Address Resolution Protocol (ARP) resolves IP addresses to Media Access Control (MAC) addresses on a local network. ARP Spoofing, also known as ARP Poisoning, is a specific On-Path (Man-in-the-Middle) attack where a malicious actor broadcasts falsified ARP messages over a Local Area Network (LAN).
…Address Resolution Protocol (ARP) resolves IP addresses to Media Access Control (MAC) addresses on a local network. ARP Spoofing, also known as ARP Poisoning, is a specific On-Path (Man-in-the-Middle) attack where a malicious actor broadcasts falsified ARP messages over a Local Area Network (LAN).
The vulnerability stems from ARP being a stateless protocol; devices implicitly trust ARP replies even if they never sent a request. In a CompTIA PenTest+ scenario, an attacker typically uses tools like Ettercap, Bettercap, or Arpspoof to flood the network with "gratuitous ARP" packets. These packets falsely claim that the attacker's MAC address is associated with the IP address of a legitimate target, most commonly the default gateway.
Once the victim's ARP cache is "poisoned" with this forged entry, all traffic intended for the internet is routed through the attacker's machine at Layer 2. This allows the attacker to intercept sensitive data (credential harvesting), modify traffic in transit, or drop packets to cause a Denial of Service (DoS). Because this happens on the local link, it bypasses standard switch traffic isolation.
To identify this vulnerability during an engagement, a pentester analyzes network traffic using protocol analyzers like Wireshark, looking for duplicate MAC addresses mapped to different IPs or excessive ARP replies. Remediation strategies often involve implementing Dynamic ARP Inspection (DAI) on network switches or using static ARP entries for critical infrastructure.
Comprehensive Guide to ARP Spoofing and Poisoning
What is ARP Spoofing? Address Resolution Protocol (ARP) is the mechanism used to map a Layer 3 IP address to a Layer 2 physical MAC address. ARP Spoofing (also known as ARP Poisoning) is a Man-in-the-Middle (MitM) attack technique where an attacker sends falsified ARP messages over a Local Area Network (LAN). The goal is to associate the attacker's MAC address with the IP address of another legitimate host, such as the default gateway, causing any traffic meant for that IP to be sent to the attacker instead.
Why is it Important? For the CompTIA PenTest+, this is a critical concept because it is the foundational step for intercepting local traffic. By successfully poisoning the ARP cache, a pentester (or attacker) can launch session hijacking, capture unencrypted credentials (like Telnet or HTTP), or modify traffic in transit. It demonstrates a significant internal vulnerability: the lack of authentication in the ARP protocol.
How it Works The attack exploits the 'stateless' nature of ARP, meaning devices often accept ARP replies without having sent an ARP request. 1. Reconnaissance: The attacker identifies the IP addresses of the victim and the default gateway. 2. Spoofing: The attacker uses a tool (e.g., Ettercap, Arpspoof, Bettercap) to send unsolicited ARP replies (Gratuitous ARP) to the victim. 3. Poisoning: The victim's device updates its ARP cache, mapping the Gateway's IP address to the Attacker's MAC address. 4. Interception: The victim sends data intended for the internet to the attacker. The attacker enables IP Forwarding to pass the data to the real gateway, keeping the connection alive while sniffing the traffic.
Exam Tips: Answering Questions on ARP Spoofing and Poisoning To answer exam questions correctly, focus on identifying the attack indicators and mitigation strategies: 1. Analyze the Output: If a question presents a log or a terminal output showing the MAC address of a known server (like 192.168.1.1) changing, or if the arp -a command shows two different IPs sharing the same MAC address, the answer is ARP Poisoning. 2. Understand the Scope: Remember that ARP is a Layer 2 protocol. It is non-routable. If a scenario asks about attacking a machine on a remote network, ARP spoofing is not the answer. It only works on the local subnet. 3. Differentiate from DNS: If the issue involves domain names resolving to the wrong IP, it is DNS Poisoning. If it involves IP addresses mapping to the wrong hardware address (MAC), it is ARP Poisoning. 4. Mitigation: The standard remediation answer for ARP spoofing is implementing Dynamic ARP Inspection (DAI) on switches or using static ARP entries for critical infrastructure. 5. Tool Awareness: Be ready to identify tools like Wireshark (for detection) and Ettercap (for execution) in multiple-choice scenarios.