows an authenticated remote cluster to modify or delete posts authored by local users or oth...

CVE-2026-10103 Authenticated remote cluster can modify or delete posts it does not own in Mattermost Connected Workspaces shared channels

Mattermost versions 11.7.x = 11.7.2, 11.6.x = 11.6.4, 10.11.x = 10.11.19 fail to verify post ownership in the shared channel inbound sync handler, which allows an authenticated remote cluster to modify or delete posts authored by local users or other remotes via crafted sync messages referencing...

CVE-2026-9571

Mattermost CVE-2026-9571 affects versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, and 10.11.x

CVE-2026-9571 Deactivated user accounts can continue to obtain valid OAuth access tokens via refresh token grant in Mattermost

Mattermost versions 11.7.x = 11.7.2, 11.6.x = 11.6.4, 10.11.x = 10.11.19 fail to invalidate OAuth refresh tokens upon user account deactivation, which allows a deactivated user or an attacker in possession of a valid refresh token to obtain new functional access tokens via the OAuth refresh token...

PT-2026-57661

Mattermost versions 11.7.x = 11.7.2, 11.6.x = 11.6.4, 10.11.x = 10.11.19 fail to validate the length and content of message attachment field values, which allows an authenticated attacker to cause a denial of service for all users in a channel via a post containing a specially crafted payload tha...

GO-2026-5818 Mattermost doesn't sanitize team member data when returned via API to users without elevated permissions in github.com/mattermost/mattermost-server

Mattermost doesn't sanitize team member data when returned via API to users without elevated permissions in github.com/mattermost/mattermost-server...

GO-2026-5845 Mattermost doesn't filter nil elements from outgoing webhook attachment payloads before processing in github.com/mattermost/mattermost-server

Mattermost doesn't filter nil elements from outgoing webhook attachment payloads before processing in github.com/mattermost/mattermost-server...

GO-2026-5833 Mattermost doesn't validate user-supplied input in API request handlers in github.com/mattermost/mattermost-plugin-github

Mattermost doesn't validate user-supplied input in API request handlers in github.com/mattermost/mattermost-plugin-github...

GO-2026-5820 Mattermost doesn't archive the channel before removing persistent notifications in github.com/mattermost/mattermost-server

Mattermost doesn't archive the channel before removing persistent notifications in github.com/mattermost/mattermost-server...

GO-2026-5836 Mattermost doesn't enforce request body size limits on plugin HTTP endpoints in github.com/mattermost/mattermost-plugin-github

Mattermost doesn't enforce request body size limits on plugin HTTP endpoints in github.com/mattermost/mattermost-plugin-github...

GO-2026-5838 Mattermost doesn't properly validate msgpack-encoded WebSocket frames before memory allocation in github.com/mattermost/mattermost-server

Mattermost doesn't properly validate msgpack-encoded WebSocket frames before memory allocation in github.com/mattermost/mattermost-server...

GO-2026-5835 Mattermost doesn't validate the TIFF IFD offset in the image header before allocating memory in github.com/mattermost/mattermost-server

Mattermost doesn't validate the TIFF IFD offset in the image header before allocating memory in github.com/mattermost/mattermost-server...

GO-2026-5839 Mattermost allows authenticated users to gain access to private repositories in github.com/mattermost/mattermost-plugin-github

Mattermost allows authenticated users to gain access to private repositories in github.com/mattermost/mattermost-plugin-github...