Information security research credited to John Martinelli @ 
SECURELI.com. (john@secureli.com)

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Affects: WordPress <=v4.4
Vulnerability: Information Disclosure
CVE-ID: Pending
Impact: Username exists disclosure on /wp-login.php

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By default, WordPress <=4.4 discloses whether a username is registered 
inside the system, allowing attackers the opportunity to do brute-force 
password cracking attacks.

One proof of concept would be to scrape Alexa.com for WordPress running 
domains, check automatically whether it confirms the username (possibly 
top 100 most popular WP usernames) exists in the WordPress users table 
using a simple RegEx boolean to determine “IF USER EXISTS”, then to send 
a Selenium automated browser to attempt logins with top 10 most popular 
passwords.

Would easily allow attackers to login and with publishing or editing 
permissions, install malware onto the WordPress installation as well as 
potentially escalate privileges on the hosting account if running 
vulnerable versions of WordPress or plugins.

**** Proof screenshot @ 
https://secureli.com/information-security-research/wordpress/ ****