What Is ARP Spoofing? How ARP Poisoning Enables MITM Attacks

• ARP spoofing: (ARP poisoning) is a network-layer man-in-the-middle attack where an attacker forges ARP messages to associate their MAC address with a victim’s IP address.
• It typically occurs on flat IPv4 LANs wired or wireless where the attacker has local network access.
• By corrupting ARP tables, the attacker can intercept, modify, or disrupt traffic (e.g., between a workstation and the default gateway).
• Impact: credential theft, session hijacking, malware injection, and denial-of-service often with low visibility.
• Why it matters: ARP spoofing breaks the implicit trust of local networks and remains a common internal network risk without segmentation or monitoring.
• Mitigations: network segmentation, switch protections e.g., DHCP snooping, Dynamic ARP Inspection, encryption TLS, and continuous traffic monitoring.

ARP spoofing or ARP cache poisoning is a Layer 2 network attack where a malicious host injects fake Address Resolution Protocol ARP messages into a local subnet to re-map IP addresses to the attacker’s MAC address. In plain terms, the attacker tells other devices I am the router or another host, causing them to send traffic through the attacker. The core reason this works is that ARP has no built-in authentication: any device can reply to an ARP request and hosts will trust it. As a result, an attacker on the same Ethernet LAN can trick victims into sending packets to the attacker’s machine. This let the attacker intercept, inspect, or alter network traffic, a classic man-in-the-middle attack.

ARP spoofing is still relevant today because most IPv4 networks rely on ARP. IPv6 networks use the Neighbor Discovery Protocol, which includes cryptographic checks, but IPv4 remains prevalent. Security teams commonly see ARP spoofing in internal or campus networks where an insider or compromised device gains local access. For example, a malware-infected workstation or rogue Wi-Fi client could poison ARP tables in its subnet to spy on other machines. According to CAIDA, on the order of 30,000 ARP poisoning attempts happen per day worldwide, underscoring its scale. In short, ARP spoofing breaks fundamental network trust and is a favorite tool for attackers carrying out local network reconnaissance or interception.

How ARP Spoofing Works

ARP spoofing exploits the ARP request/response process. Normally, when Host A wants to send a packet to IP X, it broadcasts Who has IP X? on the LAN. The true owner of IP X replies with IP X is at MAC M, and Host A caches that in its ARP table. The vulnerability is that any host can reply with a forged ARP response, and hosts will accept it. An attacker abuses this by sending fake ARP replies that bind the attacker’s MAC address to another IP, often the gateway’s IP. As a result, victims update their ARP cache to send traffic to the attacker’s MAC instead of the legitimate device.

1. Prerequisite Local Access: The attacker must be on the same IPv4 LAN wired or wireless as the targets. They typically first learn key IP addresses e.g. the router’s IP and the victim’s IP.
2. Forge ARP Replies: Using a spoofing tool or script, the attacker broadcasts unsolicited ARP responses: e.g. 192.168.1.1 the router is at AA:BB:CC:DD:EE:FF attacker’s MAC. They may do this in both directions telling the router the victim’s IP is at the attacker’s MAC, and vice versa.
3. Victims Update ARP Table: Upon receiving these forged replies, both the victim and the gateway or other hosts trust the ARP information and update their ARP caches with the attacker's MAC for the spoofed IP.
4. Traffic Interception: Now packets destined for the router or other host go to the attacker. The attacker can sniff and log them. Often the attacker then forwards the traffic to the real router, setting up the attacker’s machine as a transparent proxy so that communications continue normally, keeping the victim unaware.
5. Maintain Poisoning: Because ARP caches expire or may be corrected, attackers often send continuous or periodic spoofed ARP replies sometimes called gratuitous ARPs to maintain the false mappings.

ARP spoofing succeeds because the ARP protocol has no guardrails. Hosts accept unsolicited ARP replies at any time. For example, a pentester using tools like arpspoof or Ettercap can automate step 2, quickly linking many IPs to their own MAC. The attacker effectively pretends to be both ends of a communication see image. In effect, ARP spoofing turns the LAN into a two-ended tunnel with the attacker in the middle.

Real-World Examples

In practice, ARP spoofing is most commonly observed in internal network contexts:

• LAN MitM on Corporate/Wi-Fi Networks: An attacker on a corporate Ethernet or even a public Wi-Fi that shares the same IP subnet can poison the ARP cache of the local gateway. For instance, an attacker in a coffee shop might ARP-spoof the local router, capturing any unencrypted login credentials or session cookies sent by patrons. Similarly, in an office, malware could ARP-poison a file server’s IP to intercept data exports or inject malicious commands.
• Session Hijacking & Data Theft: Once in the middle, the attacker can simply sniff traffic. They may steal plain-text passwords, internal documents, or proprietary data from the flow. They can also hijack sessions e.g. capture a user’s web session cookie since they see all packets. Because this attack happens at the network layer, it can circumvent application-layer controls unless traffic is encrypted.
• Malware Injection or Redirects: ARP spoofing can be used to insert malicious content. For example, the attacker could substitute a malicious executable or script in transit, or redirect the victim’s requests such as DNS lookups to attacker-controlled servers. Imperva notes an attacker can push a malicious file or website to the workstation once in the path. They can even use ARP spoofing to focus a subsequent DDoS, by poisoning many IPs to point to a target’s MAC.

