Incorrect ACL get/set allowed on symlink path.

Description

All versions of Samba from 3.2.0 to 4.4.0rc3 inclusive are vulnerable to
a malicious client overwriting the ownership of ACLs using symlinks.

An authenticated malicious client can use SMB1 UNIX extensions to
create a symlink to a file or directory, and then use non-UNIX SMB1
calls to overwrite the contents of the ACL on the file or directory
linked to.

Patch Availability

A patch addressing this defect has been posted to

https://www.samba.org/samba/security/

Additionally, Samba 4.4.0rc4, 4.3.6, 4.2.9 and 4.1.23 have been issued as
security releases to correct the defect. Patches against older Samba
versions are available at https://www.samba.org/samba/patches/. Samba
vendors and administrators running affected versions are advised to
upgrade or apply the patch as soon as possible.

Workaround

Add the parameter:

unix extensions = no

to the [global] section of your smb.conf and restart smbd.

Alternatively, prohibit the use of SMB1 by setting the parameter:

server min protocol = SMB2

to the [global] section of your smb.conf and restart smbd.

Credits

This problem was found by Jeremy Allison of Google, Inc. and the Samba
Team, who also provided the fix.