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Web Spoofing: an Internet Con Game

                    Web Spoofing: An Internet Con Game

       Edward W. Felten, Dirk Balfanz, Drew Dean, and Dan S. Wallach

                          Technical Report 540-96

            Department of Computer Science, Princeton University


This paper describes an Internet security attack that could endanger the
privacy of World Wide Web users and the integrity of their data. The attack
can be carried out on today's systems, endangering users of the most common
Web browsers, including Netscape Navigator and Microsoft Internet Explorer.

Web spoofing allows an attacker to create a "shadow copy" of the entire
World Wide Web. Accesses to the shadow Web are funneled through the
attacker's machine, allowing the attacker to monitor the all of the
victim's activities including any passwords or account numbers the victim
enters. The attacker can also cause false or misleading data to be sent to
Web servers in the victim's name, or to the victim in the name of any Web
server. In short, the attacker observes and controls everything the victim
does on the Web.

We have implemented a demonstration version of this attack.

Spoofing Attacks

In a spoofing attack, the attacker creates misleading context in order to
trick the victim into making an inappropriate security-relevant decision. A
spoofing attack is like a con game: the attacker sets up a false but
convincing world around the victim. The victim does something that would be
appropriate if the false world were real. Unfortunately, activities that
seem reasonable in the false world may have disastrous effects in the real

Spoofing attacks are possible in the physical world as well as the
electronic one. For example, there have been several incidents in which
criminals set up bogus automated-teller machines, typically in the public
areas of shopping malls [1]. The machines would accept ATM cards and ask
the person to enter their PIN code. Once the machine had the victim's PIN,
it could either eat the card or "malfunction" and return the card. In
either case, the criminals had enough information to copy the victim's card
and use the duplicate. In these attacks, people were fooled by the context
they saw: the location of the machines, their size and weight, the way they
were decorated, and the appearance of their electronic displays.

People using computer systems often make security-relevant decisions based
on contextual cues they see. For example, you might decide to type in your
bank account number because you believe you are visiting your bank's Web
page. This belief might arise because the page has a familiar look, because
the bank's URL appears in the browser's location line, or for some other

To appreciate the range and severity of possible spoofing attacks, we must
look more deeply into two parts of the definition of spoofing:
security-relevant decisions and context.

Security-relevant Decisions

By "security-relevant decision," we mean any decision a person makes that
might lead to undesirable results such as a breach of privacy or
unauthorized tampering with data. Deciding to divulge sensitive
information, for example by typing in a password or account number, is one
example of a security-relevant decision. Choosing to accept a downloaded
document is a security-relevant decision, since in many cases a downloaded
document is capable of containing malicious elements that harm the person
receiving the document [2].

Even the decision to accept the accuracy of information displayed by your
computer can be security-relevant. For example, if you decide to buy a
stock based on information you get from an online stock ticker, you are
trusting that the information provided by the ticker is correct. If
somebody could present you with incorrect stock prices, they might cause
you to engage in a transaction that you would not have otherwise made, and
this could cost you money.


A browser presents many types of context that users might rely on to make
decisions. The text and pictures on a Web page might give some impression
about where the page came from; for example, the presence of a corporate
logo implies that the page originated at a certain corporation.

The appearance of an object might convey a certain impression; for example,
neon green text on a purple background probably came from Wired magazine.
You might think you're dealing with a popup window when what you are seeing
is really just a rectangle with a border and a color different from the
surrounding parts of the screen. Particular graphical items like file-open
dialog boxes are immediately recognized as having a certain purpose.
Experienced Web users react to such cues in the same way that experienced
drivers react to stop signs without reading them.

The names of objects can convey context. People often deduce what is in a
file by its name. Is manual.doc the text of a user manual? (It might be
another kind of document, or it might not be a document at all.) URLs are
another example. Is MICR0S0FT.COM the address of a large software company?
(For a while that address pointed to someone else entirely. By the way, the
round symbols in MICR0S0FT here are the number zero, not the letter O.) Was
dole96.org Bob Dole's 1996 presidential campaign? (It was not; it pointed
to a parody site.)

