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Basics of Network Security
Network
Nicolas T. Courtois
- University College London
Reading
Roadmap
2
•
Chapter 16.1. and 16.2. – Layering of Protocols
•
•
(almost not studied: IPSec Chapter 16.3.)
Not now: Chapter 16.5. SSL/TLS: will be studied later!
•
•
•
Chapter 17.1. - TCP sessions
Chapter 17.2. - DNS
Chapter 17.3. - Firewalls: Packet Filters
Nicolas T. Courtois, January 2009
NetSec
Shared Media Networks
• Shared Medium = all messages can
be received at any location.
• Example: Most modern LAN (Local Area Networks)
– WiFi and Ethernet are the most popular
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NetSec
Network
Layers
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NetSec
OSI Levels:
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© Shi Zhou
NetSec
Encapsulation /
Protocol Stacking
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NetSec
Encapsulation (typical case)
Application message
message
segment
TCP
data
TCP
data
IP TCP
data
ETH IP TCP
data
TCP
data
TCP Header
IP packet
IP Header
frame
Link (Ethernet) Header
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ETF
Link (Ethernet) Trailer
NetSec
*** + A lot more industrial standards and protocols….
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NetSec
Attacks:
Passive
/
• Attack the browser/server
Active:
•
– Sniff connections with a bank – Plugin MITM
• Attack the TCP or UDP
– Sniff port 8080
•
– TCP session hijack
• Attack the IP protocol
– Sniff IP Datagrams
•
– Spoof IP address
• Attack the LAN
– Sniff Ethernet frames
• Attack the Shared Media
– Sniff the media / RF
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•
– ARP poisoning
•
– Relay attacks
NetSec
ARP Protocol
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NetSec
MAC Address = Media Access Control
• 48 bits = 6 bytes
• Example: 0A:BC:00: 00:00:1F
• Should be fixed in hardware and never repeat
– first half is the manufacturer…
• In practice:
– can be changed in SOME hardware
– can be simulated in software!
– can repeat
• by accident OR
• manufacturer does not care…
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NetSec
Address Resolution Protocol (ARP)
• translate IP addresses
to Ethernet MAC addresses
– legacy: also used in WiFi
Each host OS has:
– a table of IP to MAC addresses
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NetSec
ARP Discovery
Your IP address is managed centrally,
by DHCP (attributed by a local DHCP server).
Not always known to other hosts on the same
LAN.
New computer on the LAN
– did not communicated before =>
• NOT in our table of IP to MAC addresses
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NetSec
Who is 192.168.1.3?
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4/24/2009
Nicolas T. Courtois, 2011
Copyright © 2009
Shah Mahmood
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NetSec
ARP Poisoning - Example
• How does it work?
– ARP request:
• Sender = 01:02:03: 01:02:03.
• Content = “ARP who is” IP=192.168.1.3 ?
– ARP reply: it’s me!
• not authenticated!
• me = MAC Address 01:02:03: 01:02:04.
– From now on, all messages directed to 192.168.1.3 will
be in fact sent to 01:02:03: 01:02:04.
• Can be the attacker.
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NetSec
ARP Poisoning
• Defenses
– static ARP table, does not change
– DHCP snooping (access control based on IP,
MAC, and port)?
– detection: Arpwatch, Reverse ARP
• Legitimate use, very frequent:
– redirect a user to a registration page before
allowing usage of the network…
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NetSec
IP Protocol
IP TCP
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data
IP Protocol
Delivers packets from 1 IP address to another
• Source: pelican = 192.168.0.40
•
Destination:
news.bbc.co.uk = 192.168.0.50
Each packet = datagram is independent of others
• delivery not acknowledged
•
can be altered
•
can be lost
•
can arrive in any order
•
source address is required
by upper-level protocols!
IP
IP TCP
data
payload
IP Header
IP Header:
• Source: 192.168.0.40
• Destination: 192.168.0.50
• Time To Live: TTL=128
• Header checksum: 0x57d1
• Etc..
Non-cryptographic checksum
Q. WHY is it good/needed?
IP
IP TCP
data
payload
Sniffing – Passive Attacks
• read interesting data
• ftp, telnet send passwords in clear!
