SANS Exam SEC504 Topic 8 Question 35 Discussion

Adam works as a Security Analyst for Umbrella Inc. Company has a Windows-based network. All computers run on Windows XP. Manager of the Sales department complains Adam about the unusual behavior of his computer. He told Adam that some pornographic contents are suddenly appeared on his computer overnight. Adam suspects that some malicious software or Trojans have been installed on the computer. He runs some diagnostics programs and Port scanners and found that the Port 12345, 12346, and 20034 are open. Adam also noticed some tampering with the Windows registry, which causes one application to run every time when Windows start.

Which of the following is the most likely reason behind this issue?