Web Security

Chapter 7

04/19/23 1Web Security

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Objectives

• Protect e-mail systems

• Identify World Wide Web vulnerabilities

• Secure Web communications

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How E-Mail Works

• Defined in RFC 822• Use two Internet protocols to send and receive

messages– Simple Mail Transfer Protocol (SMTP) handles

outgoing mail and – Either Post Office Protocol (POP3 for the current

version) handles incoming mail – Or Internet Mail Access Protocol (IMAP4 is current)

• The SMTP server on most machines uses sendmail to do the actual sending; this queue is called the sendmail queue

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E-mail Architecture

Internet

mailboxesSender Recipient

Mail Server

Mail server

Remote Access server

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E-mail Architecture

Internet

SMTP MGA

Sender Recipient

Retrieval ProtocolDelivery

Protocol

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How E-Mail Works

• Sendmail tries to resend queued messages periodically (about every 15 minutes)

• Downloaded messages are erased from POP3 server (by default)

• Deleting retrieved messages from the mail server and storing them on a local computer may make it difficult to manage messages from multiple computers

• Internet Mail Access Protocol (current version is IMAP4) is a more advanced protocol that solves many problems– E-mail can remain on the e-mail server

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How E-Mail Works (cont)

• E-mail attachments are often in binary format (word processing documents, spreadsheets, sound files, pictures, etc.)– But SMTP is text only.

• Multipurpose Internet Mail Extensions (MIME)– Rfc 1341, 1521, 1522– Non-text documents must be converted into text

format before being transmitted– Three bytes from the binary file are extracted and

converted to four text characters

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E-Mail Vulnerabilities

• Several e-mail vulnerabilities can be exploited by attackers:– Malware– Spam– Hoaxes

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Malware

• Because of its ubiquity, e-mail has replaced floppy disks as the primary carrier for malware

• E-mail is the malware transport mechanism of choice for two reasons: – Because almost all Internet users have e-

mail, it has the broadest base for attacks– Malware can use e-mail to propagate itself

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Malware (cont)

• A worm can enter a user’s computer through an e-mail attachment and send itself to all users listed in the address book or attach itself as a reply to all unread e-mail messages

• E-mail clients can be particularly susceptible to macro viruses– A macro is a script that records the steps a user

performs– A macro virus uses macros to carry out malicious

functions

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Spam

• The amount of spam (unsolicited e-mail) that flows across the Internet is difficult to judge

• The US Congress passed the Controlling the Assault of Non-Solicited Pornography and Marketing Act of 2003 (CAN-SPAM) in late 2003

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Spam (cont)

• According to a Pew Memorial Trust survey, almost half of the approximately 30 billion daily e-mail messages are spam

• Spam is having a negative impact on e-mail users:– 25% of users say the ever-increasing volume of spam

has reduced their overall use of e-mail– 52% of users indicate spam has made them less

trusting of e-mail in general– 70% of users say spam has made being online

unpleasant or annoying

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E-Mail Encryption

• Two technologies used to protect e-mail messages as they are being transported:– Secure/Multipurpose Internet Mail Extensions – Pretty Good Privacy

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S/MIME

• Secure/Multipurpose Internet Mail Extensions– Initial specification 10/95 (rfc 1847)– V2 widely deployed 3/98 (rfc 2311)– V3 available 6/99 (rfc 2633)

• Protocol that adds digital signatures and encryption to Multipurpose Internet Mail Extension (MIME) messages

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S/MIME

• Functionality– Enveloped Data (Message Privacy)– Signed Data (Digital Signatures, Tamper

Detection)– Clear-signed Data (Interoperability)– Signed and Enveloped Data (Everything!)

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Pretty Good Privacy (PGP)

• Functions much like S/MIME by encrypting messages using digital signatures

• A user can sign an e-mail message without encrypting it, verifying the sender but not preventing anyone from seeing the contents

• Originally released as freeWare. Now available as a commercial product– GNU Privacy Guard (GPG) available for free.– Two versions are generally INcompatible.