People often get context from the timing of events. If two things happen at
the same time, you naturally think they are related. If you click over to
your bank's page and a username/password dialog box appears, you naturally
assume that you should type the name and password that you use for the
bank. If you click on a link and a document immediately starts downloading,
you assume that the document came from the site whose link you clicked on.
Either assumption could be wrong.

If you only see one browser window when an event occurs, you might not
realize that the event was caused by another window hiding behind the
visible one.

Modern user-interface designers spend their time trying to devise
contextual cues that will guide people to behave appropriately, even if
they do not explicitly notice the cues. While this is usually beneficial,
it can become dangerous when people are accustomed to relying on context
that is not always correct.

TCP and DNS Spoofing

Another class of spoofing attack, which we will not discuss here, tricks
the user's software into an inappropriate action by presenting misleading
information to that software [3]. Examples of such attacks include TCP
spoofing [4], in which Internet packets are sent with forged return
addresses, and DNS spoofing [5], in which the attacker forges information
about which machine names correspond to which network addresses. These
other spoofing attacks are well known, so we will not discuss them further.

Web Spoofing

Web spoofing is a kind of electronic con game in which the attacker creates
a convincing but false copy of the entire World Wide Web. The false Web
looks just like the real one: it has all the same pages and links. However,
the attacker controls the false Web, so that all network traffic between
the victim's browser and the Web goes through the attacker.


Since the attacker can observe or modify any data going from the victim to
Web servers, as well as controlling all return traffic from Web servers to
the victim, the attacker has many possibilities. These include surveillance
and tampering.

Surveillance The attacker can passively watch the traffic, recording which
pages the victim visits and the contents of those pages. When the victim
fills out a form, the entered data is transmitted to a Web server, so the
attacker can record that too, along with the response sent back by the
server. Since most on-line commerce is done via forms, this means the
attacker can observe any account numbers or passwords the victim enters.

As we will see below, the attacker can carry out surveillance even if the
victim has a "secure" connection (usually via Secure Sockets Layer) to the
server, that is, even if the victim's browser shows the secure-connection
icon (usually an image of a lock or a key).

Tampering The attacker is also free to modify any of the data traveling in
either direction between the victim and the Web. The attacker can modify
form data submitted by the victim. For example, if the victim is ordering a
product on-line, the attacker can change the product number, the quantity,
or the ship-to address.

The attacker can also modify the data returned by a Web server, for example
by inserting misleading or offensive material in order to trick the victim
or to cause antagonism between the victim and the server.

Spoofing the Whole Web

You may think it is difficult for the attacker to spoof the entire World
Wide Web, but it is not. The attacker need not store the entire contents of
the Web. The whole Web is available on-line; the attacker's server can just
fetch a page from the real Web when it needs to provide a copy of the page
on the false Web.

How the Attack Works

The key to this attack is for the attacker's Web server to sit between the
victim and the rest of the Web. This kind of arrangement is called a "man
in the middle attack" in the security literature.

URL Rewriting

The attacker's first trick is to rewrite all of the URLs on some Web page
so that they point to the attacker's server rather than to some real
server. Assuming the attacker's server is on the machine www.attacker.org,
the attacker rewrites a URL by adding http://www.attacker.org to the front
of the URL. For example, http://home.netscape.com becomes
http://www.attacker.org/http://home.netscape.com. (The URL rewriting
technique has been used for other reasons by two other Web sites, the
Anonymizer and the Zippy filter. See page 9 for details.)

Figure 1 shows what happens when the victim requests a page through one of
the rewritten URLs. The victim's browser requests the page from
www.attacker.org, since the URL starts with http://www.attacker.org. The
remainder of the URL tells the attacker's server where on the Web to go to
get the real document.


Figure 1: An example Web transaction during a Web spoofing attack. The
victim requests a Web page. The following steps occur: (1) the victim's
browser requests the page from the attacker's server; (2) the attacker's
server requests the page from the real server; (3) the real server provides
the page to the attacker's server; (4) the attacker's server rewrites the
page; (5) the attacker's server provides the rewritten version to the


Once the attacker's server has fetched the real document needed to satisfy
the request, the attacker rewrites all of the URLs in the document into the
same special form by splicing http://www.attacker.org/ onto the front. Then
the attacker's server provides the rewritten page to the victim's browser.

