Tài liệu Information Security: The Big Picture – Part V pdf

1
Information Security: The Big Picture - SANS GIAC
© 2000
1
Information Security:
The Big Picture – Part V
Stephen Fried
2
Information Security: The Big Picture - SANS GIAC
© 2000
2
Agenda
• General Security Introduction
• Telecommunications Fundamentals
•Network Fundamentals
•Network Security
•
World Wide Web Security
• Information Secrecy & Privacy
• Identification and Access Control
• Programmatic Security
•Conclusion
If you are taking this course you undoubtedly know about the World Wide Web. As valuable, as
useful, and as important to our everyday lives as the web has become, it is full of security issues and
problems. This section will examine those issues.
3
Information Security: The Big Picture - SANS GIAC
© 2000
3
Everything You Always Wanted to
Know About Web Communications…

• Servers and Clients
• HTTP and HTML
• Stateless Communications
• Retrieving Information – GET
• Sending Information – POST
The World Wide Web has become the de facto communications medium for the Internet. Millions of people use it every day to get
information, communicate with coworkers, buy and sell goods, entertain themselves, and keep up to date with current events. However, most of
these people have very little knowledge about how the web actually works. On this slide we will give you a brief introduction to the web and tell
you everything you always wanted to know about the web but were afraid to ask. All in less than three minutes.
All computers on the web fall into one of two categories: clients or servers. Let’s start with servers. A server is a computer that contains some
sort of information that an organization wants to distribute. The server runs a special piece of software, called a web server, that takes requests
from other machines, figures out what the request is for, finds the answer to the request, and sends it back to the requesting machine. That’s
basically all a server does.
The client machine is the machine that is doing the requesting. The client runs a piece of software called a “Web browser”, or just browser for
short. Browsers take input from users, convert that input into a language the server will understand, send it off to the server over the network,
and waits for the reply. When the server sends the reply, the browser will format it and display it for the user. Simple as that. OK, it’s not really
all that simple. There may be a lot of processing that goes on behind the scenes. For example, the server may have to contact other computers to
get the information the client needs, or the client may have to run some other programs in order to properly interpret the response from the
browser, but here you have the basics: Client sends request, server responds to request.
The way clients and servers communicate on the Web is through a protocol called HTTP – the HyperText Transfer Protocol. Like any other
protocol, HTTP is just a set of standards, conventions, and notations the two systems must understand in order to communicate.
The HyperText Markup Language, or HTML, is the actual language used to develop web pages. HTML uses a set of special notations, called
tags, to tell the browser how to display a page, including things like where to center text, what fonts to use, where to place images on a page, and
so on. If you want to see examples of HTML, most browsers allow you to view the HTML source code for any page it displays.
Communication on the Web is called “stateless.” This is because each interaction between clients and servers is an independent transaction. For
example, each time you click on a web page you are starting a completely new interaction between your browser and the server. If you click on
12 different links on a page your browser will make 12 different connections to the server. There is no information about the state of any
previous transactions carried over from one transaction to the next. That’s why it is called “stateless.” We will see in the next slide how servers
and clients can be tricked into carrying state information between transactions.
There are two types of transactions that browsers can request of servers. They are called GET and POST. A GET transaction asks the server to

get some information and send it back to the browser. When you click on a simple link on a web page you are typically issuing a GET
transaction request. A POST transaction allows the browser to send some information to the server, usually information from a form the user
fills out. POST transactions send the information from the browser to the server. The server will then act on the input and send any results back
to the client. Generally, users don’t have control over whether clicking on a link on a web page will initiate a GET or POST transaction. That
decision is already coded into the web page itself.
4
Information Security: The Big Picture - SANS GIAC
© 2000
4
HTML Security
• Reading HTML Source
• Hidden Fields
• Server Side Includes
Given the open nature of the HTTP protocol, it is easy to start seeing some of the negative security issues that surround its use. On this
slide we will examine some of these problems.
The easiest way to learn HTML is to examine the HTML source code of any page you happen to visit. Most browsers have an option
to let you view the HTML source of the current page you are viewing. From there you can see all the code, fields, tags, and other
HTML elements that make up the page. You may also see some unexpected things. Many developers put information into source code
that is never meant for public viewing, thinking that regular people will never see it. When you view the code you may see things like
variable names and data values that are used internally by the web site’s programs. You may see references to the names of the site’s
developers or internal information about the organization that is running the server. You may see references to directory names where
files are stored on the web server. There may be references to user IDs or passwords for different services on the machines. If the
server is using JavaScript or some other scripting language you may see code paths that refer to options that the user would not
normally see. All this information can give an attacker a clue as to the underlying structure and organization of the server in order to
plan an attack. And it’s all there free for the looking.
Many web pages, particularly those that use input forms, make use of a feature of HTML called Hidden Fields. Like their name
implies, hidden fields reside on a web page form but they are hidden from view when the page is displayed. Hidden fields are typically
used as a method for carrying information from one form to another without requiring the user to re-enter the information on each
form. However, hidden fields can also contain values not entered by the user. For example, when a user enters a user ID on a web
form, the server might look up the user’s Social Security Number and place that in a hidden field for later use. If you look at the