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PGP (cont)

• Stores private keys on a local “keyring” file – Uses a passphrase to encrypt the keyring on

the local computer

• Passphrase:– A longer and more secure version of a

password– Typically composed of multiple words– More secure against dictionary attacks

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PGP Encryption

• Message Compression– Reduces patterns and enhances resistance to

cryptanalysis

• Session key (a one-time-only secret key – 128 bit number)– This key is a number generated from random

movements of the mouse and keystrokes typed

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PGP Services• Digital Signature

– Uses DSS / SHA or RSA / SHA – SHA-1 used to create message digest. RSA / DSS used

to encrypt digest using sender’s private key

• Message Encryption– Session Key generated.– Message encrypted with CAST-128 / IDEA / 3DES– Session key encrypted using D-H or RSA and recipient’s

public key

• Compression– Uses zip algorithm

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PGP Encryption

Meet mefor lunchTuesday

compress

Mt mfr lnchTsdy

encrypt

4#*l,$@2ugD86qL

Recipient’s Public Key

Session Key

4#*l,$@2ugD86qL

Sender

Recipient

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WWW Vulnerabilities

• WWW Phishing• Mobile Code

– Javascript, Java, ActiveX/COM

• Cookies• SQL Injection• Dynamic content can also be used by attackers

– Sometimes called repurposed programming (using programming tools in ways more harmful than originally intended)

HTML• Hypertext markup language (HTML)

– Describes the content and formatting of Web pages– Rendered within browser window

• HTML features– Static document description language– Supports linking to other pages and embedding images by

reference– User input sent to server via forms

• HTML extensions– Additional media content (e.g., PDF, video) supported through

plugins– Embedding programs in supported languages (e.g., JavaScript,

Java) provides dynamic content that interacts with the user, modifies the browser user interface, and can access the client computer environment

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Phishing• Forged web pages created to

fraudulently acquire sensitive information

• User typically solicited to access phished page from spam email

• Most targeted sites– Financial services (e.g., Citibank)

– Payment services (e.g., PayPal)

– Auctions (e..g, eBay)

• 45K unique phishing sites detected monthly in 2009 [APWG Phishing Trends Reports]

• Methods to avoid detection– Misspelled URL

– URL obfuscation

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Phishing Example

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http://www.anti-phishing.com http://www.anti-phishing.com

URL Obfuscation• Properties of page in previous slide

– Actual URL different from spoofed URL displayed in address bar

• URL escape character attack– Old versions of Internet Explorer did not

display anything past the Esc or null character

– Displayed vs. actual sitehttp://trusted.com%01%00@malicious.com

• Unicode attack– Domains names with Unicode

characters can be registered– Identical, or very similar, graphic

rendering for some characters– E.g., Cyrillic and Latin “a”– Phishing attack on paypal.com– Current version of browsers display

Punycode, an ASCII-encoded version of Unicode: www.xn--pypal-4ve.com

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http://www.anti-phishing.com http://www.anti-phishing.com

Mobile Code

• What is mobile code?– Executable program– Sent via a computer network– Executed at the destination

• Examples– JavaScript– ActiveX– Java Plugins– Integrated Java Virtual Machines

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JavaScript

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• Scripting language interpreted by the browser

• Code enclosed within <script> … </script> tags

• Defining functions:<script type="text/javascript">

function hello() { alert("Hello world!"); }</script>

• Event handlers embedded in HTML<img src="picture.gif" onMouseOver="javascript:hello()">

• Built-in functions can change content of windowwindow.open("http://brown.edu")

• Click-jacking attack<a onMouseUp="window.open(′http://www.evilsite.com′)"href="http://www.trustedsite.com/">Trust me!</a>

ActiveX vs. JavaActiveX Control• Windows-only technology

runs in Internet Explorer• Binary code executed on

behalf of browser• Can access user files• Support for signed code• An installed control can

be run by any site (up to IE7)

• IE configuration options– Allow, deny, prompt

– Administrator approval

Java Applet• Platform-independent via

browser plugin

• Java code running within browser

• Sandboxed execution

• Support for signed code

• Applet runs only on site where it is embedded

• Applets deemed trusted by user can escape sandbox

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Embedding an ActiveX Control<HTML> <HEAD><TITLE> Draw a Square </TITLE></HEAD><BODY> Here is an example ActiveX reference: <OBJECT

ID="Sample“ CODEBASE="http://www.badsite.com/controls/stop.ocx" HEIGHT="101“WIDTH="101“CLASSID="clsid:0342D101-2EE9-1BAF-34565634EB71" >

<PARAM NAME="Version" VALUE=45445"><PARAM NAME="ExtentX" VALUE="3001"><PARAM NAME="ExtentY" VALUE="2445">

</OBJECT></BODY> </HTML>

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Authenticode in ActiveX

• This signed ActiveX control ask the user for permission to run– If approved, the

control will run with the same privileges as the user

• The “Always trust content from …” checkbox automatically accepts controls by the same publisher– Probably a bad idea

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Malicious Mobile Code, by R. Grimes, O’Reilly Books Malicious Mobile Code, by R. Grimes, O’Reilly Books

