Mattermost fails to check if a custom emoji reaction exists when sending it to a post and to limit the amount of custom emojis allowed to be added in a post, allowing an attacker sending a huge amount of non-existent custom emojis in a post to crash the mobile app of a user seeing the post and to crash the server due to overloading when clients attempt to retrive the aforementioned post. 

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 Jira Plugin handling subscriptions fails to check the security level of an incoming issue or limit it based on the user who created the subscription resulting in registered users on Jira being able to create webhooks that give them access to all Jira issues.

Mattermost Jira Plugin fails to protect against logout CSRF allowing an attacker to post a specially crafted message that would disconnect a user's Jira connection in Mattermost only by viewing the message.

Mattermost fails to update the permissions of the current session for a user who was just demoted to guest, allowing freshly demoted guests to change group names.

Mattermost fails to scope the WebSocket response around notified users to a each user separately resulting in the WebSocket broadcasting the information about who was notified about a post to everyone else in the channel.

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 version 8.1.6 and earlier fails to sanitize channel mention data in posts, which allows an attacker to inject markup in the web client.

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. 

Mattermost fails to validate team membership when a user attempts to access a playbook, allowing a user with permissions to a playbook but no permissions to the team the playbook is on to access and modify the playbook. This can happen if the user was once a member of the team, got permissions to the playbook and was then removed from the team.