200-201 Network Intrusion Analysis — 50 Questions

Question 1

A security analyst reviews an alert from the IPS that shows a spike in TCP SYN packets from an external IP to multiple internal hosts on port 443. What is the most likely attack type?

Accepted Answer

SYN flood. A SYN flood attack exploits the TCP three-way handshake by sending a high volume of SYN packets to multiple hosts without completing the handshake, exhausting server resources. The alert describes a spike in TCP SYN packets from an external IP to multiple internal hosts on port 443, which matches the behavior of a distributed SYN flood targeting HTTPS services. This is the most likely attack because the IPS is detecting the initial connection attempts characteristic of a SYN flood.

Answer

Port scanning

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Man-in-the-middle

Answer

DNS amplification

Question 2

An analyst notices that a host is sending large amounts of data to an external IP address on TCP port 22 during non-business hours. What is the most likely activity?

Accepted Answer

Data exfiltration via SSH. SSH (TCP port 22) is commonly used for secure remote administration, but the scenario describes large data transfers to an external IP during non-business hours, which is a classic indicator of data exfiltration. Attackers often use SSH tunneling to bypass security controls and exfiltrate data because SSH encrypts the traffic, making it difficult for network monitoring tools to inspect the payload. The combination of high volume, external destination, and off-hours activity strongly suggests malicious data theft rather than legitimate administrative tasks.

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Remote administration

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DNS query

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FTP file transfer

Question 3

An analyst sees an alert: 'ET POLICY Outgoing HTTP Request with Suspicious User-Agent (Mozilla/5.0 compatible; MSIE 6.0; Windows NT 5.1)'. The source is an internal host that typically uses Windows 10. What should the analyst suspect?

Accepted Answer

The traffic is likely generated by malware. The User-Agent string 'Mozilla/5.0 compatible; MSIE 6.0; Windows NT 5.1' mimics Internet Explorer 6 on Windows XP (NT 5.1). Since the source host normally runs Windows 10, this outdated and mismatched User-Agent is a strong indicator of malware attempting to disguise its traffic as legacy browser activity to evade detection.

Answer

The traffic is from a web proxy

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The host is running Windows XP

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The host is running a browser update

Question 4

During an investigation, an analyst finds that an internal host has been communicating with a known malicious IP on port 445. Which protocol is most likely involved?

Accepted Answer

SMB. Port 445 is the default port for Microsoft SMB (Server Message Block) over TCP, used for file sharing, printer sharing, and other network services. Communication with a known malicious IP on this port strongly indicates SMB-based activity, such as exploitation of vulnerabilities like EternalBlue (MS17-010) or unauthorized file access.

Answer

SSH

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RDP

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HTTP

Question 5

An analyst reviews NetFlow data and sees a single internal IP communicating with many external IPs on port 53, each with small UDP packets. The internal host is not a DNS server. What is the most likely explanation?

Accepted Answer

The host is the victim of a DNS amplification attack. The internal host is not a DNS server, yet it is sending small UDP packets to many external IPs on port 53. This is characteristic of a DNS amplification attack, where the attacker spoofs the victim's IP address and sends small queries to open DNS resolvers, which then send large responses to the victim. The NetFlow data shows the victim receiving the amplified traffic, not initiating it, making C correct.

Answer

The host is acting as a DNS server

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The host is performing recursive DNS lookups

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The host is scanning for open DNS resolvers

Question 6

A security analyst detects a large number of TCP RST packets from a single external IP to various internal hosts. The internal hosts are not sending any corresponding packets. What is the most likely cause?

Accepted Answer

A TCP reset attack is being performed. A TCP reset attack (also known as a forged RST attack) occurs when an attacker sends spoofed TCP RST packets to disrupt an existing connection. The key clue is that the internal hosts are not sending any corresponding packets, indicating the RST packets are unsolicited and likely forged, which is characteristic of this attack rather than a normal network event.

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A denial-of-service attack is occurring

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A TCP connection is being established

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A port scan is in progress

Question 7

An analyst sees an alert from the IDS: 'ET TROJAN Possible Zeus Variant Outbound Connection'. What action should the analyst take first?

Accepted Answer

Investigate the source host for signs of compromise. Option C is correct because the first priority when an IDS alerts on a possible Zeus variant (a known Trojan) is to investigate the source host to confirm or rule out compromise. Zeus is a credential-stealing Trojan that often establishes outbound C2 (command-and-control) traffic; blindly blocking the IP (A) could disrupt the investigation and may not stop the malware if it uses domain flux or multiple IPs. Reimaging (D) destroys forensic evidence, and ignoring the alert (B) is negligent given the severity of Zeus. The analyst must perform host-based analysis (e.g., check processes, registry, network connections) to validate the alert before taking containment actions.