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ActiveX Security

Classic ActiveX Exploits• Exploder and Runner controls designed by Fred McLain

– Exploder was an ActiveX control for which he purchased a VeriSign digital signature

– The control would power down the machine– Runner was a control that simply opened up a DOS prompt

While harmless, the control easily could have executed format C: or some other malicious command

– http://www.halcyon.com/mclain/ActiveX/Exploder/FAQ.htm

• Quicken exploit by a German hacking club– Intuit’s Quicken is personal financial management tool– Can be configured to auto-login to bank and credit car sites– The control that would search the computer for Quicken and

execute a transaction that transfers user funds to their account

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Cookies

• Cookies are a small bit of information stored on a computer associated with a specific server– When you access a specific website, it might store information as

a cookie– Every time you revisit that server, the cookie is re-sent to the

server– Effectively used to hold state information over sessions

• Cookies can hold any type of information– Can also hold sensitive information

• This includes passwords, credit card information, social security number, etc.

• Session cookies, non-persistent cookies, persistent cookies– Almost every large website uses cookies

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More on Cookies

• Cookies are stored on your computer and can be controlled– However, many sites require that you enable cookies in order to use the

site

– Their storage on your computer naturally lends itself to exploits (Think about how ActiveX could exploit cookies...)

– You can (and probably should) clear your cookies on a regular basis

– Most browsers will also have ways to turn off cookies, exclude certain sites from adding cookies, and accept only certain sites' cookies

• Cookies expire– The expiration is set by the sites' session by default, which is chosen by

the server

– This means that cookies will probably stick around for a while

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SQL Injection Attack

• Many web applications take user input from a form

• Often this user input is used literally in the construction of a SQL query submitted to a database. For example:SELECT user FROM table

WHERE name = ‘user_input’;

• An SQL injection attack involves placing SQL statements in the user input

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SQL Syntax

• SELECT statement is used to select data FROM one or more tables in a database

• Result-set is stored in a result table

• WHERE clause is used to filter records

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SELECT column_name(s) or * FROM table_nameWHERE column_name operator value

Login Authentication Query

• Standard query to authenticate users:select * from users where user='$usern' AND pwd='$password'

• Classic SQL injection attacks– Server side code sets variables $username and $passwd from

user input to web form– Variables passed to SQL query

select * from users where user='$username' AND pwd='$passwd'

• Special strings can be entered by attackerselect * from users where user='M' OR '1=1' AND pwd='M' OR '1=1'

• Result: access obtained without password

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Securing Web Communications

• Most common secure connection uses the Secure Sockets Layer/Transport Layer Security protocol

• One implementation is the Hypertext Transport Protocol over Secure Sockets Layer

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SSL / TLS

• SSL protocol developed by Netscape to securely transmit documents over the Internet– Uses private key to encrypt data transferred

over the SSL connection

– Version 3.0 is most widely supported version

– Personal Communications Technology (PCT), developed by Microsoft, is similar to SSL

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SSL / TLS

• TLS protocol guarantees privacy and data integrity between applications communicating over the Internet – An extension of SSL; they are often referred

to as SSL/TLS

• SSL/TLS protocol is made up of two layers

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SSL / TLS

• TLS Handshake Protocol allows authentication between server and client and negotiation of an encryption algorithm and cryptographic keys before any data is transmitted

• FORTEZZA is a US government security standard that satisfies the Defense Messaging System security architecture– Has cryptographic mechanism that provides message

confidentiality, integrity, authentication, and access control to messages, components, and even systems

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HTTPS• One common use of SSL is to secure Web HTTP

communication between a browser and a Web server– This version is “plain” HTTP sent over SSL/TLS and

named Hypertext Transport Protocol over SSL

• Generally designated HTTPS, which is the extension to the HTTP protocol that supports it

• Whereas SSL/TLS creates a secure connection between a client and a server over which any amount of data can be sent security, HTTPS is designed to transmit individual messages securely

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References:

• S/MIME – PGP– RFCs 2311-2315– RFC 2015, 2440, 3156– Cryptography and Network Security – Stallings

• openPGP– RFC 4880

• TLS– RFC 2246

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Summary

• Protecting basic communication systems is a key to resisting attacks

• E-mail attacks can be malware, spam, or hoaxes• Web vulnerabilities can open systems up to a

variety of attacks• A Java applet is a separate program stored on

the Web server and downloaded onto the user’s computer along with the HTML code

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Summary

• ActiveX controls present serious security concerns because of the functions that a control can execute

• A cookie is a computer file that contains user-specific information

• CGI is a set of rules that describe how a Web server communicates with other software on the server

• The popularity of IM has made this a tool that many organizations are now using with e-mail