A security analyst reviews a packet capture and identifies the following output as…

The packet capture output displays TCP packets with the FIN, PSH, and URG flags set, as indicated by Flags[FPU]. This specific combination of flags is the signature of an Nmap "Xmas" scan (-sX). This type of scan is considered a stealth scan, designed to identify the state of ports on a target system. The scan works because RFC 793 specifies that a closed port must respond to such a malformed packet with a RST packet, while an open port should ignore it. The log shows these packets being sent to multiple ports on the same destination IP, which is characteristic of a port scan.

B. Nikto's web scan: Nikto is a web vulnerability scanner; its network traffic would primarily consist of standard HTTP requests (e.g., GET, POST), not TCP packets with this flag combination.

C. Socat's proxying traffic using the urgent flag: Socat is a utility for data transfer. While it can manipulate TCP flags, its purpose is not port scanning, and it would not generate this specific scanning pattern.

D. Angry IP Scanner output: This tool typically uses more common methods like ICMP echo requests or TCP SYN probes for scanning, not the FIN, PSH, and URG flag combination.

1. Nmap Project (Official Vendor Documentation): The official Nmap Reference Guide explicitly describes the Xmas Scan (-sX). It states

"Xmas scan... sets the FIN

PSH

and URG flags

lighting the packet up like a Christmas tree."

Source: Nmap Reference Guide

Chapter 15

Section: "TCP Xmas Scan (-sX)". Retrieved from https://nmap.org/book/scan-methods-stealth.html

2. Internet Engineering Task Force (IETF) RFC 793 (Peer-Reviewed Standard): This document defines the TCP protocol. The principle of the Xmas scan relies on the behavior defined for closed ports.

Source: Postel

J. (1981). RFC 793: Transmission Control Protocol. IETF. Section 3.4

"Establishing a connection

" implies that for a segment arriving at a closed port

"an incoming segment not containing a RST causes a RST to be sent in response."

3. University Courseware: University-level computer security courses describe Nmap's stealth scanning techniques

including the Xmas scan

as a fundamental concept in network reconnaissance.

Source: David M. Nicol

"Lecture 10: Network Probing and Scanning

" ECE 422: Computer Security I

University of Illinois Urbana-Champaign

Fall 2020. The lecture notes detail various Nmap scans

including the Xmas scan and its use of FIN

URG

and PSH flags.