When mount.ecrpytfs_private before version 87-0ubuntu1.2 calls setreuid() it doesn't also set the effective group id. So when it creates the new version, mtab.tmp, it's created with the group id of the user running mount.ecryptfs_private.
CVSS:3.0/AV:L/AC:L/PR:L/UI:N/S:C/C:N/I:N/A:L
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