Defending Your Network: A Comprehensive Guide to VLAN Hopping Attacks

Published on: October 20, 2023

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18 min read

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For German Version

VLANs are an essential component of modern networking. They are designed to enhance network efficiency and security by segmenting traffic into isolated logical networks. However, when not implemented and configured correctly, VLANs can become vulnerable to exploitation. VLAN hopping attacks are a stealthy and sophisticated category of security threats that target weaknesses in VLAN configurations. VLAN hopping is a computer security breach in which networked resources on a VLAN are attacked. A malicious party is required to initially compromise at least one network VLAN. This makes it possible for attackers to establish a foundation of activities from which they can target other VLANs linked to the network. They manipulate the VLAN infrastructure, to breach the network security. This attack type has the potential to compromise the confidentiality and integrity of data and resources, granting unauthorized access to sensitive parts of a network.

The fundamental idea behind all VLAN hopping attacks is for an attacking host on one VLAN to get access to traffic on other VLANs that would otherwise be inaccessible. VLAN hopping is accomplished through two methods: switch spoofing and double tagging. With proper switch port configuration, both attack paths can be avoided. To reduce the risk of VLAN hopping, good security hygiene should be practiced. For instance, the interfaces that are not employed should be shut down and moved to an idle VLAN. The VLANs on trunk ports should not be brought to use until absolutely essential.

The following topics are going to be covered in this article;

• What Is VLAN Hopping?
• Is VLAN Secure?
• What Are the Risks of VLAN Hopping?
• How Does VLAN Hopping Work?
• What are the types of VLAN Hopping Attacks?
• What Is a Switch Spoofing Attack?
• What Is a Double Tagging Attack?
• How to Detect VLAN Hopping?
• How to Prevent VLAN Hopping Attacks
• How to Respond to VLAN Hopping?

What is VLAN Hopping?

VLAN hopping is a network security threat where an attacker leverages vulnerabilities in VLAN configurations to gain unauthorized access to different VLANs within the same network. It's done by a method of attacking the network resources of a VLAN by sending packets to a port not usually accessible from an end system. The significance of VLAN hopping lies in its potential to compromise the integrity and confidentiality of data and resources within a network. This technique enables attackers to bypass network segmentation, which is intended to isolate traffic and enhance security. To provide security and traffic management, large organizations design their network by dividing it into multiple VLANs to separate departments. Those can be finance, research and development, marketing, or human resources for instance. In a VLAN hopping attack scenario, it won't be expected to have a breach in the human resources department, if the goal is to protect the marketing details. But by taking advantage of a switch port that is not properly configured, intruders can hop from the R&D segment to marketing for instance. In a university campus network, different VLANs are created for students, faculty, and administrative staff. Employing VLAN hopping techniques, hackers could compromise the student VLAN and gain access to faculty or administrative VLANs. This can result in having hands-on confidential research and administrative data.

Understanding VLAN hopping is paramount because it highlights the critical importance of correctly configuring and securing VLANs. VLAN hopping poses a significant risk to the confidentiality and integrity of data within a network. It can enable bad actors to steal passwords and other sensitive information from network subscribers, modify or delete data, install malware, and propagate threat vectors such as viruses, worms, and Trojans throughout a network. In addition, many regulatory standards and compliance requirements (e.g., PCI DSS, HIPAA) mandate strong network segmentation. Failure to prevent VLAN hopping can lead to compliance violations and a lack of network resilience. It underscores the need for robust security measures, such as access control lists, port security, and proper network monitoring, to thwart potential attackers and protect the integrity of network segments. Awareness of this threat is crucial for network administrators and security professionals as they work to fortify their networks against these covert attacks. To make it certain that all user ports are dedicated to access mode ports, the unused ports have to be disabled and those ports' default settings should be set as access mode. Becoming certain that it is set to accept only the VLANs that are required, is a must whilst setting a trunk port.

Is VLAN Secure?

The main idea behind VLAN security is that it provides a level of isolation and segmentation for suitable solutions for network security. However, various reasons lead to wrong perceptions about the security of VLANs. VLANs mainly create logical isolation by assigning devices to specific segments. This becomes the reason to assume that these segments are entirely insulated from one another, which is not the case. This perception can be misleading because the level of isolation depends on proper configuration, and it is not always implemented correctly. Another misunderstanding that can expose vulnerabilities is oversimplifying network security. This is done assuming there is no need for additional security measures as devices are on different VLANs. The design and implementation of VLANs may lead to VLAN hopping attacks.

