CVE-2021-3178

A flaw leak of the file handle for parent directory in the Linux kernel's NFS3 functionality was found in the way user calls READDIRPLUS. A local user could use this flaw to traverse to other parts of the file-system than mounted sub-folder.

This flaw is rated as having Moderate impact because of the attack limitation: the user can gain more access than expected only inside NFS root mount point if already have permissions for the access to this NFS sub-folder.

Also this is a known limitation of NFSv3 and there is a known and documented configuration option to avoid this. As such, this is more of an hardening rather than security issue.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

When export subdirectory of a filesystem, enable subtree_check option of the NFS server for preventing possibility of accessing outside of this export.