Since all of the URLs in the rewritten page now point to www.attacker.org,
if the victim follows a link on the new page, the page will again be
fetched through the attacker's server. The victim remains trapped in the
attacker's false Web, and can follow links forever without leaving it.


If the victim fills out a form on a page in a false Web, the result appears
to be handled properly. Spoofing of forms works naturally because forms are
integrated closely into the basic Web protocols: form submissions are
encoded in URLs and the replies are ordinary HTML Since any URL can be
spoofed, forms can also be spoofed.

When the victim submits a form, the submitted data goes to the attacker's
server. The attacker's server can observe and even modify the submitted
data, doing whatever malicious editing desired, before passing it on to the
real server. The attacker's server can also modify the data returned in
response to the form submission.

"Secure" connections don't help

One distressing property of this attack is that it works even when the
victim requests a page via a "secure" connection. If the victim does a
"secure" Web access ( a Web access using the Secure Sockets Layer) in a
false Web, everything will appear normal: the page will be delivered, and
the secure connection indicator (usually an image of a lock or key) will be
turned on.

The victim's browser says it has a secure connection because it does have
one. Unfortunately the secure connection is to www.attacker.org and not to
the place the victim thinks it is. The victim's browser thinks everything
is fine: it was told to access a URL at www.attacker.org so it made a
secure connection to www.attacker.org. The secure-connection indicator only
gives the victim a false sense of security.

Starting the Attack

To start an attack, the attacker must somehow lure the victim into the
attacker's false Web. There are several ways to do this. An attacker could
put a link to a false Web onto a popular Web page. If the victim is using
Web-enabled email, the attacker could email the victim a pointer to a false
Web, or even the contents of a page in a false Web. Finally, the attacker
could trick a Web search engine into indexing part of a false Web.

Completing the Illusion

The attack as described thus far is fairly effective, but it is not
perfect. There is still some remaining context that can give the victim
clues that the attack is going on. However, it is possible for the attacker
to eliminate virtually all of the remaining clues of the attack's

Such evidence is not too hard to eliminate because browsers are very
customizable. The ability of a Web page to control browser behavior is
often desirable, but when the page is hostile it can be dangerous.

The Status Line

The status line is a single line of text at the bottom of the browser
window that displays various messages, typically about the status of
pending Web transfers.

The attack as described so far leaves two kinds of evidence on the status
line. First, when the mouse is held over a Web link, the status line
displays the URL the link points to. Thus, the victim might notice that a
URL has been rewritten. Second, when a page is being fetched, the status
line briefly displays the name of the server being contacted. Thus, the
victim might notice that www.attacker.org is displayed when some other name
was expected.

The attacker can cover up both of these cues by adding a JavaScript program
to every rewritten page. Since JavaScript programs can write to the status
line, and since it is possible to bind JavaScript actions to the relevant
events, the attacker can arrange things so that the status line
participates in the con game, always showing the victim what would have
been on the status line in the real Web. Thus the spoofed context becomes
even more convincing.

The Location Line

The browser's location line displays the URL of the page currently being
shown. The victim can also type a URL into the location line, sending the
browser to that URL. The attack as described so far causes a rewritten URL
to appear in the location line, giving the victim a possible indication
that an attack is in progress.

This clue can be hidden using JavaScript. A JavaScript program can hide the
real location line and replace it by a fake location line which looks right
and is in the expected place. The fake location line can show the URL the
victim expects to see. The fake location line can also accept keyboard
input, allowing the victim to type in URLs normally. Typed-in URLs can be
rewritten by the JavaScript program before being accessed.

Viewing the Document Source

There is one clue that the attacker cannot eliminate, but it is very
unlikely to be noticed.

By using the browser's "view source" feature, the victim can look at the
HTML source for the currently displayed page. By looking for rewritten URLs
in the HTML source, the victim can spot the attack. Unfortunately, HTML
source is hard for novice users to read, and very few Web surfers bother to
look at the HTML source for documents they are visiting, so this provides
very little protection.

A related clue is available if the victim chooses the browser's "view
document information" menu item. This will display information including
the document's real URL, possibly allowing the victim to notice the attack.
As above, this option is almost never used so it is very unlikely that it
will provide much protection.