Legitimate uses: ARP and similar techniques are also used defensively. For example, network administrators may assign static ARP entries for critical devices hardcoding a router’s IP-to-MAC to prevent poisoning. Engineers use ARP-based monitoring like arpwatch to audit network changes. In penetration testing, ARP spoofing is simulated with permission to evaluate an organization’s network defenses. In essence, ARP itself is fundamental to any Ethernet network, but ARP spoofing specifically is an attacker’s tool ethical or malicious for gaining visibility on LAN traffic.

Why ARP Spoofing Is Important

ARP spoofing matters because it can completely undermine a network’s confidentiality and integrity. When successful, it lets attackers bypass perimeter defenses by attacking inside the LAN. They can silently capture sensitive corporate or personal data and inject harmful traffic. The impacts include stolen passwords and documents, injection of malware into communications, and even complete denial of service. As SentinelOne notes, ARP spoofing poses significant security risks: it can let attackers intercept or alter traffic at will. For example, the eavesdropping enabled by ARP poisoning can expose internal credentials and unencrypted data, and ARP flooding can paralyze network operations.

Beyond technical loss, the business implications are serious: a breach through ARP poisoning can lead to regulatory penalties, loss of customer trust, and expensive incident response. In real numbers, research indicates tens of thousands of ARP poisoning attacks happen daily in IPv4 networks. In modern networks where so many services still rely on unencrypted protocols, an attacker who successfully ARP-spoofs a gateway could capture everything from database queries to login attempts. Even if data were encrypted, ARP spoofing could disrupt encrypted streams or trick devices into using rogue DNS servers. The prevalence and simplicity of ARP spoofing means any flat, inadequately monitored LAN is at risk, making it a high priority for defenders.

Common Abuse or Misuse

Attackers misuse ARP spoofing primarily as an internal penetration tactic. After gaining a foothold on a network such as via phishing or a compromised host, an adversary can leverage ARP spoofing to escalate their access. It’s commonly used during lateral movement, allowing an attacker to pivot and sniff traffic from other machines on the LAN. ARP spoofing tools are widely available: for example, open-source utilities like arpspoof part of the dsniff suite or Ettercap automate this attack. This low barrier makes ARP poisoning a go-to for many intruders during internal reconnaissance or data exfiltration.

ARP spoofing is effective because it exploits a fundamental protocol weakness. There is no challenge/response in ARP, so hosts blindly trust updates. Attackers can spoof ARP replies even if the victim never sent a request to these unsolicited or gratuitous ARPs to continuously keep caches poisoned. Detecting malicious ARP on its own is hard because ARP traffic is normally broadcast frequently, and one spoofed packet can look like a routine update. An attacker can also throttle or mimic legitimate ARP rates to avoid raising bandwidth-based alarms.

In short, ARP spoofing is abused any time an attacker needs to eavesdrop or manipulate LAN traffic. It is often paired with MAC address spoofing changing the attacker’s NIC MAC to impersonate another device and can complement DNS or DHCP spoofing. Once active, ARP poisoning can be difficult to spot without careful monitoring, since it blends into routine network chatter. Network switches typically do not log ARP packets by default, and many hosts do not alert when an ARP table entry changes. This stealthiness is why security teams emphasize proactive controls see below rather than reactive fixes.

Detection & Monitoring

Detecting ARP spoofing requires looking for inconsistencies in ARP mappings or unusual ARP traffic patterns. Here are key approaches:

