GHSA-q22q-2rrf-m27p Mattermost allows unsolicited invites to expose access to local channels

• Go github.com/mattermost/mattermost/server/v8: >= 9.9.0, < 9.9.1 → fixed in 9.9.1 • Go github.com/mattermost/mattermost/server/v8: >= 9.5.0, < 9.5.7 → fixed in 9.5.7 • Go github.com/mattermost/mattermost/server/v8: >= 9.7.0, < 9.7.6 → fixed in 9.7.6 • Go github.com/mattermost/mattermost/server/v8: >= 9.8.0, < 9.8.1 → fixed in 9.8.1

Mattermost versions 9.9.x <= 9.9.0, 9.5.x <= 9.5.6, 9.7.x <= 9.7.5 and 9.8.x <= 9.8.1 fail to disallow unsolicited invites to expose access to local channels, when shared channels are enabled, which allows a malicious remote to send an invite with the ID of an existing local channel, and that local channel will then become shared without the consent of the local admin.