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Network Security Part II: Attacks

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Presentation on theme: "Network Security Part II: Attacks"— Presentation transcript:

1 Network Security Part II: Attacks
Layer 2 / 3 Attacks

2 Overview Layer 2 attack landscape MAC Attacks VLAN hopping attacks
ARP Attacks Spanning Tree attacks Layer 2 port authentication Other attacks SECURITY INNOVATION ©2003

3 The redundant rats nest!

4 Preliminaries All attacks and associated mitigation techniques assume a switched Ethernet network running IP If shared Ethernet is used (WLAN, Hub, etc.) the majority of these attack scenarios get much easier Obviously, if you aren't using Ethernet as your L2 protocol some of these attacks may not be appropriate. However you may be vulnerable to different ones. Rapid deployment. Attacks that are theoretical can move to the practical in a matter of days and become widely distributed in weeks. Focus will be on L2 attacks and potential solutions. SECURITY INNOVATION ©2003

5 MAC Attacks


7 What is the CAM Table? Basically a really efficient lookup table
Present on all modern switches CAM == Content Addressable Memory For more information on the CAM table and how it is updated check out or SECURITY INNOVATION ©2003

8 What is the CAM Table? This internal table looks something like this:
Port Ethernet Addresses Host or Uplink 1 01:00:af:34:53:62 Single host 2 01:e4:5f:2a:63:35 00:c1:24:ee:62:66 ... Switch or Hub 3 11:af:5a:69:08:63 00:17:72:e1:72:70 ... 4 00:14:62:74:23:5a SECURITY INNOVATION ©2003

9 B Unknown… Flood the Frame
Normal CAM Behavior I MAC A C Port 1 3 A  B Port 2 A  B MAC B Port 1 I see traffic to B! MAC A Port 3 A  B B Unknown… Flood the Frame MAC C SECURITY INNOVATION ©2003

10 Normal CAM Behavior II Port 2 MAC B Port 1 MAC A Port 3 MAC C
B  A A is on Port 1 Learn: B is on Port 2 Port 3 MAC A B C Port 1 2 3 MAC B MAC A MAC C SECURITY INNOVATION ©2003

11 Normal CAM Behavior III
A  B MAC B MAC C MAC A Port 1 B is on Port 2 I see do Not see traffic to B! MAC A B C Port 1 2 3 Port 2 Port 3 SECURITY INNOVATION ©2003

12 CAM Overflow I Theoretical attack made available to all….
macof tool since May 1999 “dsniff” by Dug Song Based on CAM Tables limited size SECURITY INNOVATION ©2003

13 CAM Overflow II Port 2 Port 1 Port 3 X is on Port 3 Y is on Port 3 MAC
B  A X  ? Y  ? Y is on Port 3 SECURITY INNOVATION ©2003

14 B Unknown… Flood the Frame
CAM Overflow III MAC X Y C Port 3 A  B Port 2 A  B Port 1 I see traffic to B! Port 3 B Unknown… Flood the Frame A  B SECURITY INNOVATION ©2003

15 63 bits of source (MAC, VLAN, misc) creates a 17 bit hash value
Catalyst CAM Tables Catalyst switches use hash to place MAC in the CAM table 1 A B C 2 D E F G 3 H I . J K 16, L M N O P Q R S T Flooded! 63 bits of source (MAC, VLAN, misc) creates a 17 bit hash value If the value is the same there are 8 buckets to place CAM entries, if all 8 are filled the packet is flooded SECURITY INNOVATION ©2003

16 MAC Flooding Switches with Macof

17 Snoop output on a non-SPAN port
CAM Table Full! Dsniff can generate 155,000 MAC entries on a switch per minute. Assuming a perfect hash function, the CAM table will be completely filled after 131,052 (approx. 16,000 x 8) entries Once table is full, traffic without a CAM entry floods on the local VLAN, but NOT existing traffic with an existing CAM entry. This attack will also fill CAM tables of adjacent switches. Snoop output on a non-SPAN port SECURITY INNOVATION ©2003

18 MAC Flooding Attack Mitigation
Port Security Capabilities are dependent on the platform Allows you to specify MAC addresses for each port, or to learn a certain number of MAC addresses per port Upon detection of an invalid MAC the switch can be configured to block only the offending MAC or just shut down the port. Port security prevents macof from flooding the CAM table. SECURITY INNOVATION ©2003

19 VLAN Hopping Attacks

20 VLAN “Hopping” Attacks
Trunk ports have access to all VLANs by default Used to route traffic for multiple VLANs across the same physical link Encapsulation can be 802.1Q or ISL Trunk Port SECURITY INNOVATION ©2003

