CVE-2026-9597 Deactivated guest accounts can authenticate via magic-link token in Mattermost REST API login endpoint

Mattermost versions 11.7.x = 11.7.2, 11.6.x = 11.6.4 fail to verify whether a guest account is deactivated before creating a session in the magic-link token login path, which allows a deactivated guest user to obtain a fully functional session via a magic-link token issued prior to deactivation...

CVE-2026-9597

Mattermost advisory MMSA-2026-00681 documents a vulnerability in Mattermost where versions 11.7.x <= 11.7.2 and 11.6.x

CVE-2026-6850

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

CVE-2026-6850 Crafted message attachment causes client-side denial of service via markdown parser regex backtracking in Mattermost

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...

CVE-2026-10106

Mattermost vulnerable versions: 11.7.x ≤ 11.7.2, 11.6.x ≤ 11.6.4, and 10.11.x ≤ 10.11.19. The issue stems from failing to verify that the channel referenced in an action cookie matches the target post’s channel, enabling an authenticated user without access to a private channel to trigger interac...

CVE-2026-10106 Unauthorized users can trigger interactive post actions in private channels via action cookie channel mismatch in Mattermost

Mattermost versions 11.7.x = 11.7.2, 11.6.x = 11.6.4, 10.11.x = 10.11.19 fail to verify that the channel referenced in an action cookie matches the channel of the target post, which allows an authenticated user without access to a private channel to trigger interactive post actions on posts in th...

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

Mattermost is affected in versions 11.7.x <= 11.7.2, 11.6.x <= 11.6.4, and 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-2026-9708

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

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

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-2026-5818 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-2026-5817 Mattermost doesn't validate file ownership and access control in github.com/mattermost/mattermost-server

Mattermost doesn't validate file ownership and access control 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...

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-2026-5836 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-2026-5845 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...