vulnerability any user could post on another user’s wall. The security researcher first reported to Facebook about the vulnerability. But, Facebook did not want to fix the issue. The security researcher then hacked Mark Zuckerberg’s Facebook page and posted on his wall to prove how serious that vulnerability can be. Facebook later fixed the issue but did not pay any bounty to the security researcher, as the security researcher had violated Facebook’s security policy. This is a good example of grey hat hacking.

I hope this helps. However, interested readers who want to know more about how different web application attacks work and how we can prevent them can refer to the book “Web Application Vulnerabilities And Prevention.”