VLAN hopping

VLAN hopping is a computer security exploit, a method of attacking networked resources on a Virtual LAN (VLAN). The basic concept behind all VLAN hopping attacks is for an attacking host on a VLAN to gain access to traffic on other VLANs that would normally not be accessible. There are two primary methods of VLAN hopping: switch spoofing and double tagging. Both attack vectors can be easily mitigated with proper switchport configuration.

Switch spoofing

In a switch spoofing attack, an attacking host imitates a trunking switch by speaking the tagging and trunking protocols (e.g. Multiple VLAN Registration Protocol, IEEE 802.1Q, Dynamic Trunking Protocol) used in maintaining a VLAN. Traffic for multiple VLANs is then accessible to the attacking host.

Mitigation

Switch spoofing can only be exploited when interfaces are set to negotiate a trunk. To prevent this attack on Cisco IOS, use one of the following methods:

1. Ensure that ports are not set to negotiate trunks automatically.

Switch(config-if)# switchport nonegotiate

2. Ensure that ports that are not meant to be trunks are explicitly configured as access ports

Switch(config-if)# switchport mode access

Double tagging

In a double tagging attack, an attacking host connected on a 802.1q interface prepends two VLAN tags to packets that it transmits. The packet (which corresponds to the VLAN that the attacker is really a member of) is forwarded without the first tag, because it is the native VLAN. The second (false) tag is then visible to the second switch that the packet encounters. This false VLAN tag indicates that the packet is destined for a target host on a second switch. The packet is then sent to the target host as though it originated on the target VLAN bypassing the network mechanisms that logically isolate VLANs from one another. However, this attack allows to send packets toward the second switch, but possible answers are not forwarded to the attacking host.

Mitigation

Double Tagging can only be exploited when switches use "Native VLANs". Ports with a specific access VLAN (the native VLAN) don't apply a VLAN tag when sending frames, allowing the attacker's fake VLAN tag to be read by the next switch.

Double Tagging can be mitigated by either one of the following actions (Incl. IOS example):

• Simply do not put any hosts on VLAN 1 (The default VLAN). i.e., assign an access VLAN other than VLAN 1 to every access port

   Switch(config-if)# switchport access vlan 2
  

• Change the native VLAN on all trunk ports to an unused VLAN ID.

  Switch(config-if)# switchport trunk native vlan 999
  

• Explicit tagging of the native VLAN on all trunk ports. Must be configured on all switches in network autonomy.

  Switch(config)# vlan dot1q tag native
  

Example

As an example of a double tagging attack, consider a secure web server on a VLAN called VLAN2. Hosts on VLAN2 are allowed access to the web server; hosts from outside VLAN2 are blocked by layer 3 filters. An attacking host on a separate VLAN, called VLAN1(Native), creates a specially formed packet to attack the web server. It places a header tagging the packet as belonging to VLAN2 under the header tagging the packet as belonging to VLAN1. When the packet is sent, the switch sees the default VLAN1 header and removes it and forwards the packet. The next switch sees the VLAN2 header and puts the packet in VLAN2. The packet thus arrives at the target server as though it was sent from another host on VLAN2, ignoring any layer 3 filtering that might be in place.

See also

• Private VLAN

References

• Boyles, Tim (2010). CCNA Security Study Guide. Sybex.