There are several ways the victim might accidentally leave the attacker's
false Web during the attack. Accessing a bookmark or jumping to a URL by
using the browser's "Open location" menu item might lead the victim back
into the real Web. The victim might then reenter the false Web by clicking
the "Back" button. We can imagine that the victim might wander in and out
of one or more false Webs. Of course, bookmarks can also work against the
victim, since it is possible to bookmark a page in a false Web. Jumping to
such a bookmark would lead the victim into a false Web again.

Tracing the Attacker

Some people have suggested that this attack can be deterred by finding and
punishing the attacker. It is true that the attacker's server must reveal
its location in order to carry out the attack, and that evidence of that
location will almost certainly be available after an attack is detected.

Unfortunately, this will not help much in practice because attackers will
break into the machine of some innocent person and launch the attack there.
Stolen machines will be used in these attacks for the same reason most bank
robbers make their getaways in stolen cars.


Web spoofing is a dangerous and nearly undetectable security attack that
can be carried out on today's Internet. Fortunately there are some
protective measures you can take.

Short-term Solution

In the short run, the best defense is to follow a three-part strategy:

  1. disable JavaScript in your browser so the attacker will be unable to
     hide the evidence of the attack;
  2. make sure your browser's location line is always visible;
  3. pay attention to the URLs displayed on your browser's location line,
     making sure they always point to the server you think you're connected

This strategy will significantly lower the risk of attack, though you could
still be victimized if you are not conscientious about watching the
location line.

At present, JavaScript, ActiveX, and Java all tend to facilitate spoofing
and other security attacks, so we recommend that you disable them. Doing so
will cause you to lose some useful functionality, but you can recoup much
of this loss by selectively turning on these features when you visit a
trusted site that requires them.

Long-term Solution

We do not know of a fully satisfactory long-term solution to this problem.

Changing browsers so they always display the location line would help,
although users would still have to be vigilant and know how to recognize
rewritten URLs.

For pages that are not fetched via a secure connection, there is not much
more that can be done.

For pages fetched via a secure connection, an improved secure-connection
indicator could help. Rather than simply indicating a secure connection,
browsers should clearly say who is at the other end of the connection. This
information should be displayed in plain language, in a manner intelligible
to novice users; it should say something like "Microsoft Inc." rather than

Every approach to this problem seems to rely on the vigilance of Web users.
Whether we can realistically expect everyone to be vigilant all of the time
is debatable.

Related Work

We did not invent the URL rewriting technique. Previously, URL rewriting
has been used as a technique for providing useful services to people who
have asked for them.

We know of two existing services that use URL rewriting. The Anonymizer,
written by Justin Boyan at Carnegie Mellon University, is a service that
allows users to surf the Web without revealing their identities to the
sites they visit. The Zippy filter, written by Henry Minsky, presents an
amusing vision of the Web with Zippy-the-Pinhead sayings inserted at

Though we did not invent URL rewriting, we believe we are the first to
realize its full potential as one component of a security attack.


The URL-rewriting part of our demonstration program is based on Henry
Minsky's code for the Zippy filter. We are grateful to David Hopwood for
useful discussions about spoofing attacks, and to Gary McGraw and Laura
Felten for comments on drafts of this paper. The figure was designed by
Gary McGraw.

For More Information

More information is available from our Web page at
http://www.cs.princeton.edu/sip, or from Prof. Edward Felten at
felten@cs.princeton.edu or (609) 258-5906.


[1] Peter G. Neumann. Computer-Related Risks. ACM Press, New York, 1995.

[2] Gary McGraw and Edward W. Felten. Java Security: Hostile Applets, Holes
and Antidotes. John Wiley and Sons, New York, 1996.

[3] Robert T. Morris. A Weakness in the 4.2BSD UNIX TCP/IP Software.
Computing Science Technical Report 117, AT&T Bell Laboratories, February

[4] Steven M. Bellovin. Security Problems in the TCP/IP Protocol Suite.
Computer Communications Review 19(2):32-48, April 1989.

[5] Steven M. Bellovin. Using the Domain Name System for System Break-ins.
Proceedings of Fifth Usenix UNIX Security Symposium, June 1995.

[6] Web site at http://www.anonymizer.com

[7] Web site at http://www.metahtml.com/apps/zippy/welcome.html

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