What is a VLAN Hopping Attack?

 

What is a VLAN Hopping Attack?

Virtual Local Area Networks (VLANs) play a crucial role in segmenting traffic for security and performance in today's interconnected networks. However, attackers can exploit misconfigurations to bypass these segmentation controls through VLAN hopping attacks.

In this blog, we’ll explore:

• What VLAN hopping is
• How VLAN hopping attacks work
• Types of VLAN hopping attacks
• How to prevent VLAN hopping

What is VLAN Hopping?

VLAN hopping is a network attack where an attacker gains unauthorized access to traffic on a different VLAN, bypassing network segmentation. This allows them to intercept, modify, or disrupt traffic for other VLANs.

VLAN hopping is possible due to misconfigured switch ports or exploiting VLAN protocols like Dynamic Trunking Protocol (DTP).

 

How Does VLAN Hopping Work?

There are two main methods attackers use to perform VLAN hopping:

• Switch Spoofing (DTP Exploitation)
• Dynamic Trunking Protocol (DTP) is enabled by default, and the interfaces of our switches will be in (dynamic auto) or (dynamic desirable) mode. It means that whenever we receive a DTP packet that requests to form a trunk.
• If a switch port is left in "auto" or "desirable" mode, an attacker can connect a rogue switch and send DTP packets to form a trunk.
• Once a trunk is established, the attacker can access all VLANs passing through that trunk.

Attack Scenario:

1. An attacker connects a rogue device (e.g., a laptop running a switch emulator) to an unsecured switch port.
2. The rogue device sends DTP frames, tricking the switch into enabling trunking.
3. The attacker now has access to all VLANs on the trunk link.
4.  Double Tagging (VLAN Stacking Attack)
5. This attack exploits 802.1Q VLAN tagging and requires the attacker to be on the native VLAN.
6. The attacker sends a frame with two VLAN tags:
7. The outer tag matches the native VLAN (which is stripped by the first switch).
8. The inner tag is the target VLAN, which the next switch processes.
9. Since the first switch removes the outer tag, the second switch forwards the frame to the target VLAN.

Attack Scenario:

• The attacker is connected to a port on the native VLAN (e.g., VLAN 1).
• They craft a packet with two VLAN tags:
• Outer tag: VLAN 1 (native VLAN)
• Inner tag: VLAN 20 (target VLAN)
• The first switch strips the outer tag and forwards the frame.
• The second switch sees only the inner tag (VLAN 20) and sends it to the victim’s VLAN.

Note: Double tagging is a one-way attack (only egress traffic is affected) and works only if the attacker is on the native VLAN.

 

How to Prevent VLAN Hopping Attacks

To secure your network against VLAN hopping, follow these best practices:

1. Disable DTP (Dynamic Trunking Protocol)
2. Configure switch ports as access ports unless trunking is explicitly needed.

Use the following commands on Cisco switches:

Use a Dedicated Native VLAN

• Avoid using VLAN 1 (default native VLAN) for user traffic.
• Assign an unused VLAN as the native VLAN for trunk ports:

 

Enable Port Security

• Restrict the MAC addresses allowed on a port to prevent unauthorized devices.

 

Disable Unused Ports

• Shut down unused switch ports to prevent unauthorized access.

 

Implement Private VLANs (PVLANs)

• Use PVLANs to further isolate devices within the same VLAN.

Monitor and Audit VLAN Configurations

Regularly check switch configurations for unauthorized trunking.

Use tools like SIEM (Security Information and Event Management) to detect anomalies.

 

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Conclusion

VLAN hopping is a serious security threat that can bypass network segmentation, allowing attackers to intercept sensitive traffic. By disabling DTP, securing trunk ports, and using proper VLAN configurations, organizations can significantly reduce the risk of VLAN hopping attacks.

Best Practices Recap:
✔ Disable DTP on all access ports.
✔ Change the default native VLAN.
✔ Enable port security and disable unused ports.
✔ Regularly audit VLAN configurations.

By implementing these measures, you can strengthen your network security and prevent VLAN hopping exploits.

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Have you encountered VLAN hopping in your network? Share your experiences in the comments! 🚀

#NetworkSecurity #CyberSecurity #VLAN #Hacking #InfoSec