Mattermost versions 9.8.x <= 9.8.0, 9.7.x <= 9.7.4, 9.6.x <= 9.6.2 and 9.5.x <= 9.5.5, when shared channels are enabled, fail to use constant time comparison for remote cluster tokens which allows an attacker to retrieve the remote cluster token via a timing attack during remote cluster token comparison.

Mattermost versions 9.5.x <= 9.5.5 and 9.8.0 fail to properly sanitize the recipients of a webhook event which allows an attacker monitoring webhook events to retrieve the channel IDs of archived or restored channels.

Mattermost versions 9.8.0, 9.7.x <= 9.7.4, 9.6.x <= 9.6.2 and 9.5.x <= 9.5.5 fail to prevent users from specifying a RemoteId for their posts which allows an attacker to specify both a remoteId and the post ID, resulting in creating a post with a user-defined post ID. This can cause some broken functionality in the channel or thread with user-defined posts

Mattermost versions 9.5.x <= 9.5.5 and 9.8.0 fail to sanitize the RemoteClusterFrame payloads before audit logging them which allows a high privileged attacker with access to the audit logs to read message contents.

Mattermost versions 9.5.x <= 9.5.5 and 9.8.0, when using shared channels with multiple remote servers connected, fail to check that the remote server A requesting the server B to update the profile picture of a user is the remote that actually has the user as a local one . This allows a malicious remote A to change the profile images of users that belong to another remote server C that is connected to the server A.

Mattermost Desktop App versions <=5.7.0 fail to correctly prompt for permission when opening external URLs which allows a remote attacker to force a victim over the Internet to run arbitrary programs on the victim's system via custom URI schemes.

Mattermost Desktop App versions <=5.7.0 fail to disable certain Electron debug flags which allows for bypassing TCC restrictions on macOS.

Mattermost versions 9.5.x <= 9.5.3, 9.6.x <= 9.6.1, 8.1.x <= 8.1.12 fail to restrict the audience of the "custom_playbooks_playbook_run_updated" webhook event, which allows a guest on a channel with a playbook run linked to see all the details of the playbook run when the run is marked by finished.

Mattermost versions 9.5.x <= 9.5.3, 9.7.x <= 9.7.1, 9.6.x <= 9.6.1 and 8.1.x <= 8.1.12 fail to check if the email signup configuration option is enabled when a user requests to switch from SAML to Email. This allows the user to switch their authentication mail from SAML to email and possibly edit personal details that were otherwise non-editable and provided by the SAML provider.

Mattermost versions 9.5.x <= 9.5.3, 9.6.x <= 9.6.1 and 8.1.x <= 8.1.12 fail to perform proper input validation on post actions which allows an attacker to run a playbook checklist task command as another user via creating and sharing a deceptive post action that unexpectedly runs a slash command in some arbitrary channel.