• ARP Table Anomalies: One telltale sign is multiple IP addresses mapping to one MAC in a host’s ARP cache. For example, running arp -a and seeing two different IPs with the same MAC means one is likely spoofed. Regularly auditing ARP caches on critical hosts manually or via scripts can catch poisoning.
• Network Traffic Analysis: Intrusion detection systems or packet capture can identify suspicious ARP behavior. Tools like arpwatch or custom IDS rules monitor ARP announcements. For instance, a sudden burst of gratuitous ARP replies or an IP changing MAC unexpectedly can trigger alerts. Modern NIDS/NSM platforms may include signatures for ARP cache poisoning e.g. Snort’s arp-inspector that flag if a host repeatedly claims a gateway’s IP.
• Endpoint Logging: On Windows, enabling Sysmon and logging NetworkConnection creation events can flag when one MAC starts serving multiple IPs. Likewise, Windows security event logs may show ARP cache changes if baselines are monitored. On Linux, enabling auditd with monitoring of the setsockopt/ioctl syscalls on netlink ARP table changes can record when ARP entries are programmatically modified. Correlating these logs can reveal ARP updates that don’t match expected behavior.
• Switch and DHCP Logs: If the network uses features like DHCP Snooping or Dynamic ARP Inspection DAI, the switch itself can detect mismatches. DAI compares ARP packets against a trusted table e.g. from DHCP leases and can log or drop invalid ARP replies. Reviewing switch logs after enabling such features can show blocked ARP attacks.
• Encrypted vs. Unencrypted Traffic: As a sanity check, note that an ARP poisoner can’t see encrypted payloads. A sudden failure to monitor certain traffic e.g. all HTTP is now over HTTPS might hide evidence of sniffing, so focusing on metadata session starts, certificate alarms may be needed.

In practice, defenders look for ARP irregularities across endpoints and the network. MITRE recommends watching for multiple IP addresses resolving to a single MAC or unusual gratuitous ARPs. A combination of logs Sysmon/auditd, periodic ARP cache snapshots, and network alerts is most effective. Blind spots include hosts that disable logging or segmented devices that aren’t monitored. Because false positives like DHCP updates or legitimate failovers can be noisy, organizations often tune thresholds or maintain known-good baselines e.g. expected gateway MAC addresses to reduce alerts.

Mitigation & Prevention

Preventing ARP spoofing relies on both network design and host controls:

• Static ARP Entries: In small or very secure environments, statically mapping MACs to IPs on critical hosts can eliminate spoofing for those devices. For example, permanently assigning the router’s MAC to its IP on each server’s ARP table ensures the server won’t accept forged ARPs for the gateway. This is administratively heavy and impractical at scale, but effective for key systems.
• Switch Security Features: Modern managed switches often support Dynamic ARP Inspection DAI or ARP validation. When enabled, the switch checks ARP packets against a known IP–MAC list learned via DHCP Snooping or static bindings and drops mismatches. Similarly, port security can lock each switch port to a single MAC address, preventing an attacker from simply plugging into the network and claiming another identity. These controls effectively neutralize common ARP attacks in Ethernet LANs.
• Network Segmentation and Isolation: Keeping sensitive devices on separate VLANs or subnets limits the blast radius. Since ARP poisoning cannot cross a routed boundary, an attack in one subnet won’t affect another. As Varonis notes, an attack in one subnet cannot impact devices in another if networks are properly segmented. Thus, dividing the internal network into well-segmented zones and using firewalls or routers between them is a critical defense.
• 802.1X and Access Control: Ensuring only authorized devices join the LAN prevents rogue hosts from even attempting ARP attacks. Using port-based Network Access Control 802.1X means a malicious client must pass authentication before it can poison ARP. This addresses the core requirement of ARP spoofing: physical or link-layer access. Without being on the switch port or Wi-Fi SSID, ARP spoofing is moot.
• Encryption: While encryption doesn’t stop ARP messages, it mitigates their impact. Using VPNs or SSL/TLS for all communications means that even if an attacker intercepts packets, they cannot decipher the contents. Imperva advises that encrypted tunnels make the data worthless for an ARP spoofing attacker. In practice, promoting end-to-end encryption HTTPS, SSH, etc. ensures that ARP poisoning will at worst disrupt connectivity but not expose sensitive data.
• Monitoring and Response: As noted above, continual monitoring using tools like arpwatch, IDS, and EDR logs is vital. If poisoning is detected, network teams can isolate the suspected host. Periodic ARP scans and alerts on changes help in quick remediation. Regularly updating switch and host OS patches is also important, as some newer systems may offer ARP-hardening features.

By combining these controls static mappings, switch enforcement, strict network design, and encryption organizations can significantly reduce ARP spoofing risk. No single measure is perfect, but together they make ARP poisoning far less effective and easier to detect. For example, Varonis summarizes ARP prevention tips see image to highlight exactly these strategies: static ARPs, switch security, physical/network isolation, and encryption.

