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About your assignment 5 -layers Model Application Layer(HTTP, DNS, . . .) TCP Layer(add sequence number to packets) IP Layer(for addressing) Data Link Layer(Ethernet, PPP) Physical Layer(wire, wireless, optical fibre, . . . ) Hacking Common security problems virus

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About your assignment l.jpg

About your assignment

  • 5 -layers Model

Application Layer(HTTP, DNS, . . .)

TCP Layer(add sequence number to packets)

IP Layer(for addressing)

Data Link Layer(Ethernet, PPP)

Physical Layer(wire, wireless, optical fibre, . . . )


Hacking l.jpg

Hacking

  • Common security problems

    • virus

      • malicious computer program that makes copies of itself and attaches those copies to other programs

    • Trojan horse

      • a program that appears to have one ubiquitous function, but actually has a hidden malicious function


Hacking3 l.jpg

Hacking

  • Dictionary attack

    • try all possible words in a dictionary and compare with the encrypted password in the server

    • need to get the password file first

password

mary

Password file

One-way

encrypting

function

xo12u50zag

dictionary


Denial of service attack l.jpg

Denial of Service Attack

  • Sends fake IP packets to the web server

    • server sends an reply to the client, waiting for the HTTP message to arrive

    • but the source address is faked, so the server has to wait forever

  • a server can only support a limited number of concurrent requests, if all these requests are faked, then the server cannot do any useful work


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Denial of Service Attack

  • How to prevent it?

    • charge the packets !

    • check the IP address at the source


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Denial of Service Attack

  • Details of the attack

    • client > initial packet with a sequence no to server

    • server> return its own sequence no to client

    • client> OK, I got your sequence number

    • client> the request

    • server> return the requested file

    • client> disconnect

    • server> disconnect


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Christmas 1994

  • 25th December, 1994, 14:09:32 . . .

    • Security expert Tsutomu Shimomura was on a skiing trip

    • meanwhile his computer in was break-in by a hacker

  • 15th February, 1995

    • By tracing the traffic log, Kevin Mitnick, the most wanted computer outlaw, was caught

  • Paperbacks and movie about the story

    • Takedown by Shimomura and John Markoff

    • The Fugitive Game by Jonathan Littman

    • Takedown (movie) by Miramax


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IP spoofing

  • Security hole in remote login (rlogin)

    • authentication based on IP address

    • password is not required

  • IP spoofing attack

    • host A: attacker

      host B: the machine to be attacked

      host C: the machine impersonated by the attacker, C is trusted by B

    • A sends the 1st packet to host B using the false IP source address (host C)

    • B checks the IP, verifies it, find nothing wrong, so let the hacker login without checking the password


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B (machine under attacked)

1st

2nd

C

(friend of B)

A (hacker)

3-way handshakes

  • Problem: 3-way handshakes

    • 1st segment: A --------------> B

    • 2nd segment: A <-- ------------ B

    • 3rd segment: A --- ACK ---> B

    • BUT B sends the 2nd segment to C, so that A cannot return the ACK to B without knowing the sequence number used by B


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Sequence number prediction

  • BUT the ISN of B can be guessed!!

  • First, the hacker makes a real connection to B

    • because the connection is real, the 2nd segment sent by B will be received by the hacker

    • the 2nd TCP segment contains the number of B

    • after getting the number of B, the hacker launches the attack

    • return the 3rd packet using the guessed ISN

    • how to guess the number?

    • the number is incremented by 128000 for every new connection


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Sequence number prediction

  • so that for two consecutive connection requests, the ISN will be differed by 128,000

  • if there is no other connection request to the server during that time, the ISN can be guessed accurately and easily

  • the best time to attack is when there is little connection requests to the server

    • Christmas

  • so after sending the 1st packet to B,

    A sends the the 3rd packet to B with the guessed number


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SYN flooding

  • but the story does not end here, note that when B sends the 2nd packet to C, C will return a packet (reset) and spoils the attack

  • To prevent C from returning the packet, A sends a large number of SYN packets (the first packet) to C using dead site as source address

  • C receives a lot of connection requests and opens up many half-open connections (waiting the the 3rd packet to return)

  • but since the source is a non-existing host, the 3rd handshake never come back

  • prevent C from receiving anything from B


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