Mattermost versions 9.9.x <= 9.9.0 and 9.5.x <= 9.5.6 fail to validate the source of sync messages and only allow the correct remote IDs, which allows a malicious remote to set arbitrary RemoteId values for synced users and therefore claim that a user was synced from another remote.
CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:N/I:L/A:N
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