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CIT 612 Fundamentals of Computer Security Mohammed A. Saleh 30 th December 2009 1.

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1 CIT 612 Fundamentals of Computer Security Mohammed A. Saleh 30 th December 2009 1

2 Web Attacks and Internet Vulnerabilities With interest in computer and network security, you would need to know about the Internet Focuses on the internet protocols that keep the internet humming We will look at vulnerabilities of several of these services, as well as exploits that can be used to attack them 2

3 About the Internet When many people think of the Internet, the first thing that comes to mind is often the World Wide Web The Internet is composed of many different connection schemes called protocols All the protocols transmit over a common system of communication called Transmission Control Protocol/Internet Protocol (TCP/IP) Protocols include: File Transfer Protocol (FTP)  allows rapid, reliable transfer of data files between repositories, called FTP servers, and between computers with FTP client software installed, called FTP clients. 3

4 Cont … Hypertext Transfer Protocol (HTTP)  allows users to access pages of text that are marked up using a special format called the Hypertext Markup Language (HTML)  HTML tags are inserted into a web document to indicate the desired font, color, and position of text, and it facilitates linking to different different web sites, files, or pages Simple Mail Transfer Protocol (SMTP)  The SMTP service allows a standardized method of electronic mail transmission Domain Name Service (DNS)  The Domain Name Service resolves the easy-to-read names familiar to Internet users, such as, to the Internet Protocol addresses that actually guide information around the network, such as 4

5 Cont … Dynamic Host Configuration Protocol (DHCP)  DHCP automatically provides an Internet Protocol (IP) address, such as 172.16.32. 15, to a computer on a local area network  An IP address is required to communicate with other network devices that exist beyond the immediate proximity of the computer requesting the address 5

6 Packets, Addresses and Ports To increase the reliability of communications, messages are chopped up into chunks of uniform length, called packets A packet-switched network gives each packet an individual address label and then shoots it out onto the network, trusting that each packet will eventually make it to its destination There are mechanisms deployed to determine whether packets sent are corrupted or delayed, and to facilitate retransmission of replacements. 6

7 Cont … So how do packets help make networks and the Internet reliable?  a packet travels over the circuit quickly If it goes missing, its replacement can be retransmitted without taking a long time  it is understood that packets may take one of several possible routes to their destination there is a possibility that packets may actually spend part of their journey traveling in parallel 7

8 More concepts An Internet Protocol (lP) address describes a location on the network An IP is also known as a network address or a logical address The use of logical address allow the network to route packets to the correct part of the network IP addresses generally take the form of four numbers, separated by periods, in which each number is between 1 and 255  For instance, Each device on the other hand a hardware address, known as the MAC address 8

9 More concepts From a security perspective, It is important that packets are identified by function, what they are intended to do. This allows them to be switched to the correct location by inspection without having to open them up and examine their contents Packets use the port number to state the packet function Each IP address comes with roughly 65,000 port numbers Different types of network traffic use different ports. 9

10 Cont … Question: Why do you need to understand this multitier system of addressing? Answer: most network attacks in some way involve falsely manipulating or replacing the IP address, MAC address, or Port In fact, one of the most important tools used today for network safety, the firewall, is based almost entirely on recognizing suspicious or invalid combinations of addresses and ports 10

11 Network Protocols A protocol is a defined procedure for interconnecting and interacting It determines how data are transported over the Internet, or over a LAN that uses TCP/IP, provide a variety of services Some protocols move web pages, some move email, some move files, and some move streaming media Many of the most important network protocols, which also happen to be most commonly attacked, are the protocols needed to make communication over a network possible. 11

12 Data Navigation Protocols Which is the fundamental network protocol? It describes how packets will navigate from network to network The IP also provides a fragmentation and reassembly function Fragmentation means that if a message, or datagram, is too long, an IP packet can be split into smaller chunks for transmission through the network Reassembly on the hand mean putting messages back together when it gets to its final destination What IP does not do is keep track of whether messages actually make it to where they are going 12

13 Cont … IP treats each piece of a message as an independent entity unrelated to any other message A message is also referred to as an Internet datagram The IP must link up with several other protocols to insure reliable end-to-end delivery and retransmission of missing messages For reliable delivery IP works with another data navigation protocol known as Transmission Control Protocol (TCP) TCP wraps itself around the IP packet and provides the information needed to see a packet through multiple hops to its destination and determines if all packets made the trip 13

14 Cont … TCP can figure out which packets were lost and order up replacements Another protocol used for delivery is User Datagram Protocol (UDP) one that moves packets with SPEED sacrificing end-to-end delivery The File Transfer Protocol mentioned previously operates using TCP All the data travels reliably over the network, and the transmission is not finished until the packets have all made the trip and been reassembled in order at the destination 14

15 Cont … TCP can detect errors because each packet uses a cyclical redundancy check (CRC), which is like a parity or checksum, to check itself. A checksum is a mathematical mechanism that detects errors in transmission  adds up the numeric value of all the characters transmitted and seeing if the total is the same at both ends of the link If IP needs to report errors to the sender, it uses helping protocols from a suite called Internet Control Message Protocol (ICMP) 15

16 Data Navigation Protocols Attacks These four protocols: IP, TCP, UDP, and ICMP are the basis for Internet communications They are also the basis of many attacks that use the Internet or of attacks against the Internet itself How does TCP assure reliability?  by introducing sequence numbers and acknowledgments to IP For TCP to start transmission, it has to establish a link, between the sender and recipient, by using a three-way handshake Once the handshake is complete both parties can start communicating 16