21 Dynamic Trunk Protocol
What is DTP? Automates ISL/802.1Q trunk configuration Operates between switches Not supported on 2900XL or 3500XL DTP synchronizes the trunking mode on link ends DTP state on ISL/1Q trunking port can be set to “Auto”, “On”, “Off”, “Desirable”, or “Non-Negotiate”. Dynamic Trunk Protocol DST MAC 0100.0ccc.cccc SNAP Proto 0x2004 SECURITY INNOVATION ©2003

22 Basic VLAN Hopping Attack
Trunk Port Trunk Port A station can spoof as a switch with ISL or 802.1Q signaling (DTP signaling is usually required as well, or a rogue DTP speaking switch) The station is then member of all VLANs Requires a trunking favorable setting on the port SECURITY INNOVATION ©2003

23 Double Encapsulated 802.1q VLAN Hopping Attack
Note: Only works if trunk has the same native VLAN as the attacker 802.1q, 802.1q 802.1q, Frame Frame Strip off First, and Send Back out Send double encapsulated 802.1Q frames Switch performs only one level of decapsulation Unidirectional traffic only Works even if trunk ports are set to off SECURITY INNOVATION ©2003

24 Double Encap 802.1Q Ethereal Capture
Outer Tag, Attacker VLAN Inner Tag, Attacker VLAN SECURITY INNOVATION ©2003

25 Disabling Auto-Trunking
Defaults change depending on switch; always check. SECURITY INNOVATION ©2003

26 Security for VLANS and Trunking
Always use a dedicated VLAN ID for all trunk ports Disable unused ports and put them in an unused VLAN Be paranoid: Do not use VLAN 1 for anything Set all user ports to non-trunking (DPT Off) SECURITY INNOVATION ©2003

27 ARP Attacks

28 ARP Refresher An ARP request message should be placed in a frame and broadcast to all computers on the network Each computer receives the request and examines the IP address The computer mentioned in the request sends a response; all other computers process and discard the request without sending a response. V Z Y X W V Z Y X W V Z Y X W SECURITY INNOVATION ©2003

29 Gratuitous ARP 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 is a broadcast packet (like an ARP request) Host W: Hey everyone I’m host W and my IP address is: and my MAC address is 12:34:56:78:9A:BC V Z Y X W SECURITY INNOVATION ©2003

30 Misuse of Gratuitous ARP
ARP has no security or ownership of IP or MAC address What if we did the following? Host W broadcasts I’m with MAC 12:34:56:78:9A:BC (Wait 5 seconds) Host Y .2 Host W .4 Host X .3 /24 .1 SECURITY INNOVATION ©2003

31 Hands On Example Host X and Y will likely ignore the message unless they currently have an ARP table entry for When host Y requests the MAC of the real router will reply and communications will work until host W sends a gratuitous ARP again Even a static ARP entry for on Y will get overwritten by the gratuitous ARP on some OSs (NT4 and Win2k) Host Y .2 Host W .4 Host X .3 /24 .1 SECURITY INNOVATION ©2003

32 Dsniff ARP Spoofing MAC flooding Selective sniffing

33 Hands On - Arpspoof SECURITY INNOVATION ©2003

34 Arpspoof All traffic now flows through machine running dsniff in a half-duplex manner Port security does not help Note that the attack could be generated in the opposite direction by spoofing the destination host when the router sends its ARP request Attack could be more selective and spoof just one victim SECURITY INNOVATION ©2003

35 Supports more than 30 standardized/proprietary protocols
Selective Sniffing Once the dsniff box has started the arpspoof process, the magic begins: Supports more than 30 standardized/proprietary protocols FTP, Telnet, SMTP, HTTP, POP, poppass, NNTP, IMAP, SNMP, LDAP, Rlogin, RIP, OSPF, PPTP, MS-CHAP, NFS, YP/NIS, SOCKS, X11, CVS, IRC, AIM, ICQ, Napster, PostgreSQL, Meeting Maker, Citrix ICA, Symantec pcAnywhere, NAI Sniffer, Microsoft SMB, Oracle SQL*Net, Sybase, Microsoft SQL SECURITY INNOVATION ©2003

36 SSL/SSH Interception Using dnsspoof all web sites can resolve to the dsniff host IP address: Once that happens you can proxy all web connections through the dsniff host SECURITY INNOVATION ©2003

37 SSL/SSH Interception Using dsniff (webmitm) most SSL sessions can be intercepted and bogus certificate credentials can be presented SECURITY INNOVATION ©2003

38 SSL/SSH Interception Upon inspection they will look invalid but they would likely fool most users invalid SECURITY INNOVATION ©2003

39 The Evolution of dsniff: Ettercap
Similar to dsniff though not as many protocols supported for sniffing Can ARP spoof both sides of a session to achieve full-duplex sniffing Allows command insertion into persistent TCP sessions Menu driven interface SECURITY INNOVATION ©2003