Some other potential weaknesses that come with VLANs are as follows:

• Misconception of VLAN objective: VLANs are primarily not intended for security. Instead, they are handy for network management, including network segmentation. They can't be at the level of a firewall or encryption, when it comes to protecting the network. They are a good aid in traffic isolation and network performance.
• Misconfiguration: VLANs that are not configured in a proper way might result in security flaws. Common mistakes include misunderstanding the direction of traffic in VLAN ACLs, not testing or troubleshooting ACLs after applying them, and poor switch configuration.
• Limited Layer 2 protection: VLANs operate at the data link layer (layer 2) of the OSI network model. Most layer 2 attacks can be more easily executed and go undetected compared to layer 3 attacks. Considering that switches often lack the ability to track and detect attacks, it makes VLANs more susceptible to certain types of security threats and makes it easier for attackers to perform malicious acts without detection.
• Tool availability: There are several known applications that provide potential attackers with the tools to penetrate VLANs and cause chaos. These tools are easy to find and download from the internet.
• Inherent weaknesses: VLANs have inherent weaknesses, such as CAM table overflow/MAC attacks and Address Resolution Protocol (ARP) attacks. Switches, which are responsible for implementing VLANs, do not have built-in mechanisms to detect and prevent attacks on VLANs.
• Poor switch configuration: Many VLAN security vulnerabilities stem from misconfigurations in network switch settings, such as allowing unauthorized access to VLANs or not properly securing trunk links. These configuration errors can lead to VLAN hopping and other security breaches and VLAN hopping attacks.

VLANs are not inherently secure, and there are several factors that contribute to the common misconception of their security. Saying that VLANs still remain capable of offering some amount of security. They just have to be properly set up and managed. They have the potential to improve network performance and ease management. They can contribute to user mobility. Firewalls, access controls, and authentication protocols are some examples of them being employed in conjunction with other security measures. They are a valuable tool for network management and they can help in traffic isolation, enhance network performance, and provide flexible user mobility. To enhance VLAN security, it is crucial to properly configure switches, implement access controls, and monitor and manage VLAN traffic. Robust security measures should be implemented like access control lists (ACLs), port security, and encryption protocols.

What Are the Risks of VLAN Hopping?

VLAN hopping attacks pose significant risks to network security, potentially compromising the integrity, confidentiality, and availability of data and resources. Most network IT frameworks now incorporate VLAN configuration. Organizations depend more and more on VLANs for secure network communications, therefore any security gaps might result in threats to your privacy, data access and manipulation, and security risks. In addition to this intrusion and infiltration, attackers can access crucial network settings, passwords, and any type of private information. The system will be vulnerable to theft, change, and even removal of data if your VLAN architecture is compromised.

Here's an overview of the risks associated with VLAN hopping attacks:

• Confidentiality: VLAN hopping can enable unauthorized access to sensitive data and resources, potentially leading to data breaches. For example, an attacker could gain access to a finance department's VLAN, exposing financial records or customer data.
• Availability: In some cases, VLAN hopping attacks can disrupt network availability. If an attacker destabilizes network segments or misconfigures VLANs, it can lead to network outages. For instance, a successful VLAN hopping attack could cause a disruption in healthcare operations by compromising the network segment for critical medical equipment.
• Integrity: Attackers who obtain access to your VLANs can quickly have hands on the data stored on the resources connected to the VLANs. They gain access to network segments, which can lead to the manipulation of data and resources. For instance, an attacker could access the administrative VLAN of a university, altering grades or compromising administrative records. In a hospital network, critical machine security or patients' confidential records can be compromised.
• Hopping Attacks: Switching between VLANs exposes your network to a number of security risk factors. Harmful agents like worms and trojan horses can be spread throughout the network by breaking into and hopping VLANs. In addition, attackers who successfully hop between VLANs can use this access as a launchpad for further attacks, potentially compromising other systems and networks within an organization.

How Does VLAN Hopping Work?

VLAN hopping attacks are frequently carried out by knowledgeable attackers. They are mostly aware of network architecture and security flaws, at an advanced level. Financial gain, espionage, or even sometimes just to challenge the security shield of the networks may inspire these attackers. VLAN hopping is simplified by knowing the fundamental design of VLANs and the strategies used by attackers. Here's a basic analogy to explain how VLAN hopping works. It's an example of how switch vulnerability and spoofing attacks occur. Consider a school with different classrooms for each grade level. Each classroom is like a VLAN, and the students are like the traffic on the network. Only students in a particular grade level are allowed to enter their classroom, just like only traffic in a particular VLAN is allowed to pass through a switch. A vulnerability in securing the door, or a breach in identity cards can both cause an individual who is not a student, or at least enough level to pass, to pass through doors and get into classes.

