Permissions problem with cifs.mount

hi,

I have the following permission problems with cifs.mount : a share on a VNXe (EMC NAS) is accessed by two RHEL 5.9 accounts (authenticated by Active Directory); One account has read+wite permission to the share , the second one has only read permission. Both accounts uses the following mount command:

mount -t cifs --verbose -o sec=krb5,cifsacl,multiuser //NAS/share/mount_point

First, the multiuser option is rejected (but the mounting is done). Moreover both accounts can write to the share, ignoring the NTFS permissions. What is going on ?