40 It Doesn’t Get Much Easier…

41 ARP Spoof Mitigation: Private VLANs
Only One Subnet! Promiscuous Port Promiscuous Port Primary VLAN Community VLAN Isolated VLAN PVLANs isolate traffic in specific communities to create distinct “networks” within a normal VLAN Note: Most inter-host communication is disabled with PVLANS turned on Community ‘A’ Community ‘B’ Isolated Ports SECURITY INNOVATION ©2003

42 ARP Spoof Mitigation Some IDS systems will watch for an unusually high amount of ARP ARPWatch is a freely available tool that will track IP/MAC address pairings Consider static ARP for critical routers and hosts (potential administrative pain) SECURITY INNOVATION ©2003

43 Spanning Tree Attacks

44 A switch is elected as Root
Spanning Tree Basics STP purpose: To maintain loop-free topologies in a redundant Layer 2 infrastructure A switch is elected as Root Root selection is based on the lowest configured priority of any switch A F F Root A ‘Tree-Like’ loop-free topology is established from the perspective of the root bridge F F B B X F STP is very simple. Messages are sent using Bridge Protocol Data Units (BPDUs). Basic messages include: configuration, topology change notification/acknowledgement (TCN/TCA); most have no “payload”. Avoiding loops ensures broadcast traffic does not become storms SECURITY INNOVATION ©2003

45 Spanning Tree Attacks and Methods
Standard 802.1d STP takes seconds to deal with a failure or root bridge change (ha ha ha… DoS served here) Generally only devices affected by the failure notice the issue PortFast and UplinkFast can greatly improve this Sending BPDUs from the attacker can force these changes and create a DoS condition on the network As a link with macof: the TCN message will result in the CAM table aging all entries in 15 seconds if they do not communicate (the default is 300 seconds) Easy to create the DoS condition. Depending on the topology it could yield additional packets for the attacker Spanning-tree PortFast causes a port to enter the spanning-tree forwarding state immediately, bypassing the listening and learning states. You can use PortFast on switch ports connected to a single workstation or server to allow those devices to connect to the network immediately, rather than waiting for the port to transition from the listening and learning states to the forwarding state. UplinkFast provides fast convergence in the network access layer after a spanning-tree topology change using uplink groups. An uplink group is a set of ports (per VLAN), only one of which is forwarding at any given time. Specifically, an uplink group consists of the root port (which is forwarding) and a set of blocked ports (not including self-looped ports). The uplink group provides an alternate path in case the currently forwarding link fails. TCN topology change notification BRCONFIG OpenBSD System Manager's Manual BRCONFIG(8) NAME brconfig - manipulate bridge interfaces DESCRIPTION The brconfig utility retrieves kernel state of bridge interfaces and al- lows user control of these bridges. Bridge devices create a logical link between two or more Ethernet interfaces or encapsulation interfaces (see gif(4)), which will selectively forward frames from each interface on the bridge to every other interface on the bridge. This can be used to iso- late traffic between sets of machines on the same segment and to provide a transparent filter for ip(4) datagrams. In the first synopsis, the -a flag will cause brconfig to list the status of all bridges in the system. In the second, its command line consists of the name of a bridge and a set of operations to be performed on that bridge. The commands are executed in the order they were specified. If no command is specified in the second synopsis, the brconfig will display status information about the bridge. With the third synopsis, rules for filtering Ethernet MAC addresses can be added to a bridge. SECURITY INNOVATION ©2003

46 Spanning Tree Attack Example I
Access Switches Root Send BPDU messages to become root bridge STP Attacker F X B STP SECURITY INNOVATION ©2003

47 Spanning Tree Attack Example II
Access Switches Send BPDU messages to become root bridge The attacker then sees frames he shouldn’t MITM, DoS, etc. all possible Ant attack is very sensitive to the original topology, trunking, PVST, etc. Although STP takes link speed into consideration, it is always done from the perspective of the root bridge. Taking a Gb backbone to half duplex 10 Mb has been verified. Requires the attacker to be dual homed to two different switches (with a hub, it can be done with just one interface on the attacking host) Attacker F X B Root SECURITY INNOVATION ©2003

48 Knowledge Applied X B GE FE F Access Switch Root Attacker
STP Attacker F X B Root Access Switch FE GE Goal: See traffic on the backbone but interesting hosts have static ARP entries and are very chatty (macof will likely never steal their CAM entry) Step 1: MAC flood access switch Step 2: Run bridging software (i.e. brconfig) on attacking host; advertise as a priority zero bridge Attacker becomes root bridge Spanning tree recalculates GE backbone becomes FE Cam table on access switch is full (from macof); there is no room at the inn for the chatty servers. Traffic is flooded. SECURITY INNOVATION ©2003

