HACKING ON L2 DEVICES

Why We need Layer 2 security?

OSI layer was build to allow different layers to work without knowledge of each other.

Layer 2 can be very weak link in the network.

If any one of the layer hacked, communications are compromised.

Topics

1. MAC Attacks2. VLAN hopping attacks3. ARP attacks4. Spanning Tree Attacks5. DHCP Starvation Attack

MAC ATTACKS

What is MAC and CAM ?

MAC is 48 Bit L2 address

1234.5678.9ABC

First 24 bits is manufacture code Assigned by IEEE00-50-56XX-XXXX

Second 24 bits is specific interface,Assigned by ManufactureXXXX-XXC0-00-01

Content Addressable memory (CAM) table stores information such as MAC addressed available on physical ports with their associated VLAN parameters.

http://www.nirsoft.net/utils/mac_address_lookup_find.html

Normal CAM operation (1/3)

Normal CAM operation (2/3)

Normal CAM operation (3/3)

How Bypass the CAM?

1. Due to hardware restrictions, CAM has fixed size memory

2. Different switches has different size of CAM table

3. Overload the CAM to bypass the MAC table restriction

CAM Overflow attack

CAM Overflow attack

CAM Overflow attackExample output of macof tool

Duration of this attack

• 63 bits of source (MAC, VLAN, misc) creates 17 Bits of hash value and it will get stored in the CAM table.

• In a Cisco Catalyst 5650, we can store appox. 131,000 CAM entries

• Dsniff macof tool can create 1,55,000 MAC entries in a minute.

How to overcome this attack?

We can overcome this attack by enabling switch port security such as1. Static Secure MAC Addresses2. Dynamic Secure MAC addresses3. Sticky secure MAC addresses.

Limitations:

1. A secure port can’t be a SPAN port2. A secure port cannot be an 802.1X port.3. A secure port cannot belong to an EtherChannel port-channel

interface.

Cisco# conf t Cisco(config)# interface fastethernet0/1 Cisco(config-if)# switchport mode access Cisco(config-if)# switchport port-security Cisco(config-if)# switchport port-security maximum 5Cisco(config-if)# switchport port-security violation restrict Cisco(config-if)# switchport port-security mac-address aaaa.aaaa.aaaa Cisco(config-if)# switchport port-security mac-address bbbb.bbbb.bbbb

VLAN Hopping Attack

What is mean by Trunk Port?

• Trunk port have access to all the VLANs by default.• It used to route traffic of multiple VLANs across the same

physical link.• Encapsulation can be 802.1Q or ISL (Cisco preparatory)

Rogue Trunk / Switch Spoof attack

1. A computer can spoof as a switch with 802.1Q or ISL signaling.2. DTP signaling is required.3. Requires trunking favorable setting on the switch port.

Double encapsulated VLAN attack

1. Attacker sends double tagged 802.1Q frames2. Switch can perform only one level of decapsulation, so the frame

will be forwarded as per the second tag.3. VLAN hopping occurs

The Yersinia

Security practices to avoid the attacks• Always use dedicated VLAN ID for all trunk ports• Disable Unused ports and put them on unused VLANs• Don’t use VLAN1 for anything• Set DTP off

ARP Attacks

An ARP request messageshould be placed in a frameand broadcast to allcomputers on the network

Each computer receives therequest and examines theIP address

The computer mentioned inthe request sends aresponse; all othercomputers process anddiscard the request withoutsending a response

ARP recall

Gratuitous ARP is used by hosts to announce their IP address to the local network and avoid duplicate IP addresses on the network; routers and other network hardware may use cache information gained from gratuitous ARPs

Gratuitous ARP

ARP Spoofing• ARP has no security on IP / MAC addresses

• Host W broadcasts I’m 1.2.3.1 with MAC 12:34:56:78:9A:BC in regular interval

• When the host x requests the MAC of gateway, it will be overwritten by the gratuitous ARP packet

Even a static ARP entry for 1.2.3.1 on Y will get overwritten by the Gratuitous ARP on some Oss.

Dsniff Output

Sniffed outputSniffed credentials by Cain and Abel

ARP Spoof Mitigation

• Some IDS systems will watch for an unusually high amount of ARP traffic

• ARPWatch, AntiARP tools are available to avoid ARP spoofing

• Static ARP on critical systems• ARP firewall feature is implemented in some Cisco devices.

STP Attacks

STP BasicsSTP is used to avoid loops and broadcast storms

Messages are sent using Bridge Protocol Data Units (BPDUs). Basic messages include: configuration, topology change notification/acknowledgment. (TCN/TCA)

Standard 802.1d STP takes 30-45 seconds to deal with a failure or Root bridge change.

Sending BPDUs from the attacker can force these changes and create a root bridge change.

Now attacker can see any frames

MITM, DoS, etc. all possible

STP Attack Mitigation

We can avoid this attack by enabling

Root guardBPDU guard in the switch

DHCP attack

DHCP Starvation Attack

• This attack performing by broadcasting number of DHCP requests by spoofed source MAC

• If enough request flooded onto the network, the attacker can completely exhaust the address space allocated by the DHCP servers for an indefinite period of time.

DHCP Starvation Attack

• Yersinia is used here for DHCP starvation attack.

Mitigation of DHCP Starvation attack• Port security should be enabled.• DHCP snooping to be enabled

Cisco(config)#interface range GigabitEthernet1/0/1 - 48Cisco(config-if)#description Access PortsCisco(config-if)#switchport port-securityCisco(config-if)#switchport port-security maximum 4Cisco(config-if)#switchport port-security aging time 5Cisco(config-if)#switchport port-security aging type inactivityCisco(config-if)#switchport port-security violation shutdownCisco(config-if)#exitCisco(config)#interface GigabitEthernet1/0/49Cisco(config-if)#description Uplink to DHCP ServerCisco(config-if)#ip dhcp snooping trustCisco(config-if)#exitCisco(config)#ip dhcp snoopingCisco(config)#ip dhcp snooping vlan 1-10Cisco(config)#ip dhcp snooping database tftp://remotehost.company.com/Ciscodhcpsnoop.txtCisco(config)#ip dhcp snooping verify mac-addressCisco(config)#endCisco#

Thank You