Alice
Network
Defenses:
•encryption,
•improved routing
Bob
NetSec
Tools for Network Sniffing
•
•
•
•
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tcpdump, windump
snort
wireshark
etc…
Nicolas T. Courtois, 2011
NetSec
Hub vs. Switch
• Sniffing through a hub
– a hub forwards all traffic to all connected ports
– Easy sniffing for anyone
• is sniffing with a router/switch possible?
YES: tool = dsniff
– flooding attacks,
• saturate memory, becomes a hub
– use ARP poisoning (studied later)
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NetSec
Ethereal Sniffer Example
TCP, HTTP and Connection…
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NetSec
DNS
=Domain Name Server(s)
www.darpa.mil = 192.5.18.19
IP TCP
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data
NetSec
Simple DNS Attack – Race Condition
• attacker observes UDP DNS query sent to
well known server on well known port
•first response wins
•second response is silently dropped
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NetSec
DNS Hijacking = DNS Redirection
Route packets to the wrong server,
A number of commercial ISPs use DNS
hijacking for their own purposes, such as
displaying advertisements…
In the UK, the Information Commissioner's Office have acknowledged that this practice contravenes the EC
Directives on Data Protection and Privacy and Electronic Communications (PECR)… but refused to act
against companies which do it, claiming it does little actual harm…
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NetSec
Next Level: DNS Cache Poisoning: 1234
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NetSec
Secure DNS [2010]
DNSSEC = Digitally sign DNS data + use PKI
In July 2010 DNSSEC was integrated in all
the Internet root servers.
What remains: all usual problems with the PKI…
Main problem: how do I know an authentic PK for any entity?...
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NetSec
Other IP Attacks
IP TCP
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data
NetSec
IP Spoofing
• Easy, false source address
router
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NetSec
Record and Playback
• Easy… false source address again
• can be detected… or not.
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NetSec
Squatting
• Means:
Simulating a local machine (relay)
For example to access the Intranet. Etc.
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NetSec
Interconnection of Networks
= InterNet
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NetSec
Internet = Public Network and Need for NAT
• Local address space
– vs. global Internet address space…
– there is not enough IPv4 addresses
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NAT =
Network Address Translation
Nicolas T. Courtois, 2011
NetSec
NAT
• Q:
What is the security advantage of NAT?
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NetSec
NAT
• Q:
What is the security advantage of NAT?
• Hides computers IP addresses port numbers and their
connections in internal networks
– asymmetry: local computers know more…
• Forces businesses to use routers and firewalls
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NetSec
Detailed example
Observations?
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NetSec
Detailed example
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NetSec
Detailed example
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NetSec
Firewalls
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NetSec
Business Need
• Security
– prevent intrusion
– prevent data leaks
– prevent compromised machines
from spreading viruses to the Internet
– other
• Cost of using the network: many businesses PAY per
megabyte => block access to P2P, Skype, VoIP etc…
• Productivity
– How do employees spend their time?
• Liability
– Avoid legal risks!!!
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NetSec
Firewall
• Perimeter defence:
– Divide the world into safe inside(intranet) and dangerous
outside world (Internet)
– Prevent anything bad from entering the inside
• Block communications which are risky or just
unnecessary
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NetSec
Sorts of firewalls:
• Stateless packet filter
• Dynamic packet filter = Stateful
– smarter, knows if this is likely to be a response to
a message really sent (connection was initiated
or not)
• Transport and application-layer firewalls
• VPN and IPSEC
• SSL/TLS
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not firewalls anymore rather
methods to ‘securely’ circumvent
them OR/AND to avoid the need to
have firewalls
NetSec
Stateless Firewalls = Packet Filters
• Allow or block IP packets based on their IP
header fields and TCP/UDP port numbers
– Look at:
•
•
•
•
protocol (TCP/UDP/ICMP),
source and destination IP address,
source and destination port,
TCP flags etc
• Major trend: modern firewalls are aware of
more and more higher-level protocols!!!