In the first half of the 2000s, after it was proven to be a security issue in certain network switch setups, VLAN hopping gained popularity. They have become more popular in recent years as networks have become more complicated, but they were already there for a good amount of time. There were further chances for attackers to benefit from network systems' weaknesses and obtain possession of private information in time. On the other hand, the prevention methods got more sophisticated and it's harder to perform such attacks through new technology network equipment. VLAN hopping attacks can have serious outcomes and they are not always easy to detect and steer clear of. A number of things that can be done to reduce the risk are covered in the following sections.

What are the Types of VLAN Hopping Attacks?

VLAN hopping is a network attack approach that allows an attacker to navigate from one VLAN to another without having the rights. This can be accomplished by leveraging the network's security flaws. Another way can be by utilizing social engineering tactics to deceive people into disclosing sensitive information. VLAN hopping attacks are mainly classified into two types:

1. Switch spoofing: Switch spoofing is an attack that includes convincing a switch that the intruder's machine is a trustworthy one. It can be accomplished through the transmission of fake MAC addresses or by abusing defects in the firmware of the switch. A switch spoofing attack works by sending forged MAC addresses to the switch. The idea is to convince the switch that the attacker's device is recognized like a server, printer, or a piece of hardware. Once the switch has been compromised, the attacker is free to transmit packets to any VLAN on the network. Steps involved in a switch spoofing attack are given below:

   1. A network device is accessed by the attacker.
   2. The attacker uses a tool to sniff MAC addresses on the network.
   3. A MAC address of a trusted device on the network is picked by the intruder.
   4. The intruder changes the MAC address of their device to the MAC address of the trusted device.
   5. Traffic is sent to the switch with the forged MAC address.
   6. The switch is tricked into thinking that the attacker's device is a trusted device and forwards the traffic to any VLAN on the network.

2. Double tagging: This type of attack involves sending traffic with two VLAN tags. The first tag represents the intruder's active VLAN, while the second represents the target one. The first tag will be deleted by the switch, but not the second tag. It will permit the traffic to be routed to the desired VLAN. Steps involved in a double tagging attack are given below:

   1. The attacker gains access to a device on the network.
   2. An Ethernet frame with two VLAN tags is created by the intruder.
   3. The Ethernet frame is sent to the switch.
   4. After removing the first VLAN tag, the switch directs traffic to the desired VLAN.

What is a Switch Spoofing Attack?

Switch spoofing is a type of VLAN hopping attack where an attacker impersonates a network switch to gain unauthorized access to multiple VLANs within a network. It takes advantage of vulnerabilities in the way switches communicate and make decisions. In switch spoofing, or switch spoof approach, attackers can exploit a switch's trust relationship with a neighboring switch by pretending to be a trunked switch. They send Dynamic Trunking Protocol (DTP) packets to the switch, attempting to negotiate a trunk connection. If the switch is configured with the default "dynamic auto" or "dynamic desirable" switchport mode, the attacker can establish a trunk connection and gain access to all VLANs on the network. As a result, they can trick the neighboring switch into sending traffic from multiple VLANs over to their own port.

Here's an example of how a switch spoofing attack works:

1. The setup and attacker's goal: A network switch can be considered as the traffic cop of a network, directing data traffic. This switch connects different departments (VLANs) in a building. Each department has its own separate room, which is ensured by the switch. The attacker's aim is to get into multiple rooms (VLANs) without the permission of the cop, which is the network switch.

2. Fake identity and instructions: The attacker pretends to be the network switch. By employing a phony badge, the attacker's device sends fake messages, called spanning tree protocol Bridge Protocol Data Units (STP BPDUs), that tell the genuine network switch that it is the cop with higher priority now. The network switch receives these fake messages and believes the attacker's device is the new cop to organize the traffic. It then makes the attacker's device the "root bridge" for the VLAN.

3. Trunk mode and hopping: Using its higher access privileges, the attacker's device tricks the switch into turning the switch port into trunk mode. Trunk mode allows data to flow between VLANs. The normal switch, now taking orders from the attacker, opens the doors between rooms (VLANs). With the port in trunk mode, the attacker's device can now access multiple VLANs and conduct unauthorized activities.

How to Prevent Switch Spoofing Attacks?

Network trunks are lines that help convey numerous signals simultaneously to give network access between two points. Trunking is an extremely vital element of the VLAN. VLAN trunking is nothing but a bridge between two devices that carry more than one VLAN. With VLAN trunking, the configured VLAN is accessed from the entire network. Attacks that use switch spoofing are based on switches with incorrect trunking setups.

