password hash in shadow file

[Adam Weremczuk <adamw@matrixscience.com>]

Hi all,

I've just spotted that on one of my old wheezy servers root entry in 
/etc/shadow was updated just over 3 weeks ago.

The root password is still the same and the lastchanged count is much 
higher than 3 weeks.

The difference I've noticed is the hashed password string being much longer.

It's now prefixed with $6$ (SHA-512 algorithm) comparing with $1$ (MD5) 
before the change.

My first suspect was a security patch but the system was not updated 
around that time.

Has anybody seen this before and could explain?

Thanks
Adam