Mattermost fails to properly validate a gif image file, allowing an attacker to consume a significant amount of server resources, making the server unresponsive for an extended period of time by linking to specially crafted image file.

Mattermost fails to properly validate markdown, allowing an attacker to crash the server via a specially crafted markdown input.

Mattermost fails to verify channel membership when linking a board to a channel allowing a low-privileged authenticated user to link a Board to a private channel they don't have access to,

Mattermost Boards fail to properly validate a board link, allowing an attacker to crash a channel by posting a specially crafted boards link.

Mattermost fails to disable public Boards after the "Enable Publicly-Shared Boards" configuration option is disabled, resulting in previously-shared public Boards to remain accessible.