Vlan Hopping

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Hello, i have read many doc about this exploit but there are any

I hnow that this exploit exist in 3 ways :

1) Basic=> The attacker spoof a switch and gains the trunked states of
the switch's port. Rely on auto-negotiate feature turned ON. This ways
is simple to understand and to block

2) Complex 1 => This attack is described on
http://www.sans.org/resources/idfaq/vlan.php and to work need that the
attacker and the trunk share same native vlan ( ex. VLAN 10 ). In this
doc. the attacker send on the access port ( VLAN 10 ) a tagged frame
with a VLAN-ID of target VLAN ( ex. VLAN 20 ) . The switch takes frame
and forward it on trunk port without native tag (10). The other switch
(connected via-trunk) read VLAN-ID(20) and forward frame on the access
vlan 20.

In this scenario my doubts is :

- Why the first SW accepts tagged frame ?
Is this behavior an anomaly of work ?

- Why the last switch that receives native frame on trunk port reads
the VLAN-ID ? Is this normal or anomaly ? I think that sw does'nt read
VLAN-ID because the frame on trunk is native .

2)Complex 2 => In other docs per ex:
there is an attack called " Double-Encapsulated 802.1Q ". In this
exploit the conditions are similar to the
precedent but the attacker need to insert two VLAN-ID ( outer,inner ).
If this case work then the first switch read VLAN-ID on access port and
forward frame on trunk ( strip off first VLAN-ID ) . This behavior is
different that
precedent case . Why the switch forward this frame according to VLAN-ID

on the access-port ? Is this behavior another anomalies ?

Sorry about lenght of post but i want to understand if this
vulnerability were resolved or not .


Giuseppe Citerna

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