7PK - Security Features
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.
CWE-254 - Security Features
Security features are integrated into the application infrastructure to protect its resources and the trusted environment against a known exploit, threat, or vulnerability. However, flawed, disabled, or absent security features expose the application to a variety of attacks. This is a multidimensional vulnerability with broad potential impact. It can lead to defacement, confidentiality compromise, system takeover, etc.