17 Three way handshake: Step 1: client host sends TCP segment to server m specifies initial seq # m no data Step 2: server host receives segment, replies with ACK segment m server allocates storage m specifies server initial seq. # Step 3: client receives segment ACK, replies with ACK segment, which may contain data TCP Connection establishment 26 client segment (SEQ Client#) server segment (SEQ Server#), ACK ACK

18 Cont A hacker trick is to open up a session (begin a communication) with a system under attack, receive an acknowledgment, and then leave the connection half- completed Tying up resources and memory on the attacked device Do this enough times and  systems can hang up or cease functioning  denying services to legitimate users  can crash Why is IP such a pushover?  Simple: Because it's not being used for that which it was built  The military wanted a network protocol that would survive a worst case scenario something along the lines of global thermonuclear war 18

19 Cont …  The network needed to pass traffic to every location smoothly and efficiently, and to be able to reconfigure itself around bad routes and sudden outages Instead, the Internet became an "information superhighway" that led to economic growth, prosperity, and jobs.  a tool of enhanced communications, helping to bring the entire human family closer together  Along the ‘highway’ came robbers  These are unintended consequences against which the Internet was never fortified 19

20 Cont …  The network needed to pass traffic to every location smoothly and efficiently, and to be able to reconfigure itself around bad routes and sudden outages Instead, the Internet became an "information superhighway" that led to economic growth, prosperity, and jobs.  a tool of enhanced communications, helping to bring the entire human family closer together  Along the ‘highway’ came robbers  These are unintended consequences against which the Internet was never fortified 20

21 Other Internet Protocols Protocols can be subjected to attack The easiest way to attack these datagrams is by monitoring the network using a packet sniffer Packet sniffer monitors and decodes packets, allowing the attacker to gather information about the network and the devices and persons attached to it A more sophisticated attack would be:  change the contents of a datagram (data modification)  make it appear as if it came from a different party (spoofing) On the other hand packet sniffers are useful tools for network administrators because they allow you to see what protocols are on the network 21

22 File Transfer Protocol Designed to promote sharing files by connecting machines reliably and efficiently Remote access of computers became more commonplace Flaws of FTP However, the FTP protocol is subject to abuse  In the first place, it transmits in the clear without encryption shielding Attackers can just sit and listen to a network connection  FTP is also very subject to anonymous access highly desirable in many environments, where to regulate access requires issuing passwords to every applicant 22

23 Simple Mail Transfer Protocol Designed to transfer email messages reliably and efficiently It does this by setting up a channel between the initial sender and a receiver Once the transmission channel is established, the mail sender issues a MAIL command If the mail receiver can accept mail, it responds with an OK reply Mail that can't be delivered because of incorrect or invalid addresses are returned with a note from whichever mail server determined the problem 23

24 Cont … Flaws of SMTP Email is normally transmitted in the clear  means that a host that pretends to be an email relay can access all email that passes through it  mail could then be copied or modified  When an attacker suspects that a user or administrator is getting suspicious, it is relatively easy to disconnect the relay and lay low It is very easy to create an email message that looks as if it was sent from someone other than the true sender  an create problems in its own right  also makes it easy to formulate an attack that sends tens of thousands of emails out to various addresses on the Internet, email bombing 24

25 SMTP and Spam The ability to spoof a return address and easily mail the same message to multiple recipients has lead to the uncontrolled outbreak of junk email, or spam Spam, by some accounts, represents up to 50% of email traffic and is popular for one reason: email is dirt cheap  Email has few costs: scraping up a few million email addresses off newsgroups and chain letters is not really that hard  Launching and sending such messages is largely automatic Other recipients will resent the intrusion and request not to be disturbed as for invalid addresses, email systems will send back notices to the sender to this effect Tracking spammers down requires a lot of detective work  The best way to cope is to use the Antispam features of your email client software to filter undesired email into the recycle bin before you even see it. 25

26 Domain Name Service DNS is used to resolve a friendly name, such as, to an IP address, such as DNS is needed because while the Internet runs with IP addresses, people tend to think in words DNS is not usually the first step in address resolution To save time and prevent wasted bandwidth, a table of address and their URLs is usually stored on the local machine Your computer starts at this table when you make a web request When your local machine cannot find where to send a web request, it contacts the nearest DNS server, which tells the computer every thing it knows about the desired IP address 26

27 Cont … If the address is unknown at the DNS server, that DNS server consults the next DNS server up the chain,, until your address is found Flaws of DNS First, if you seed the local machine's cache with incorrect data, it sends the user's communications to the wrong place, including possibly a decoy site of the attacker's own design Second, if you pollute the database of one of the nation's big DNS servers, you may shut down a major portion of the Internet, which is always good for achieving status in the cracker underworld 27

28 Cont … Third, if you deny access to the DNS server that provides address resolution to a population of users, say company networks, then your users are not going to be able to contact web sites for which they do not already have IP addresses Poisoning the DNS system doesn't only slow down or prevent the access of web pages and services  Mail may not work  Remote file systems may be rendered inaccessible, and  Network printing may go down. Essentially everything that involves an external communication is at risk when DNS fails. 28

29 Summary Internet protocols have opened up a host of security issues The protocols were meant to be robust, not secure The trust by which computers are able to interact and complete complicated business processes turns out to be the undoing of much business 29

30 Questions

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