CVE-2021-20316

A flaw was found in the way Samba handled file/directory metadata. This flaw allows an authenticated attacker with permissions to read or modify share metadata, to perform this operation outside of the share.

As per upstream, "Prior to Samba 4.15.0, patches for this are not possible due to the prior design of the Samba VFS layer, which used pathname-based calls for most meta-data operations. A two and a half year effort was undertaken to completely re-write the Samba VFS layer to stop the use of pathname-based calls in all cases involving reading and writing of metadata returned to the client. This work has finally been completed in Samba 4.15.0."

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.

Do not enable SMB1 (please note SMB1 is disabled by default in Samba from version 4.11.0 and onwards). This prevents the creation of symbolic links via SMB1. If SMB1 must be enabled for backward compatibility then add the parameter:

unix extensions = no

to the [global] section of your smb.conf and restart smbd. This prevents SMB1 clients from creating symlinks on the exported file system.

However, if the same region of the file system is also exported using NFS, NFS clients can create symlinks that potentially can also hit the race condition. For non-patched versions of Samba, we recommend only exporting areas of the file system by either SMB2 or NFS, not both.