Answer

Block the IP address on the firewall

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Ignore the alert as a false positive

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Reimage the host immediately

Question 8

A host is infected with malware that uses DNS tunneling to exfiltrate data. Which type of analysis would best detect this activity?

Accepted Answer

DNS log analysis. DNS tunneling encodes exfiltrated data within DNS queries or responses, often using TXT or A record types to bypass network security controls. DNS log analysis is the most direct detection method because it reveals anomalous patterns such as unusually long domain names, excessive NXDOMAIN responses, or high volumes of DNS traffic to a single external server, which are hallmarks of tunneling activity.

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Windows event log analysis

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Firewall log analysis

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NetFlow analysis

Question 9

An analyst observes that an internal host is sending ICMP echo requests with payloads containing random data to an external IP. The payload size is larger than typical. What is the most likely technique?

Accepted Answer

ICMP tunneling. ICMP tunneling encapsulates non-ICMP data (e.g., command-and-control traffic) within ICMP echo request/reply packets. The random payload data and larger-than-typical payload size are hallmarks of this technique, as the attacker uses the ICMP protocol to bypass firewalls and exfiltrate data or establish covert communication.

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Ping of death

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Traceroute

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Smurf attack

Question 10

Which TWO types of network traffic should be analyzed to detect a data exfiltration attempt via HTTP? (Choose two.)

Accepted Answer

HTTP request headers. HTTP request headers contain metadata such as User-Agent, Content-Type, and custom headers that can be manipulated to encode and exfiltrate data. The HTTP request body carries the payload, such as POST data, where stolen information can be embedded in form fields, JSON, or XML. Analyzing both allows detection of anomalous patterns indicative of data exfiltration.

Answer

ICMP echo requests

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DNS query responses

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TCP three-way handshake

Question 11

Which THREE indicators are commonly found in network traffic that suggest a host is part of a botnet? (Choose three.)

Accepted Answer

Connections to known IRC servers on non-standard ports. Option A is correct because botnets often use IRC (Internet Relay Chat) for command and control (C2) communication. Attackers configure IRC servers on non-standard ports (e.g., TCP 6667–6669 are common, but botnets may use ports like 8080, 8443, or random high ports) to evade detection by security tools that monitor default IRC ports. The presence of persistent IRC connections to unusual ports is a strong indicator of botnet activity.

Answer

Large file downloads from external servers

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Frequent DNS queries to legitimate corporate DNS servers

Question 12

Which TWO network behaviors suggest an ARP spoofing attack is occurring? (Choose two.)

Accepted Answer

Packets originating from a MAC address that does not match the IP's legitimate MAC. Option C is correct because in an ARP spoofing attack, the attacker sends forged ARP replies that associate their own MAC address with the IP address of a legitimate host (e.g., the default gateway). This causes packets destined for that IP to be sent to the attacker's MAC, creating a mismatch between the source MAC in the packet and the legitimate MAC address for that IP. Detecting such mismatches is a key indicator of ARP cache poisoning.

Answer

A high number of TCP RST packets

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A single host sending numerous ARP requests

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An increase in broadcast ARP traffic

Question 13

Refer to the exhibit. The analyst sees two IDS alerts from the same source. What should the analyst conclude?

Accepted Answer

The host is likely infected with malware. The correct answer is C because the IDS alerts indicate the same source IP is communicating with a known malicious domain (evil.com) using a suspicious user-agent string. This pattern of repeated connections to a known bad destination is characteristic of malware beaconing or command-and-control (C2) traffic, not a false positive or benign activity.

Answer

The alerts are false positives because the user-agent is common

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The host is being scanned

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The host is downloading a large file

Question 14

Refer to the exhibit. A firewall log shows denied TCP traffic from an internal host to an external IP on consecutive ports. What type of activity is indicated?

Accepted Answer

Port scanning. The firewall log shows denied TCP traffic from an internal host to an external IP on consecutive ports. This sequential pattern of connection attempts to multiple ports on the same target is a classic indicator of a port scan, where an attacker probes for open ports to identify potential services to exploit. The firewall's deny action confirms the traffic was blocked, but the behavior itself is characteristic of reconnaissance activity, specifically a TCP connect scan.

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Worm propagation

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Denial of service

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Data exfiltration

Question 15

Refer to the exhibit. An analyst sees repeated ICMP echo requests from a host to the broadcast address. What is this an example of?