HTML source for the page with the hidden field you will see that information. Unfortunately, so will anyone else that may be sniffing
the network when that page is transmitted.
Another neat tool is the use of a technology called Server Side Includes. Server Side Includes are small pieces of code that are
embedded in HTML documents. When a Web server begins to display a web page it will go line by line through the code interpreting
the HTML commands. When it comes upon a Server Side Include line it stops and does whatever the include says. For example, it
might insert text from a different file like a copyright notice or policy statement. It might insert today’s date and time to be displayed
on the page. Or, and this is the scary part, it might run a separate program and insert its output into the HTML document. This is scary
because if the included program has a bug, or the attacker can manipulate the program to run some malicious code, the potential exists
for the attacker to compromise the server and gain unauthorized access or obtain confidential information.
Now, despite these shortcomings, and some others we will examine shortly, nobody is saying that we should do away with HTML. But
security practitioners need to take extra care when developing, implementing, or reviewing HTML systems to reduce the likelihood
that information in source code or the use of hidden fields and server side includes do not have a negative effect on the serveror the
organization.
5
Information Security: The Big Picture - SANS GIAC
© 2000
5
Input Validation
• All programs are driven by their input
–“garbage in, garbage out”
• Normal input -> normal results
• Abnormal input ->
unknown
results
• No validation in standard HTML
• Need to check
all
form input before
processing
Many web pages use forms or some other user input as a way of interacting with the user. The user may need to

enter a term for searching or enter a user ID and password to gain entry to a particular site. Or, more commonly,
the user will need to enter information into a form like a credit card number or merchandise numbers. The server
will then send this information to some other program for processing. The processing program, like any other
computer program, relies on this input to drive its functions. There is an age-old axiom in computers that says
“garbage in, garbage out.” This means that if the user enters bad input into the program they will get bad output.
Computer programs by and large don’t handle bad input very well. They do great with normal, expected input.
As long as the user works with the program in the ways that the designer anticipated, everything goes along just
fine. But when a user acts in a way that the designer did not anticipate, either accidentally or maliciously, the
program will not act predictably. In fact, the results of this action are generally unknown. If you haven’t figured
this out already, computer security people hate when things act in unknown, unpredictable ways. That’s because
it makes it difficult, if not impossible, to protect the system. SYN floods, fragmentation attacks and the Ping of
Death are all examples of what happens when a system receives input it did not expect.
Plain vanilla HTML also has no built-in methods for validating user input. There are no variable checks or data
validation rules built into HTML to prevent bad input from happening. If you are using a scripting language to
develop your pages you can build validation routines into your forms, but if you want to stick with plain HTML
you are out of luck.
That’s why you need to pay particular attention to any web pages, or any program for that matter, that requires
user input. You need to ensure that all input is validated for correctness. What does “validated” mean? It means
that you need to check that the input is correct for the type of information being requested. If you are looking for
a Social Security number, make sure that there are no letters entered by the user. If you are requesting a piece of
text that should be 10 characters long, make sure the user doesn’t enter 500 characters of text.
Beyond simple type and length validation, you also need to check the input to see if it matches the type of
information you are expecting. For example, if you normally only sell 2 or 3 of a particular items is it normal for
a user to order 999 of that item? Is the name on the customer’s credit card different from the name on the
shipping address? Things like this can be a clue to possible unauthorized activity or fraud.
6
Information Security: The Big Picture - SANS GIAC
© 2000
6
Cookies

