Everything old is new again. Here's how Nmap helps you identify hosts that are easy to attack so you don't have to brute force them:

  TCP Sequence Prediction
  
  Systems with poor TCP initial sequence number generation are vulnerable to 
  blind TCP spoofing attacks. In other words, you can make a full connection to 
  those systems and send (but not receive) data while spoofing a different
  IP address. The target's logs will show the spoofed IP, and you can take 
  advantage of any trust relationship between them. This attack was all the
  rage in the mid-nineties when people commonly used rlogin to allow logins 
  to their account without any password from trusted IP addresses. 
  Kevin Mitnick is alleged to have used this attack to break into Tsutomu Shimomura's 
  computers in December 1994.
  
  The good news is that hardly anyone uses rlogin anymore, and many operating 
  systems have been fixed to use unpredictable initial sequence numbers 
  as proposed by RFC 1948. For these reasons, this line is only printed in 
  verbose mode. Sadly, many vendors still ship vulnerable operating systems 
  and devices. Even the fixed ones often vary in implementation, which 
  leaves them valuable for OS detection purposes.
  [..]
  Further details about sequence tests are provided in the section called 
  “TCP ISN greatest common divisor (GCD)” [1].

https://nmap.org/book/osdetect-usage.html [1] https://nmap.org/book/osdetect-methods.html#osdetect-gcd

A fun look at sequence number PNRGs of various (old) OSes: http://lcamtuf.coredump.cx/newtcp/