The Unofficial Web Hack FAQ

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Section 01

The Browser

01-1. What is "unsafe" about my browser?
01-2. What is vulnerable about history, bookmark, and cache files?
01-3. What other browser files are important?
01-4. Can you tell me more about the "cookie" file?
01-5. How can I protect my browser files?
01-6. Are there any default browser holes?
01-7. What about Internet Explorer?

01-1. What is "unsafe" about my browser?

There are two main areas regarding security around a browser -- reading your private files and manipulating you into a compromising situation.

A few files provide a lot of information about yourself. These include cache files, the history file, and your bookmarks. By examining someone's cache, history, and bookmarks you can learn a lot about a person. Usually if you are a typical home user running Windows, this is not a problem. But if you are storing your Netscape directory on a server, the server could be compromised and then anything in cache and history is in the hands of someone else. Every access. Submitted forms, including those to change passwords on servers whose service you are paying for.

Being manipulated the other hot area. You can be tricked into supplying user IDs and passwords, reveiling personal information like SS# and credit card information, or even be presented with misinformation to cause you to act in a way to cause a vulnerability to arise. If your browser supports HTML 3.0 extensions and Java, your history file, cache, and other files be plucked from your hard drive. Your machine could be used as a mechanism to attack other resources behind your firewall, sending critical info to an offsite hacker.

01-2. What is vulnerable about history, bookmark, and cache files?

We'll cover all three. First the history file.

The default color for a clickable link is blue. Once you've clicked on it and visited the link, it's purple. While the colors may be different depending on what is specified in the HTML, the way your browser keeps track of this information is via the history file.

Since the default is 30 days to expire a link, typically you can see the last 30 days worth of web surfing by examining the history file. "Hmm, Fred keeps looking at a particular set of stocks, does he know something I don't? Hey, Martha keeps looking at lesbian sites, what would her homophobic boss say about that?" Get the idea?

Here's an example -

%2B+%2Bmicrosoft+%2Bstock+%2Bprice+%2B+takeover+%2Brumor+%2Bapple ,
fmt=.&q=%2bapple+%2bmacintosh+%2bhack {1

If this was from the history file of someone at Microsoft, this might be quite interesting, even valuable.

Bookmarks are a problem for the same reason the history file is a problem. It shows what sites you are regularly looking at. If you are bookmarking sites which require passwords to enter, a quick look in the cache will possibly reveil that password, or at least the account ID.

The info gained from here can also be used for social engineering purposes, and can be quite useful. For example, you could determine the user was interested in aquariums and rare fish. This information could be used to assist in guessing a password.

The cache is your browser's way of making things a little easier on your access time, the server your accessing, and the network in general. What happens is that when you access a web page, a copy of the page and any graphics used on that page are stored locally. That way when you access the page again, your browser can pull up the local copy instead of always accessing the network. This saves time and bandwidth. When you reload, your browser compares the cached local file to the one on the server you are accessing and pulls down the latest one. Most browsers will also cache queries and form submittals as well.

If you are looking for dirt on someone, looking for credit card information, or just want to find out what someone's been up to, check their cache. Every query to a search site like AltaVista is stored in cache. Typically every form submittal including accesses to pages requiring an ID and password will be there, unless a site has tagged an HTML document NOT to be cached.

The cache is typically located in a subdirectory underneath the browser's working directory called "cache" or possibly ".netscape-cache", depending on your OS and browser version. Otherwise it may be stored in a temporary directory. For example, IBM's Web Explorer for OS/2 will store it's cached files in c:\tcpip\tmp, and is flushed before each run of the program.

Here is an example from the cache's index file on a Unix workstation, with names changed to protect the innocent ;-)

n   b
10&fmt=.&q=%2bhack+%2bnt+%2bserver               E1           

	   4   (       
        rE1              10/cache31DF457002CC693.html          
				 .   "
r/welcome.html               1    J          14/cache31DF18940
   text/html          J

Very interesting. Here are three entries. The first this user is trying to get NT hacking info from AltaVista, the second this user is trying to get signed onto a site called, and finally the user looks like they got in. The three cache files are:


You could view these files with a browser, as they are local copies of the web pages. If 31DF457002CC693.html had a password in it and it was unreadable, you could still do the following -

SSL (see section 02-9 for info on SSL) documents are stored in cache, depending on browser version and settings. In Netscape 3.0 and later this problem is resolved. But still it exists as a problem in Microsoft Explorer 3.0. The files are located in \WINDOWS\Temporary Internet Files. For more details on this, see

01-3. What other browser files are important?

The cookie file (typically cookie.txt, only found on Netscape and Internet Explorer) is a file used to store information about your browser and Web server connection. Since the hit is "connectionless" -- a one time hit until you click on a link or submit a form -- the cookie file is used to track information about your session with a server. This way a server can track info about you during your visit, by giving you a cookie. The cookie would typically track info such as which page you've been to or how you answered a question on a previous form, and due to the connectionless state it keeps the cookie on the client.