To reduce the risk of switch spoofing attacks, switch ports should be configured to not negotiate trunks dynamically by disabling Dynamic Trunking Protocol(DTP). Additionally, switch ports should be configured manually so that they are not intended to be trunks as access ports. Some best practices to prevent switch spoofing attacks are as follows:

• Disable auto trunk negotiation settings in network switches. This can be done by disabling DTP in all ports. This setting should be left enabled only if necessary for day-to-day network functioning.
• Explicitly configure non-trunk ports as access ports. This results in fewer available penetration-ready ports for switch spoofing attacks.
• Follow the instructions to close or disable any ports that are not in use as of that moment.

What is a Double Tagging Attack?

A double tagging attack is a type of VLAN hopping attack where an attacker sends network traffic with two VLAN tags, confusing network switches and gaining unauthorized access to a different VLAN. In double tagging attacks, which are called C-Trunk attacks, the attacker is connected to an interface in access mode with the same VLAN as the native untagged VLAN on the trunk. The attacker sends frames with double VLAN tags. Those would be a frame with two 802.1Q tags, where the "inner" VLAN tag is the target VLAN, and the "outer" VLAN tag is the native VLAN. The first tag matches the attacker's VLAN, and the second tag matches the target VLAN. When the switch receives the frame, it removes the first (native VLAN) 802.1Q tag and forwards the frame with the second 802.1Q tag on its trunk interface(s). If the network switch doesn't properly handle double tagging, it can allow the attacker to "jump" from the native VLAN to the target VLAN effectively. It would forward the frames to the target VLAN to grant unauthorized access as a result.

How to Prevent Double Tagging Attacks?

Double tagging works by using the IEEE 802.1Q standard to break through the logical isolation of different VLANs. To reduce the risk of a double tagging approach or to mitigate it, configure the native VLAN of all trunk ports to an unused VLAN ID with no hosts. To make the network more secure, steer clear of using native VLAN 1, which is the default. Some tips to minimize or prevent double-tagging attacks are listed below:

• Isolate the network's default VLAN from host access.
• On trunk ports, enable native VLAN explicit tagging.
• On trunk ports, assign a unique ID to the native VLAN that is distinct from the other user VLANs.
• Never assign access ports to the native VLAN.

How to Detect VLAN Hopping?

Detecting VLAN hopping activities is critical for maintaining network security. The following strategies, practical tips, tools, and network monitoring techniques can be employed for detecting VLAN hopping:

• Use network monitoring tools to analyze traffic patterns and identify any anomalies or suspicious activities such as multiple or mismatched VLAN tags, unknown MAC addresses, or unusual protocols or ports. Unexpected communication from one VLAN to another is a red flag.
• Analyze the VLAN tags in packet headers by employing network analyzers, Wireshark for instance. Intrusion detection systems (IDS) tools can help. Employ network-based IDS to monitor traffic and look for unusual patterns or unauthorized VLAN transitions. If an IDS detects an attempt to jump between VLANs, it can trigger an alert or block the suspicious activity.
• Inspect unknown MAC addresses in traffic flow by employing ARP tables or MAC address tables to identify and track them.
• Train and educate network staff and users on VLAN hopping risks, detection, and detection strategies.
• Regularly update and patch network devices with the latest firmware and security patches.
• Configure port security on switches to limit the number of MAC addresses allowed on a specific port. When an attacker tries to manipulate the VLAN by introducing a new MAC address, port security can disable the port or generate an alert. Keep a close eye on VLAN configuration changes. Unauthorized changes can signal an attempt to manipulate VLAN settings for hopping.
• Regularly review switch logs for suspicious activities, like excessive MAC address changes or port violations. Sudden, unusual spikes in such activities can be indicative of VLAN hopping attempts.

By implementing these strategies and techniques, network administrators can effectively detect VLAN hopping attempts and respond promptly to maintain the security and integrity of their networks. An IDS can be useful in a university campus network when an attacker tries to hop from the student VLAN to the faculty VLAN for instance. Or, in a company network, when an attacker tries to connect a rogue device to a switch port within the marketing VLAN, the properly configured switch will detect the new MAC address associated with the rogue device, if port security is enabled. The switch can give an alert or disable the port by itself. One more use case is in a hospital's network when an attacker hops from a guest VLAN to the critical healthcare unit VLAN. Because the admin reviews switch logs frequently, a sudden increase in MAC changes on the switch is noticed. This can be automated using network monitoring tools.

How to Prevent VLAN Hopping Attacks?

