Introduction

In today’s world of networking, Virtual Local Area Networks (VLANs) are an essential tool for creating secure, logically segmented networks that improve performance, reduce congestion, and enhance security. However, despite their benefits, VLANs can be susceptible to various types of cyberattacks. One such attack is known as VLAN hopping, a method used by attackers to gain unauthorized access to a different VLAN than they are assigned to. This type of attack can compromise network integrity and lead to potential security breaches.

In this blog, we will delve into the causes and mechanics of VLAN hopping, specifically focusing on which feature of a switch makes it vulnerable to these types of attacks. We will explore the underlying vulnerabilities that can allow attackers to bypass VLAN boundaries and discuss how you can protect your network from such threats.

What is VLAN Hopping?

VLAN hopping is a technique used by cybercriminals to send traffic from one VLAN to another, bypassing the security mechanisms that are in place to isolate the VLANs. Essentially, the attacker “hops” from one VLAN to another, which is supposed to be isolated by network design. This could allow an attacker to access sensitive data, services, or devices that would otherwise be inaccessible.

VLAN hopping can be done in two main ways:

Double Tagging (802.1Q Tunneling)
Switch Spoofing

In order to understand what makes VLAN hopping possible, it’s essential to explore the features of switches that can inadvertently make a network vulnerable.

Vulnerable Feature on a Switch: The primary feature of a switch that makes it susceptible to VLAN hopping attacks is VLAN Trunking. Specifically, the 802.1Q trunking protocol that is commonly used to carry multiple VLANs across a single link between switches.

VLAN trunking allows multiple VLANs to be carried over a single physical connection. The 802.1Q standard tags the frames that are transmitted over the trunk, indicating which VLAN they belong to. However, if this trunking configuration is not correctly implemented, attackers can exploit it.

1. Double Tagging (802.1Q Tunneling) Attack:

Double tagging occurs when an attacker sends a specially crafted Ethernet frame with two VLAN tags. The first tag identifies the VLAN where the attacker’s device resides, and the second tag is the target VLAN that the attacker wants to hop into.

In a typical scenario, the switch strips the first VLAN tag and forwards the packet with the second tag. However, in a vulnerable network setup, if the switch doesn’t properly validate the VLAN tags, it will forward the packet to the target VLAN, effectively allowing the attacker to bypass VLAN isolation.

How Does Double Tagging Work? Here is how a VLAN hopping attack via double tagging works:

The attacker sends a frame with two VLAN tags: the outer tag refers to the attacker’s VLAN, and the inner tag points to the target VLAN.
The first switch that receives the frame removes the outer VLAN tag and forwards the frame over the trunk link.
The second switch, which is connected to the trunk, sees the frame with the inner VLAN tag and forwards it to the target VLAN, despite the attacker’s device not belonging to that VLAN.

This attack works when a network is misconfigured to trust untagged or improperly tagged frames.

2. Switch Spoofing:

Switch spoofing is another method used to exploit the vulnerability associated with VLAN trunking. In this case, the attacker configures their device to appear as a legitimate switch and negotiates a trunking connection with the switch they want to attack.

How Does Switch Spoofing Work?

The attacker’s device sends a Dynamic Trunking Protocol (DTP) message to the target switch, pretending to be a switch.
The target switch, trusting the DTP message, allows the attacker’s device to form a trunk connection.
Once the trunk is established, the attacker can send traffic for different VLANs, bypassing the VLAN isolation and potentially gaining unauthorized access to sensitive networks.

Why are Trunking and DTP Vulnerabilities Dangerous?

Both of these vulnerabilities are dangerous because they rely on the switch’s default trust settings and improper configuration. In most cases, network administrators may neglect to secure trunk ports properly or may fail to disable DTP on ports that should not be trunking. This oversight can open the door for attackers to exploit VLAN hopping attacks.

Mitigating VLAN Hopping Attacks: While VLAN hopping attacks can be concerning, there are several best practices you can implement to minimize the risk:

Disable Dynamic Trunking Protocol (DTP): DTP is used to automatically negotiate trunking between switches. However, it can also be exploited by attackers for switch spoofing. To mitigate this, disable DTP on all access ports by using the command
Configure Trunk Ports Manually: Ensure trunk ports are manually configured and only allow the necessary VLANs to traverse the trunk link. Use the switchport trunk allowed vlan command to limit which VLANs can be carried over the trunk.
Use VLAN Access Control Lists (VACLs): VLAN Access Control Lists provide another layer of security by filtering traffic between VLANs. Implementing VACLs helps restrict traffic flow and can prevent unauthorized access.
Use Private VLANs (PVLANs): PVLANs provide additional isolation within a VLAN, preventing unauthorized devices from communicating with each other. This feature is useful in highly sensitive environments.
Disable Unused Ports: Disable any unused switch ports to reduce the attack surface. If there are ports that don’t need to be active, ensure they are turned off to prevent unauthorized access.
Set Up Port Security: Port security can limit the number of MAC addresses learned on a switch port, effectively preventing unauthorized devices from connecting to the network.
Use 802.1X Authentication: Implementing 802.1X authentication for network access can help ensure that only authorized devices can connect to the network and use VLAN resources.

Conclusion

In conclusion, VLAN hopping attacks represent a serious security risk for networks that rely on VLANs for segmentation. The VLAN trunking feature, specifically the use of 802.1Q tagging and Dynamic Trunking Protocol, can make switches vulnerable to these types of attacks if not configured properly. By following best practices such as disabling DTP, manually configuring trunk ports, using VLAN Access Control Lists, and implementing port security, you can significantly reduce the risk of VLAN hopping and safeguard your network from unauthorized access.

Implementing these security measures is crucial for maintaining the integrity of your network infrastructure and ensuring that VLANs serve their purpose in creating isolated, secure segments within your network.