The CookieMonster attack is a man-in-the-middle exploit where a third party can gain HTTPS cookie data when the "Encrypted Sessions Only" property is not properly set. This could allow access to sites with sensitive personal or financial information. It is a Python based tool, developed by security researcher Mike Perry. Perry originally announced the vulnerability exploited by CookieMonster on BugTraq in 2007. A year later, he demonstrated CookieMonster as a proof of concept tool at Defcon 16.