Mattermost does not strip embeds from metadata when broadcasting posted events.
This allows users to include arbitrary embeds in posts, which are then broadcasted via websockets. This can be exploited in many ways, for example to create permalinks with fully customizable content or to trigger a client Side Denial of Service (DoS) by sending a permalink with a non-string message.
The advisory metadata references the appropriate go pseudo version available from pkg.go.dev
Mattermost versions 9.11.x <= 9.11.0, 9.10.x <= 9.10.1, 9.9.x <= 9.9.2 and 9.5.x <= 9.5.8 fail to properly authorize requests when viewing archived channels is disabled, which allows an attacker to retrieve post and file information about archived channels. Examples are flagged or unread posts as well as files.
Mattermost versions 9.11.x <= 9.11.2, and 9.5.x <= 9.5.10 fail to protect the mfa code against replay attacks, which allows an attacker to reuse the MFA code within ~30 seconds
Mattermost versions 9.10.x <= 9.10.2, 9.11.x <= 9.11.1, 9.5.x <= 9.5.9 fail to sanitize user inputs in the frontend that are used for redirection which allows for a one-click client-side path traversal that is leading to CSRF in Playbooks
Mattermost versions 10.0.x <= 10.0.0 and 9.11.x <= 9.11.2 fail to properly query ElasticSearch when searching for the channel name in channel switcher which allows an attacker to get private channels names of channels that they are not a member of, when Elasticsearch v8 was enabled.
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 10.4.x <= 10.4.2, 10.3.x <= 10.3.3, 9.11.x <= 9.11.8, 10.5.x <= 10.5.0 fail to restrict bookmark creation and updates in archived channels, which allows authenticated users created or update bookmarked in archived channels
Mattermost versions 9.11.X <= 9.11.1, 9.5.x <= 9.5.9 icorrectly issues two sessions when using desktop SSO - one in the browser and one in desktop with incorrect settings.