Large-scale brute force attack on WP sites going on now

I use the Wordfence security plugin on my sites, and just received this email from Wordfence, after getting alerts all morning via email about multiple attempts to login to the admin area with false usernames:

As of 11am eastern time this morning we are monitoring the largest distributed brute force attack on WordPress installations that we've seen to date. The real-time attack map on www.wordfence.com became so busy that we've had to throttle the amount of traffic we show down to 4% of actual traffic.

A brute force attack is when an attacker tries many times to guess your username password combination by repeatedly sending login attempts. A distributed brute force attack is when an attacker uses a large number of machines spread around the internet to do this in order to circumvent any blocking mechanisms you have in place.

If you're using the free or paid version of Wordfence you should have the option to "Participate in the real-time Wordfence security network" under 'Other options' enabled. This will immediately block any attack originating from an IP address that has attacked other WordPress sites using Wordfence. This is an effective defense against this kind of attack.

We recommend that until this passes you monitor your WordPress websites closely for unusual activity including logins, account creation or changes to the public facing website.