• HTTP is “stateless” – no context information
• Cookies provide “state” and context
• Can only hold information you’ve given to server
• Can only be exchanged with originating server or
domain
• Beware of cross-site sharing (e.g. DoubleClick)
• Can block cookies if desired
One of the interesting things that we mentioned before about the HTTP protocol is that it is stateless. By “stateless” we mean that each
transaction is an independent unit with no relation to any transactions that came before or after it. When you request a web page your
computer connects to the server, gets the page, then closes the connection. The next time you request a page your computer makesa new
connection to the server.
Unfortunately, many web applications, like shopping or information retrieval systems, require that information be passed from one page to
the other. How do you accomplish this in a stateless system? The answer is a protocol called “cookies.” A cookie is a small piece of
information that a server will send to a browser. What does this information contain? Well, almost anything the server wants it to. It might
be the product numbers and prices for things you want to order from a site. It might be your user ID or customer number on a particular
site. It can be anything that the site needs you to store from page to page.
Cookies are actually a pretty neat technology, and nicely solve a major problem in the original HTTP protocol. However, many people
don’t like cookies. There are a couple of reasons for this. The first is that the user has no control over the information stored in cookies.
Since the content of the cookie is controlled by the server you have no way of knowing what’s in it. Also, if a site puts sensitive
information in a cookie, like a Social Security number or a credit card number, unless they take steps to hide that information (using
encryption, for example) that information will be available to everyone on the network as the cookie is transmitted back and forth between
the browser and the server.
Some people object to cookies on privacy principles. They believe that cookies are somehow magically taking information from you or
your computer and spreading that information around the Internet. Most of these fears are based on a lack of understanding of how cookies
really work. First off, cookies can only contain information that you’ve already given to the web server or the company you are dealing
with. There is no way the site can know your home address or credit card number unless you have already given it to them. So you’ve
already given up some of your privacy before the cookies even entered into the picture. Secondly, cookies can only be sent to and from the
server or domain that originally created the cookie. There is no way that a cookie from xyz.com can be shared with a server from abc.com.
This last point, however, while technically true, has found a wrinkle lately. It is true that one company’s server can not share a cookie with
another company’s server. But what if one company were able to distribute cookies on ALL servers? This is exactly what a company

called DoubleClick has done. You’ve probably seen their advertisements on web pages you’ve visited. DoubleClick rents space on web
pages for advertisements. So, for example, when you visit the web page for acme.com you will see an ad that is actually generated by
DoubleClick from the DoubleClick server. The cookies generated by that ad are shared between the browser and DoubleClick, not the
browser and Acme. Then, when you go to widgets.com you may see another DoubleClick advertisement. Again, you will share a cookie
with DoubleClick, not Widgets.com. In this way, the DoubleClick service can begin to collect information on what sites you have visited
over the Internet. Many privacy advocates are extremely worried about this practice.
If you are really worried about cookies you can take steps to protect yourself. In most browsers you can set an option to prevent the
downloading of cookies to your browser. There are also a number of shareware add-on utilities that let you selectively block cookies based
on various criteria.
7
Information Security: The Big Picture - SANS GIAC
© 2000
7
SSL
• Protocol for encrypting network traffic
• Operates at Transport Layer
• Operates on port 443
•How it works
– Client connects to server
– Server indicates need for SSL
– Client and server exchange crypto keys
– Secure session begins
• Not a guarantee of security
Plain, generic HTTP is fine for open, non-secret communications, but some applications require more privacy than that
provided by HTTP. For example, you may want to keep your credit card information or information about your bank accounts
secret over the Internet. For these types of applications, there is the Secure Socket Layer protocol, or SSL.
SSL is a general-purpose protocol for encryption of network traffic. Although it is most commonly associated with HTTP
traffic, SSL operates at the Transport Layer of the TCP/IP stack and can be used with many different application protocols. Any
program that uses TCP can be modified to use SSL. General HTTP traffic typically operates on port 80. When SSL is enabled
on a connection it usually runs on port 443.

When a client connects to a web server, the server will generally indicate whether SSL is required for that page. If it is required,
the client and the server will negotiate to determine what type of encryption the session will use. Generally, the strongest
algorithm that the two programs support will be selected.
The client and the server will then exchange encryption keys. These are the codes that will enable the two to encrypt messages
back and forth. Once the keys have been exchanged, all further communications between the client and the server are encrypted.
I have left out a LOT of detail here about the specifics of the key exchange and the use of certificates to validate the identityof
the client and the server, but most of it is unimportant in order to gain a high-level understanding of the process. What’s
important to remember is that all sensitive information that is to be transmitted over the web should require SSL to be enabled.
You can tell if SSL is enabled on a web page by looking at the bottom of your browser. In Internet Explorer there will be a
small icon of a lock in the lower right corner. In Netscape there will be a small lock in the lower left corner. Other browsers
may have other indicators, but they all mean the same thing – your information is being protected with encryption.
Please note that the use of SSL does not guarantee that your information is secure from all prying eyes. SSL only secures data in
transit over the network. Even then, it is possible that someone will capture the information as it is transmitted and decrypt the
packets. The likelihood is reduced, particularly if you are using strong encryption, but it is possible. Also, SSL does not protect
your information once it reaches the destination computer. If that computer stores the information in a publicly accessible area
or an attacker gains unauthorized access to that computer, your information is still vulnerable.
8
Information Security: The Big Picture - SANS GIAC
© 2000
8
Secure Electronic Transactions
(SET)
• Developed by Visa, MasterCard, Microsoft,
Netscape
• Specific-purpose protocol
• Secures credit and debit card transactions
•Services provided
– Authentication
– Confidentiality
– Message Integrity

