LookupUser is an extension to retrieve information about a user such as email address, user ID and the user preferences, i.e. everything that the user has specified in their preferences.
Jun 8 2017
@lewiscawte I'm more concerned about EditAccount, which poses stronger risks.
Yes, there are security and/or privacy issues. The applicable part here being "privacy issues". Which is why only staff can use these tools to enable us to do our jobs. We take privacy seriously, and these tools are not used unless it is necessary. LookupUser was originally written by one of the key MediaWiki developers, who is currently employed by the Wikimedia Foundation, so will have been written with security and privacy concerns in mind.
May 1 2017
Oops, moved upstream instead: https://phabricator.wikimedia.org/T164231