49 STP Attack Mitigation Don’t disable STP, introducing a loop would become another attack. BPDU Guard Disables ports using portfast upon detection of a BPDU message on the port Globally enabled on all ports running portfast Root Guard Disables ports who would become the root bridge due to their BPDU advertisement Configured on a per port basis SECURITY INNOVATION ©2003

50 VLAN Trunking Protocol (VTP)
Used to distribute VLAN configuration among switches VTP is used only over trunk ports VTP can cause more problems than it solves, consider if it is really needed If needed use the VTP MD5 digest: SECURITY INNOVATION ©2003

51 Potential VTP Attacks After becoming a trunk port, an attacker could send VTP messages as a server with no VLANs configured. All VLANs would be deleted across the entire VTP domain Disabling VTP: SECURITY INNOVATION ©2003

52 Other Attacks

53 Cisco Discovery Protocol (CDP)
Runs at layer 2 and allows Cisco devices to chat with one another Can be used to learn sensible information about the CDP sender (IP address, software version, router model….) CDP is in the clear and unauthenticated Considering disabling CDP, or being very selective in its use in security sensitive environments (backbone vs user port may be a good distinction) SECURITY INNOVATION ©2003

54 CDP Attacks Besides the information gathering benefit CDP offers an attacker, there was a vulnerability in CDP that allowed Cisco devices to run out of memory and potentially crash if you sent it tons of bogus packets. Problem was due to a software implementation problem. A flaw in the memory allocation for the CDP process (basically there was no upper limit). SECURITY INNOVATION ©2003

55 DHCP Starvation Attacks
Anyplace where macof works, you can DoS a network by requesting all of the available DHCP addresses With or without the DoS, an attacker could use a rogue DHCP server to provide addresses to clients Since DHCP responses include DNS servers and default gateway entries, guess where the attacker would point these unsuspecting users? All the MITM attacks are now possible SECURITY INNOVATION ©2003

56 PVLANs Work Drop Packet
Private VLAN Attacks I Attacker Mac:A IP:1 Victim Mac:B IP:2 Router Mac:C IP:3 Promiscuous Port Isolated port S:A1 D:B2 X PVLANs Work Drop Packet SECURITY INNOVATION ©2003

57 Private VLAN Attacks II
Promiscuous Port Isolated port Attacker Mac:A IP:1 PVLANs Work Drop Packet Router Mac:C IP:3 S:A1 D:C2 Victim Mac:B IP:2 S:A1 D:B2 S:A1 D:B2 S:A1 D:B2 Routers Route: Forward Packet Only allows unidirectional traffic (Victim will ARP for A and fail) If both hosts were compromised, setting static ARP entries for each other via the router will allow bi-directional traffic Most firewalls will not forward the packet like a router This is not a PVLAN vulnerability as it enforces the rules! SECURITY INNOVATION ©2003

58 PVLAN Attack Mitigation
Setup ACL on ingress router port: All known PVLAN exploits will now fail VLAN ACL could also be used SECURITY INNOVATION ©2003

59 Multicast Brute-Force Failover Analysis
Nice Try M-cast Send random Ethernet multicast frames to a switch interface attempting to get frames to another VLAN SECURITY INNOVATION ©2003

60 Random Frame Stress Attack
Nice Try Frame Send random frames to a switch interface attempting to get frames to another VLAN SECURITY INNOVATION ©2003

61 Switch Management Management can be your weakest link
All the great mitigation techniques we talked about arent worth much if the attacker telnets into your switch and disables them Most of the network management protocols are insecure (syslog, SNMOP, TFTP, Telnet, FTP, etc.) Consider secure variants of these protocols as they become available (SSH, SCP, SSL, OTP etc.). Where impossible, consider out of band management. Always use a dedicated VLAN ID for all trunks Be paranoid: do not use VLAN 1 for anything Set all user ports to non trunking SECURITY INNOVATION ©2003

62 Hacking Cisco Cisco Bugtraq Vulnerabilities 1998 - 3 1999 - 5
2002 (est) SECURITY INNOVATION ©2003

63 Hacking Routers Example Exploits: HTTP Authentication Vulnerability
using a URL of where $NUMBER is an integer between 16 and 99, it is possible for a remote user to gain full administrative access. NTP Vulnerability By sending a crafted NTP control packet, it is possible to trigger a buffer overflow in the NTP daemon SNMP Parsing Vulnerability Malformed SNMP messages received by affected systems can cause various parsing and processing functions to fail, which results in a system crash and reload. In some cases, access-list statements on the SNMP service do not protect the device SECURITY INNOVATION ©2003

64 When a router is hacked it allows an attacker to
Hacking Routers When a router is hacked it allows an attacker to DoS or disable the router & network… Compromise other routers… Bypass firewalls, IDS systems, etc… Monitor and record all outgoing an incoming traffic… Redirect whatever traffic they desire… SECURITY INNOVATION ©2003

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