–Linkage
Protocols like SSL are designed to be general purpose protocols. This means that they can be used in a variety of applications under a
variety of different circumstances. In some instances, however,it is better to have an application-specific protocol. This is a protocol that
is designed with a particular purpose in mind. Such an application is the exchange of credit and payment information over the Internet.
This type of information can be highly sensitive and the need to keep it confidential is great. For this reason the Secure Electronic
Transaction protocol, or SET, was developed.
SET was developed by a number of large players in the credit card and computer industries, including Visa, MasterCard, Microsoft, and
Netscape. It was designed to handle the specific problems of transmitting credit and debit card information. For example, SET handles
issues like validating credit card numbers, checking the customer’s authorization to use the credit card, authorizing the transaction with
the bank, and processing the transaction. SET provides an integrated system that handles the entire transaction, including card
authorization and finalization of the sale. SET has a number of mechanisms that protect the customer, the merchant, and the bank. For
example, the protocol hides the actual credit card number from the merchant, instead sending it directly to the bank. Also, the bank does
not know the actual merchandise purchased by the customer, protecting the privacy of the customer’s purchases.
SET provides four basic services that protect transactions.
Authentication: All the parties to the transaction are authenticated using digital signatures. We will learn more about digital signatures
later when we discuss cryptography.
Confidentiality: The transaction is encrypted so that Internet eavesdroppers can not capture the data and discover the details of the
transaction.
Message Integrity: The transaction can not be tampered with by attackers. Thus, they can not alter the account numbers or payment
amounts involved in the transaction.
Linkage: SET allows a message sent by one party to the transaction (either the customer, the merchant, or the bank) to contain an
attachment that can be read only by another specified party. This allows the first party to verify that the attachment is correct without
being able to read the contents of the attachment. This is very important for the privacy reasons stated above.
SET has many advantages over plain SSL in that it covers the entire transaction from end to end. If plain SSL were used, the credit and
validation information would be exposed at many different points along the way, leaving the information available for attackers or data
thieves. This is, in fact, what happened in 1994 when an attacker broke into the Netcom Internet Service Provider and stole thousands of
credit card numbers that were stored on Netcom’s computers.
Although it seems like the perfect answer to credit exchanges on the Internet, use of SET in the real world has been slow in coming.
Hopefully, in the near future, its use will increase as more companies implement it as part of the on-line ordering systems and more
customers see its advantages and begin demanding it for their personal transactions.

9
Information Security: The Big Picture - SANS GIAC
© 2000
9
Common Gateway Interface
(CGI)
• Allows web pages to
do
something instead of
just returning pages
• Extends the capabilities of a web server
• Creates many exposures on server
– Leaking information
– Performing unauthorized transactions
– Executing unintended programs
• Common Mistakes
– Misuse of command interpreters
– Bad memory management
– Passing unchecked parameters to system
For all the hype surrounding it, HTTP is still pretty much a dumb protocol. By that I mean it really only does one thing – once you make a
request for a web page, HTTP gets the page from the server and delivers it to your browser. Not too exciting, huh? Well, early users of the
Web didn’t think so either. They wanted a way to interact with Web servers. They wanted the servers to do something instead of blindly
returning pages. To make that happen, they invented the Common Gateway Interface, or CGI.
CGI is a method of extending the web server’s abilities by executing programs on the server and returning the results back to the user. CGI
scripts can generate pages dynamically based on the information obtained during their execution. Some examples of CGI programs that have
been written include database transaction systems, computer games, financial transaction systems, and even vending machine ordering.
However, CGI is a very primitive process for handling such interaction, and it may create a large number of vulnerabilities on the server in
which it is used. For example, if the results of the CGI execution are not filtered before being sent to the user, the use of CGI programs can
lead to the leakage of information about the system or its data. Because CGI has few built-in data checking mechanisms, it can be relatively
easy for a user to falsify the information sent to the CGI program, increasing the potential for the execution of unauthorized or fraudulent

