LUN (Logical Unit Number) Masking is an authorization process that makes a LUN available to some hosts and unavailable to other hosts.
LUN Masking is implemented primarily at the HBA (Host Bus Adapater) level. LUN Masking implemented at this level is vulnerable to any attack that compromises the HBA.
Some storage controllers also support LUN Masking.
LUN Masking is important because Windows based servers attempt to write volume labels to all available LUN's. This can render the LUN's unusable by other operating systems and can result in data loss.