V-273692

Discussion

Double encapsulation can be initiated by an attacker who has access to a switch port belonging to the native VLAN of the trunk port. Knowing the victim's MAC address and with the victim attached to a different switch belonging to the same trunk group, thereby requiring the trunk link and frame tagging, the malicious user can begin the attack by sending frames with two sets of tags. The outer tag that will have the attacker's VLAN ID (probably the well-known and omnipresent default VLAN) is stripped off by the switch, and the inner tag that will have the victim's VLAN ID is used by the switch as the next hop and sent out the trunk port.

Check Content

Review the switch configurations and examine all user-facing or untrusted switch ports.

device#show vlans
Total PORT-VLAN entries: 2
Maximum PORT-VLAN entries: 1024

Legend: [Stk=Stack-Id, S=Slot]

PORT-VLAN 222, Name Access, Priority level0, On
 Untagged Ports: (U1/M1)   1   2   3   4   5   6   7   8   9  10  11  12 
 Untagged Ports: (U1/M1)  13  14  15  17  18  19  20  21  22  23  24  25 
 Untagged Ports: (U1/M1)  26  27  28  29  30  31  32  33  34  35  36  37 
 Untagged Ports: (U1/M1)  38  39  40  41  43  44  45  46  47  48 
 Untagged Ports: (U1/M2)   1   2   3   4   5   6   7   8 
 Tagged Ports: (U1/M2) 1  
 Mac-Vlan Ports: None
     Monitoring: Disabled

PORT-VLAN 333, Name trunk, Priority level0, Off
 Untagged Ports: None
   Tagged Ports: (U1/M2) 1   
 Mac-Vlan Ports: None
     Monitoring: Disabled
device#

If all user-facing or untrusted ports are not configured as access (i.e., untagged) ports, this is a finding.