transactions. Finally, since many CGI programs use underlying command interpreters (like Perl or a UNIX shell), the potential exists for an
attacker to run programs not intended by the designers of the system. This is a popular method of gaining unauthorized administrative access
on web servers.
There are several common mistakes that many CGI developers make when writing their programs. The first is misuse of command
interpreters. As mentioned before, many CGI programs use command interpreters that are called by the CGI program. Since there is no
direct linkage between the CGIO program and the command interpreter, the interpreter has little way of validating the information it is being
sent. If an attacker can find a way to pass random system commands to the interpreter they have the potential to successfully compromise
the computer.
Another common mistake is the lack of attention paid to memory management. As we will see later on when we discuss buffer
overflows, a common method of attack is to send a program more information than it was designed to handle. If the information reaches a
certain peak, or if it is carefully crafted, it has the ability to crash the server, often leaving the attacker with administrator privileges on the
computer. Also, if the program itself does not pay close attention to the resources it is using, it can potentially consume all the available
resources of the computer, again leaving it exposed to compromise.
The final common mistake, and the one that is also the most preventable, is passing unchecked user input to CGI programs. Many of the
most successful attacks have been based on the fact that a CGI program did not check the information entered by the user. In some cases,
users are able to enter privileged system commands as input to web forms and the computer will blindly execute them without even a virtual
glance.
CGI programs can add a great deal of flexibility to your web site. But, like any enabling technology, it has a negative side that must be
checked before proceeding blindly with its implementation. Also, CGI is a relatively old protocol, designed back when the web was still in
its infancy. There are more modern alternatives to CGI that have addressed some of CGIs shortcomings. Unfortunately, they have also
introduced some of their own
.
10
Information Security: The Big Picture - SANS GIAC
© 2000
10
Active Content
• Programs that interact in a network
environment
•Java

•ActiveX
• Balancing risks vs. gain
It used to be that computer programs were fairly simple. You ran them, they did some work on your computer, you got the results, and you
were done. Then as network computing took off, we began to see client/server programs. You ran them, they interacted with a server
somewhere on the network, they did the work on the server, you got the results, and you were done. Then with the advent of the web, we
started seeing the use of CGI programs to do the work. However, with both client/server and CGI, much of the work was being done on the
remote computer. This placed a very heavy burden on the server. It would be nice if the work could be performed locally on your machine,
just like in the olden days. The server wouldn’t be so burdened and you could probably get the work done faster.
Enter Active Content. “Active Content” is a term commonly used for program code that is embedded in the contents of a web page. When
the page is accessed by a web browser, the embedded code is automatically downloaded and executed on the user’s workstation. Other terms
that are sometimes used to describe active content include executable content, active code, or mobile code. Active Content can be thought
of as CGI: The Next Generation.
Two of the most common examples of active content are Java and ActiveX. Java is a programming and execution environment originally
developed by Sun Microsystems. It was designed for developing programs that run on many different types of devices. One of the features
of Java’s portability is that a special type of Java program, called an applet, can be embedded in a web page’s HTML code and run on a
user’s machine. ActiveX is the term Microsoft uses for its active content components. ActiveX components are called “controls” and, like
Java, are downloaded to the user’s computer where they are executed.
What do Java applets & ActiveX controls do? Well, almost anything their designer can dream of. They can show 3-D animations, play
videos, run simulations, wander through file systems, e-mail your friends, delete files, send your secret information to attackers, launch
denial of service attacks, spread viruses… well, you get the picture. Active content can be extremely useful for unleashing the power of
network communications, but there are a lot of security risks involved with these technologies. Because they run locally on your computer,
they can often do almost anything you can do as an ordinary user.
Of course, all these technologies claim to have security mechanisms to prevent evil things from happening, but almost all of them have
flaws or bugs that allow bad things to happen anyway. We’ll examine the security models of Java and ActiveX in the next few slides.
If running these types of programs is risky, why do it at all? Well, because they are so cool. And that’s basically what it comes down to.
Many of these programs do incredibly useful, valuable, or entertaining things. The value they may add to an application or a website is
extremely high to users of the program. This points to one of the classic tradeoffs in security: balancing the risks of an action against the
benefits of that action. We’ve all heard of the experiment where the mouse was given an electric shock every time it tried to get some
cheese. Despite the risk of the shock the benefit of the cheese was so important to the mouse it was willing to risk it.
It would be easy to tell people “turn off all active content in your browser” and many security people do just that. But that is unrealistic in

today’s world. What is a more realistic approach is to turn off active content for most sites. When you have a site that you really need or
want access to, turn it on temporarily for that site. Then, turn it off again when you leave the site. It’s a bit cumbersome, but it does give a
nice balance of security and convenience.