How to contact a user who wrote a review to my app?

I have published an app in the App Store. Now one of the customers/buyer wrote a review regarding a technical issue.

How can I contact that person? In the review I can only see the author's pseudonym.

Since a few days, you now have the ability to answer to reviews, right from iTunes Connect.

Go to My Apps > YourApp > Activity > Reviews and you will be able to click on Answer for each review.

Your answers will be available directly from the App Store for people running on iOS 10.3, and I think the reviewer also receive a mail or notification that you answered.

It a huge step from Apple to finally offer this !

It's not possible. If you wish to help users you could list an email adress in your app and/or let them immediately email to it. Apple does not allow you to reply to user reviews in the app store.