A flaw was found in the Linux kernel's OverlayFS subsystem in the way the user mounts the TmpFS filesystem with OverlayFS. This flaw allows a local user to gain access to hidden files that should not be accessible.
https://ubuntu.com/security/CVE-2021-3732
https://github.com/torvalds/linux/commit/427215d85e8d1476da1a86b8d67aceb485eb3631