– How? Many ways… they inspect packets…
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• Method 0: inbound smtp uses port 25
Nicolas T. Courtois, 2011
NetSec
Stateless Firewalls = Packet Filters
• Packet filter is defined as a rule table
–
–
–
–
Linear list of rules
Each rule consist of conditions and an action
For each packet, the first matching rule is found
Two possible actions:
• allow (accept, permit, bypass) or
– maybe also allow and log
• block (drop, deny),
– Or maybe block and log
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NetSec
Routers and Packet Filters
• Firewall rule table is similar to a routing table,
with the option of dropping some packets
• Most routers implement a packet filter firewall
– Need: very fast real-time filters…
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NAT
NetSec
Packet Filter Example 1
Inbound email to SMTP server 1.2.3.10
* = any
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- for each packet, the first
matching rule is found
NetSec
TCP and UDP
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NetSec
TCP and UDP
• TCP = Transmission Control Protocol
– session and connection between two parties
– port 1081 <=> port 80
• UDP =User Datagram Protocol
– No need to open / close connections
– Just transmit / broadcast /receive some data…
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NetSec
TCP Handshake – SYN and ACK
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Copyright © 2009
Shah Mahmood
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NetSec
More Firewalls
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NetSec
Packet Filter Example 2
Allow only outbound connections:
The flag ACK in the second rule will drop out packets without ACK.
=> ONLY ONE CASE: It becomes NOT possible to initiate TCP
connections (SYN=1 ACK=0) from the inside.
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NetSec
Dynamic Firewalls
• Stateful filter: change filtering rules based on
previously seen packets…
– no limit to how complex they can be…
• Problem: outbound TCP or UDP packets
create “a hole” for inbound packets on the
same connection…
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NetSec
*Tricky Questions
• We put ACK=1 in our rule WHEN we want to
block the first packet of this connection,
the one which initiates it.
• Now who initiates the FTP data connection?
Second stage in the ftp protocol. Port 21.
– the server! NOT the client
– (more details on the next slide)
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NetSec
*Tricky Question About FTP (past exams)
traditional “active
mode” FTP
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NetSec
**Additional Remarks About FTP
Note that 20, 21 are “PRIVILEGED” ports:
a program running in the userland will NOT
typically be allowed to use them…
Later FTP servers developed so called “passive mode” where the data
connection is done differently,
for example to circumvent firewalls blocking the server initiating the connection,
and to allow web browser to connect to FTP sites as well…
These “passive mode” connections use unprivileged ports of both sides, which
makes them even harder to “firewall” and (prevent unintended usage)…
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NetSec
Layering the Defences
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NetSec
DMZ = De-Militarized Zone
Common two-firewall configuration for isolating
publicly-accessible services from the Internet
but also from the Intranet
•
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a hardened ”bastion host” in the DMZ to connect with ssh from the outside!
Nicolas T. Courtois, 2011
NetSec
Firewall Circumventing
• Remember: inbound smtp uses port 25
– WHY?
– IANA-attributed = Internet Assigned Numbers Authority.
– so that these things CAN be filtered by firewall…
• New applications want to gain popularity => developers and
users always DO THEIR BEST to circumvent firewalls
– all done ”over web” = port 80,
– or, better, “secure web” = port 443
– P2P protocols, Skype, TeamViewer… want or need to
circumvent firewalls…
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NetSec
VPN and
IPSEC
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NetSec
Business need: Connect two offices
• Company with multiple offices:
– Both parties have a firewall
– leased line between offices would be expensive.
– use the Internet: very cheap BUT dangerous/insecure
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NetSec
VPN = Virtual Private Network
• As secure as a dedicated connection
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NetSec
IPSEC
• A sort of “state of the art” low-level VPN system.
– standard to secure the Internet at the IP level
– secures network links, point-2-point connections on the way,
• does NOT provide user-2-user or application-2-application security
• Integrity + authenticity for ALL IP packets
• Not yet widely used, mandatory for IPv6
–
–
–
–
everybody uses just SSL (at higher level).
hard to make it work with existing firewalls etc..
