Who is former Twitter security chief Peiter Zatko and why is he testifying before US Congress?

Peiter Zatko, the former security chief at Twitter, will testify before US Congress on complaints that the microblogging site has been negligent about privacy and security

from https://www.firstpost.com/explainers/who-is-former-twitter-security-chief-peiter-zatko-and-why-is-he-testifying-before-us-congress-11244911.html