Attacks that use VLAN hopping benefit from incorrect VLAN topology setup and misconfigured VLANs. Most of the time the feature that makes VLANs usable and preferable like logical partitioning, logical grouping, and high management offerings, also invites security risks when not applied properly. However, the risk of VLAN hopping attacks is significantly reduced by applying the proactive measures mentioned below and maintaining vigilance in the roadmap of network security. Ongoing monitoring, periodic audits, and a proactive approach to security are essential to achieve this purpose. Securing your network against VLAN hopping requires careful configuration and a combination of best practices. Here are some implementation tips for preventing VLAN hopping:

• Monitor and manage VLAN traffic: Regularly review and update VLAN configurations, monitor for any unusual activity, and integrate accounting and log management processes to track configuration changes.
• Properly configure switches: Ensure that switch-to-switch links only accept tagged frames, host ports do not accept tagged frames, and trunk ports do not have a native VLAN.
• Disable unused access ports and DTP: Disable unused switch ports as they can be a potential entry point for attackers. Disable Dynamic Trunking Protocol (DTP) on switch ports to prevent attackers from dynamically negotiating trunks with the switch.
• Enable VLAN pruning: Enable VLAN pruning on your switches to limit the propagation of unnecessary VLAN traffic, including the spread of the attack.
• Implement VLAN access control lists (VACLs): VACLs are used to filter traffic between VLANs and prevent unauthorized communication. Configure VACLs to allow only necessary traffic and block any suspicious or unauthorized traffic. Techniques like 802.1x for port authentication are utilized for access control and strict membership rules for VLANs can be enforced.
• Implement port security: Configure port security features on switches to limit the number of MAC addresses allowed on a port and prevent unauthorized devices from connecting.
• Educate staff: Train and educate network staff and users on awareness of VLAN hopping risks and detection methods.
• Keep network switches up-to-date: Regularly update and patch network devices with the latest firmware and security patches. Learn from past attacks and document the important details. It can be used to prevent possible future threats.
• Keep VLAN device inventory: Keeping an all-encompassing inventory of all the network switch ports and the associated interfaces is essential for managing VLAN devices. Enable effective performance monitoring to track their crucial performance metrics and utilization in real time.
• Layer 2 mapping: To keep current layer 2 maps of network VLANs, their connections, and related network resources, use a dependable networking mapping tool. This aids in your ability to quickly implement issue mitigation measures as needed and comprehend the logical organization of your network. You can separate important VLANs from other VLANs in your network, for instance, by consulting the layer 2 maps of your present network.
• Port management: To enable in-depth insights into your network traffic, deploy a switch port mapping and management solution to drill down to the physical- and traffic-level aspects of the switch ports and associated interfaces.
• Safety Standards: Making certain that your network complies with the most recent safety standards improves the resilience of your network as opposed to VLAN hopping. For example, meeting PCI requirements increases the security of your logical network design by a number of levels, making it difficult for intruders to obtain critical data by VLAN hopping.

How to Respond to VLAN Hopping?

A swift, coordinated, and effective response to a VLAN hopping attack is crucial for mitigating potential damage and minimizing or preventing potential further intrusions. By following a well-defined incident response plan, documenting the incident, and addressing vulnerabilities, you can strengthen your network's security posture and minimize the impact of such incidents in the future. Here are a few steps for how to respond to such an incident, including the isolation of the compromised VLAN, applying the response protocols, and reporting the incident.

1. The first response should be to isolate or disconnect compromised devices and affected VLANs or switches to prevent the attacker from moving further through the network. This can be done by disabling the associated port(s) or, if feasible, shutting down the entire VLAN to prevent further unauthorized access. The network security team and system admins should be notified. If sensitive data has been exposed or there is a risk of data breach, consider informing affected parties in compliance with data protection regulations. In case the attack poses a significant security threat or is suspected as a criminal offense, it can be reported to the law enforcement. Shareholders can also be notified if there is any. Effective communication is key to a coordinated response.
2. The second step is to conduct a thorough analysis to determine the root cause. Then identify the attack vector by reviewing logs and security measures for any misconfigurations or vulnerabilities. Address the ones that led to the attack to prepare for remediation. Preserve any relevant logs, system alerts, or information that can help in the investigation and identification of the attacker. Trace the origin and destination of VLAN hopping packets, including physical or logical switch ports. Then implement security improvements and best practices to prevent future incidents. Review and adjust VLAN configurations to ensure that the attack vector has been closed. Double-check port settings, access control lists, and security settings.
3. The final step is to collect and preserve evidence like logs in the event of a VLAN hopping attack and documentation. It can help determine the motive, method, and impact of the attack and for analysis and future prevention. A post-incident review helps to assess the effectiveness of your response and identify areas for improvement. Document the incident thoroughly, including the time of detection, actions taken, and any potential impact on network resources.