As an information security expert, you have been assigned to perform a blind penetration test for the systems of HDA Inc. After some research, you were able to get hold of email IDs of the few employees of the HDA Inc. You created a fake email ID which resembles the email ID of HR of HDA Inc. From that email ID, you sent an email to the employees asking them to open the attached link to read the revised payroll process. One of the employees opens the link and through specially designed malware you were able to gain access to the HDA’s network and successfully conducted penetration testing. What type of attack was used by you for penetration testing?