IPSEC has no defenses against network traffic analysis
Gollman: IPSEC violates two design principles
of the Internet: network should be:
• stateless: in IPSEC network nodes to work synchronized
• unreliable: however in IPSEC there are problems if packets are lost…
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NetSec
How IPSEC Works
• Integrity + authenticity for ALL IP packets
– Based on symmetric key cryptography, complex standard
– Key management: the hard part. IKE = Internet Key Exchange
• Yet another encapsulation
ESP = Encapsulating Security Protocol: E+A
•
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NetSec
TCP
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NetSec
TCP = Transmission Control Protocol
– Connect port 1081 at host1
– to port 80 at host2
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NetSec
TCP Basics
One application message
TCP
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data
TCP
data
TCP
data
TCP = Transmission Control Protocol
• Connection-oriented, preserves order
– Sender
• break data into numbered packets
– Receiver
• acknowledge reception
• lost packets are resent!
• reassemble in correct order!
Book
Mail each page
Reassemble book
1
19
1
5
1
TCP Header
Header:
• Source Port: 1081
• Destination Port: 80
• Checksum: 0xa858
• Flags…
•
etc..
TCP
data
Example:
Connections and TCP Packets - Security
• predictable
sequence numbers
– 32 bits
– should start at random
(each OS does it differently,
many are NOT random)
– then grow by 1
for every byte transmitted
• at least one flag
is always SET
***Simplified TCP State Diagram
NetSec
Session Hijacking
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NetSec
Session Hijacking
Idea:
• I do not know the password,
• or do not have the right certificate and keys
• or do not have the tamper-resistant token….
=> I will join in the middle of existing session!
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NetSec
Session Hijacking
• host-based session hijacking
– root in Unix => can read and write local terminal devices?
– network card firmware rootkit
– etc…
• network-based session hijacking
– against TCP protocol
– against HTTP
– Q: why all banks have the ‘logout’ button even if they have a
very short timeout?
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NetSec
Attacks on TCP
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NetSec
TCP Handshake – SYN and ACK
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Copyright © 2009
Shah Mahmood
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NetSec
Attack – SYN Flooding
many fake
IP source
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Copyright © 2009
Shah Mahmood
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NetSec
TCP Session Hijacking
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NetSec
TCP Hijacking [Morris 1984]
• attacker spies on transmission between victim and
server, e.g. a telnet session with a packet sniffer
• waits for authentication to take place (pwd => OK
or any other credentials like OTP token)
• wants to hijack the session later!
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NetSec
TCP Hijacking Particulars
• The attacker observes the sequence numbers,
they are easy to guess:
– with high probability it is incremented by 1 MSS (maximum segment size).
• At a certain moment forges a packet to insert attacker’s
commands
• Does it work?
– the first ‘fake’ packet is accepted if it is received first (race condition)
• May be enough to execute the attack, like buy or transfer money with one click
– but THEN (and only then… it may be too late), another packet with the same
sequence number will be received, the attack will detected OR
synchronization will simply be lost.
• To make it really work to hijack the connection completely
and keep it active for unlimited time, MUST be combined
with ARP poisoning OR routing/MITM attacks
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NetSec
TCP Hijacking Defences
VERY IMPORTANT in the normal TCP:
The initial sequence number must be chosen at random.
Good practice:
See RFC1948 = Defending Against Sequence Number Attacks
More advanced solutions:
• Citing RFC1948: “Clearly, the proper solution is cryptographic
authentication” BUT… “it will quite a long time before that is
deployed” (tcpcrypt = proposed in July 2010).
• There also some other ‘imperfect’ solutions like TCP cookies…
(TCPCT proposal, 2009).
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NetSec
HTTP Session Hijacking
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NetSec
HTTP Session Hijacking
• HTTP is a stateless protocol
• Sessions are established using:
– [ephemeral=session] cookies.
• small text files, stored on user computer, containing
useful information for user settings on a server
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NetSec
HTTP Session Hijacking
(contd.)
• Obtain Cookies: two main methods
– Using MITM using packet sniffers
• or sniffers at any other place level or protocol layer…
– Browser attacks,
• e.g. malicious Firefox plug-ins
• Then the connection can be intercepted!
– Demo: Gmail @ Blackhat 2007.
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NetSec
Risks from Session Hijacking
Modify:
• emails,
• ftp file downloads,
• http responses
• DNS zone transfers,
• etc..
• Carry out MITM attacks on SSL or weak cryptographic protocols.
– often work because users ignored the warnings…
• Denial of service attacks, e.g. reset the connection
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NetSec
HTTPS Session Hijacking???
Q: Why and how SSL prevents this?
See part 6
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