Related Concepts

• ARP Cache Poisoning: This is simply another name for ARP spoofing. Both terms refer to maliciously filling a host’s ARP cache with incorrect entries.
• Man-in-the-Middle MITM Attacks: ARP spoofing is a classic LAN-level MITM attack. It belongs to a broader class of techniques where an attacker interposes between two parties similarly, DNS spoofing is an application-layer MITM.
• DNS Spoofing DNS Cache Poisoning: Conceptually similar, DNS spoofing corrupts a resolver’s DNS entries so a domain name maps to a malicious IP. While ARP poisoning works at Layer 2 for IP–MAC resolution, DNS poisoning works at Layer 3/4 for hostname resolution. Both enable redirection of traffic to attacker-controlled machines.
• MAC Address Spoofing: This refers to a host changing its own MAC address to mimic another device. It is often a precursor to ARP spoofing, the attacker’s NIC MAC is typically set to match a known device or router. Though not an attack by itself, MAC spoofing is related and can be used to facilitate ARP or network access attacks.
• IPv6 Neighbor Discovery Protocol NDP: IPv6 uses NDP instead of ARP. NDP includes optional cryptographic verification SEND that prevents classic ARP spoofing. As Imperva notes, IPv6’s NDP uses cryptographic keys to verify host identities, effectively mitigating this vulnerability. Thus, pure IPv6 networks are not vulnerable to ARP poisoning in the same way.
• Network Segmentation: A key defense against many LAN attacks is segmentation. By restricting broadcast domains, network segmentation ensures that an ARP spoof in one segment does not affect others. Proper segmentation complements controls like dynamic ARP inspection.

FAQs

• How is ARP spoofing different from ARP caching?

ARP caching is a normal function where a host remembers IP–MAC mappings. ARP spoofing cache poisoning is an attack where false mappings are inserted. They differ in intent and origin of ARP entries.

• Can I detect ARP spoofing on Windows or Linux?

Yes. On Windows, tools like Sysmon can log ARP updates EventCode 22 or you can periodically run arp -a to look for duplicate MAC addresses. Linux’s arp or ip neigh can be inspected similarly. Enabling auditd rules to watch for ARP table changes or running network monitors like arpwatch can help. A clue is seeing two IPs with the same MAC in the ARP table.

• Does ARP spoofing work on cloud or Wi-Fi networks?

ARP spoofing requires Layer 2 LAN access. In typical public cloud VPCs AWS, Azure, ARP is handled by the provider’s software-defined network, so customers generally cannot perform ARP spoofing there. On a private Wi-Fi or office LAN, it works like on Ethernet the wireless AP bridges to the same subnet. It only fails if the network isolates you at Layer 3 or uses IPv6.

• Are there common tools for performing ARP spoofing?

Yes. Widely used tools include arpspoof from the dsniff suite and Ettercap, which automate sending forged ARP replies. Tools like Cain & Abel Windows also have ARP spoof modules. These are intended for testing and auditing but can be misused by attackers.

• How can I protect my network from ARP spoofing?

Key defenses include: enabling Dynamic ARP Inspection on switches, using static ARP entries for critical devices, segmenting the network into separate subnets, and enforcing port security e.g. 802.1X. Also, encrypt your traffic VPN/HTTPS so that even if ARP is poisoned, intercepted data is unusable. Monitoring tools like arpwatch and endpoint logging also help detect attacks early.

• Does IPv6 eliminate ARP spoofing issues?

ARP itself does not exist in IPv6; it’s replaced by Neighbor Discovery. IPv6’s NDP can be secured to SEND to prevent spoofing. In practice, many threats shift to other layers. But yes, the classic IPv4 ARP poisoning attack is not directly applicable in IPv6-only networks.

• What is a gratuitous ARP and is it bad?

A gratuitous ARP is an unsolicited ARP announcement a device makes e.g. when an IP changes or to detect conflicts. It is normally legitimate. Attackers abuse gratuitous ARPs by sending fake ones repeatedly to poison others’ ARP caches. So gratuitous ARP is not inherently malicious, but it is the mechanism often used in ARP spoofing.

ARP spoofing is a potent local network attack that exploits the trust-based nature of ARP. In clear terms, it allows an insider attacker to impersonate other devices at Layer 2 and transparently capture or manipulate traffic. Because ARP lacks authentication, any IPv4 LAN is potentially vulnerable if proper controls aren’t in place. The good news is that defenders have many options: strict switch security DAI, port locking, network segmentation, static ARP for key machines, and encryption of traffic. By understanding how ARP spoofing works and maintaining visibility on ARP activity as with intrusion detection and network monitoring, security teams can prevent a covert breach from turning into a full compromise. In summary, ARP spoofing remains important to address for any organization that relies on flat Ethernet subnets, and mitigating it relies on layered defenses at the switch, host, and policy levels.

About the Author

Mohammed Khalil is a Cybersecurity Architect at DeepStrike, specializing in advanced penetration testing and offensive security operations. With certifications including CISSP, OSCP, and OSWE, he has led numerous red team engagements for Fortune 500 companies, focusing on cloud security, application vulnerabilities, and adversary emulation. His work involves dissecting complex attack chains and developing resilient defense strategies for clients in the finance, healthcare, and technology sectors.