Is careers.who.int a legit website?
Did you receive an email from someone with an address @careers.who.int? Did you find some information or an offer on a website starting by http(s)://careers.who.int?
Official source: Official UN website
What does that mean?
This address is an official source that you can trust.
What should you do?
Use the website as usual. You can trust what you read on this website.