Accepted Answer

Smurf attack. A Smurf attack sends ICMP echo requests to a network broadcast address with the source IP spoofed to the victim's address. All hosts on the network reply to the victim, overwhelming it with traffic. This is a classic amplification-based denial-of-service attack.

Answer

Ping sweep

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ICMP tunneling

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Denial of service

Question 16

You are a security analyst for a financial institution. Over the past hour, the intrusion detection system has generated multiple alerts for outbound traffic from a single internal host (10.0.0.50) to various external IP addresses on port 443. The alerts indicate that the host is making HTTPS connections to IPs that are associated with known command and control servers. Additionally, the host has been observed making DNS queries for domains that are algorithmically generated (e.g., rgj3k2.example.com, fh7d8s.example.net). The host is a Windows 10 workstation used by an employee in the accounting department. The employee reports that they have not noticed any unusual behavior, but they did click on a link in a phishing email yesterday. The network administrator confirms that the host's firewall rules allow outbound HTTPS traffic. You have access to endpoint logs, network flow data, and packet captures. Which course of action should you take FIRST?

Accepted Answer

Isolate the host from the network to prevent further C2 communication. Option A is correct because the immediate priority when confirmed C2 communication is detected is to contain the threat by isolating the host from the network. The combination of outbound HTTPS connections to known C2 servers and algorithmically generated domain (AGD) DNS queries strongly indicates active malware infection. Isolating the host (e.g., via network access control or switch port shutdown) stops data exfiltration and further command reception, which is the first step in incident response containment before any analysis or remediation.

Answer

Analyze packet captures to determine the full extent of the compromise

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Block all outbound HTTPS traffic from the network

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Reimage the host immediately to remove the malware

Question 17

You are a security analyst for a medium-sized enterprise. You notice that the network monitoring system has flagged an unusual amount of traffic between two internal hosts: 192.168.1.10 (a file server) and 192.168.1.20 (a workstation in the sales department). The traffic is occurring on port 445 (SMB) and is happening outside of normal business hours. The volume of data transferred is significantly higher than typical usage. The file server logs show that the sales workstation has been accessing a large number of files in quick succession. The sales employee reports that they have been working late, but they cannot explain the high volume of file access. You have access to the file server logs, network flow data, and the workstation's event logs. The workstation has antivirus software installed that is up to date. What should you do FIRST?

Accepted Answer

Isolate the workstation from the network immediately. Option A is correct because the anomalous SMB traffic on port 445, occurring outside business hours with a high volume of file access in quick succession, strongly indicates a ransomware or data exfiltration attack. Isolating the workstation immediately contains the threat, preventing lateral movement and further encryption or exfiltration of sensitive data. This aligns with the first step in incident response: containment before analysis.

Answer

Reimage the workstation to ensure it is clean

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Run a full antivirus scan on the workstation

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Analyze network flow data to identify the destination of the data

Question 18

A security analyst observes a sudden spike in outbound traffic from a critical server to an external IP address on TCP port 443. The server is a web application server that normally only receives inbound connections. Which type of intrusion is most likely occurring?

Accepted Answer

Command-and-control (C2) communication from malware on the server. A sudden spike in outbound traffic from a server that normally only receives inbound connections is a classic indicator of command-and-control (C2) communication. Malware on the server often establishes outbound HTTPS (TCP 443) connections to a C2 server to exfiltrate data or receive instructions, bypassing firewalls that typically allow outbound web traffic.

Answer

Distributed denial-of-service (DDoS) attack from the server

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Brute-force attack on the server's SSH service

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SQL injection attack against the server

Question 19

An analyst needs to determine if a host is infected with malware that is attempting to contact a known malicious domain. Which log source is most appropriate for this analysis?

Accepted Answer

DNS server logs. DNS server logs are the most appropriate source because they record all DNS queries made by hosts on the network. If a host is attempting to contact a known malicious domain, the DNS query for that domain will appear in the logs, allowing the analyst to identify the infected host by its source IP address and the timestamp of the query.

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Syslog from the host

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NetFlow records

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Data loss prevention (DLP) logs

Question 20

Which TWO of the following are indicators of a network intrusion? (Choose two.)

Accepted Answer

A sudden increase in DNS queries to unknown domains from a single host. Option D is correct because a sudden spike in DNS queries to unknown domains from a single host is a classic sign of DNS tunneling or command-and-control (C2) activity. Attackers often use DNS to exfiltrate data or communicate with external servers by encoding data in DNS queries, bypassing traditional firewall rules that allow DNS traffic.

Answer

High bandwidth usage during business hours

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A single failed login attempt from an internal user

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Regular ICMP echo requests to external hosts