As per upstream report:
All versions of Samba prior to 4.13.13 are vulnerable to a malicious client using an SMB1 or NFS symlink race to allow a directory to be created in an area of the server file system not exported under the share definition. Note that SMB1 has to be enabled, or the share also available via NFS in order for this attack to succeed.
Clients that have write access to the exported part of the file system under a share via SMB1 unix extensions or NFS can create symlinks that can race the server by renaming an existing path and then replacing it with a symlink. If the client wins the race it can cause the server to create a directory under the new symlink target after the exported share path check has been done. This new symlink target can point to anywhere on the server file system. The authenticated user must have permissions to create a directory under the target directory of the symlink.
Samba 4.14 is not affected, as parts of the VFS code have already been rewritten in Samba 4.14. The VFS rewrite was completed in Samba 4.15. This is why CVE-2021-43566 only affects Samba versions < 4.13.16.
This bug is now closed. Further updates for individual products will be reflected on the CVE page(s):