While ports 137-139 were known technically as "NBT over IP", port 445 is "SMB over IP". (SMB is known as "Samba" and stands for "Server Message Blocks".)
After all of the trouble the personal computer industry has had with Microsoft's original Windows NetBIOS ports 137 through 139, it is difficult to imagine or believe that Microsoft could have actually made things significantly worse with their replacement port 445 . . . but they did.
Whereas the great vulnerability originally created by Windows file sharing was that hackers could perhaps gain remote access to the contents of hard disk directories or drives, the default exposure of the Internet server Microsoft silently installed into every Windows 2000 system (where port 445 first appeared), allows malicious hackers to remotely log onto the computers of unsuspecting users across the Internet and more recently, though the use of some clever and readily available freeware tools (PsExec from SysInternals) to silently upload and run (in the remote user's computer) any programs of their choosing without the computer's owners ever being aware.
As you might imagine, malicious hackers have been having a field day scanning for port 445, then easily and remotely commandeering Windows machines. Even several hackers I have spoken with are unnerved by the glaring insecurities created by port 445. One chilling consequence of port 445 has been the relatively silent appearance of NetBIOS worms. These worms slowly but methodically scan the Internet for instances of port 445, use tools like PsExec to transfer themselves into the new victim computer, then redouble their scanning efforts. Through this mechanism, massive, remotely controlled Denial of Service "Bot Armies", containing tens of thousands of NetBIOS worm compromised machines, have been assembled and now inhabit the Internet.
Dealing with Port 445
Needless to say, you do NOT want port 445 exposed to the Internet. Like Windows port 135 (which is a whole different problem) port 445 is deeply embedded in Windows and can be difficult or impossible to safely close. While its closure is possible, other dependent services such as DHCP (dynamic host configuration protocol) which is frequently used for automatically obtaining an IP address from the DHCP servers used by many corporations and ISPs, will stop functioning.
For the security reasons described above, port 445 has been causing so many problems that many ISPs are taking security matters into their own hands and blocking this port on behalf of their users. If our port checking shows your port 445 as "stealth" while you are not being otherwise protected by a NAT router or personal firewall, your ISP is probably preventing port 445 traffic from reaching you.
If you really want 445 closed
Any NAT router or personal firewall should be able to block port 445 from the outside world without trouble.
Trojan Sightings: Lioten