No this doesn't seem like a problem, except that since JavaScript can write info to this cookie before it is sent back to the server, limited info can be gathered about a user -- typically the email address. So occassionally the cookie.txt file will contain interesting info, usually not.

An example of how this cookie file could be used is illustrated here:

The other important file is that pull-down in Netscape that showed the last 10 or so sites you've visited. This is typically located in the netscape.ini file in the [URL History] section. A clever Java applet could grab this info and ship it offsite, or if you've compromised a server where everyone has their config files in user directories, you can get to this information.

A couple of other directories that contain interesting files are the MAIL and NEWS subdirectories for Netscape. The MAIL directory will of course contain not only your inbox if you're using Netscape as your email app, but log every email sent out from your browser whether you are using Netscape for email or not. The file is typically called Sent, and is turned on for logging by default.

It is interesting to note that while it is trivial to send fakemail via Netscape (simply make the changes to the return address and send), the outgoing message is stored in that MAIL directory by default in most browsers. While fakemail is still pretty easy to track down, having a copy of the message on your machine that you don't know about is pretty damning evidence in my book.

01-4. Can you tell me more about the "cookie" file?

As stated in the previous section, the cookie file, "cookie.txt", is a file used to store information about your browser and Web server connection.

You find cookies in the Netscape environment. The limits are as follows - you can only have 300 cookies total, each cookie has a limit of 4KB (which works out to about 1.2MB), a single site can only have a max of 20 cookies in your cookie.txt file, and a web server can only access a user's cookie if that cookie.txt entry contains the web server's domain.

A cookie entry has the following in it -

NAME=VALUE; expires=DATE; path=PATH; domain=DOMAIN_NAME; secure

The name is the name of the cookie, and the value is the value of the cookie itself. The expires date, if present, is when the cookie expires. If there is no expiration date then the cookie is only kept on the client during the current session. The path and domain indicate which URLs can access certain cookies, and the secure keyword is used when a cookie needs to be sent over a secure link.

So how do we access this info? Using Java (these examples will not work alone, call from your own Java program).

First let's retrieve a cookie by name:

// This function is used by the GetCookie function...
function getCookieVal (offset) {
 var endstr=document.cookie.indexOf(";", offset);
 if (endstr==-1)
 return unescape(document.cookie.substring(offset, endstr));
// ...and this function returns the requested cookie.
function GetCookie (name) {
 var arg = name + "=";
 var alen = arg.length;
 var clen = document.cookie.length;
 var i = 0;
 while (i < clen) {
  var j = i + alen;
  if (document.cookie.substring(i, j) == arg)
   return getCookieVal (j);
  i = document.cookie.indexOf(" ", i) + 1;
 return null; // return null if no cookie by that name

Now to set cookie information with this function:

// The first 2 args are used, the rest are optional. If you skip an
// arg give it a null value. 
function SetCookie (name;value) {
 var argv = SetCookie.arguments;
 var argc = SetCookie.arguments.length;
 var expires = (argc > 2) ? argv[2] : null;
 var path = (argc > 3) ? argv[3] : null;
 var domain = (argc > 4) ? argv[4] : null;
 var secure = (argc > 5) ? argv[5] : false;
 document.cookie = name + "=" + escape (value) + 
  ((expires == null) ? "" : ("; expires=" + expires.toGMTString())) +
  ((path == null) ? "" : ("; path=" + path)) +
  ((domain == null) ? "" : ("; domain=" + domain)) +
  ((secure == true) ? "" : "; secure" + "");

Finally let's delete a cookie by name:

// This funxtion uses the GetCookie function above.
function DeleteCookie (name) {
 var exp = new Date();
 exp.setTime (exp.getTime() -1); // set cookie to expire and browser will
                                 // remove it at end of session
 var cval = GetCookie (name);
 document.cookie = name + "=" + cval + "; expires=" + exp.toGMTString();

01-5. How can I protect my browser files?

Well, you could disable cache (or set its size to zero) but that would certainly hurt performance. Usually flushing your cache at the end of a session or before visiting a site that's unknown would be good. Setting your history file preference to zero or wiping the file at the end of the session is also okay.

Don't put stupid stuff in your bookmark file ;-)

You can edit your cookie.txt file, removing any cookies and then using your local operating system make the cookie.txt file read only.

Disable the logging of outgoing email messages, unless you don't have a problem with anyone reading them.

A site can learn a lot about you, even without Netscape or Java. Take a look at With extra logging options a site can log your OS, browser, e-mail address, hostname, and last site visited. This is NOT using JavaScript either. Some companies use this info to build mailing lists, and track all of this info. To prevent this use's site as a "proxy" to surf anonymously. Instructions are at the anonymizer site, and it is currently a free service.

If most of this is Greek to you, and you simply read this FAQ because you are afraid of computer bad guys, go to and look in the Privacy section for a product called NSClean. This product allows you to clean up any or all of these files, and is fairly easy to use (I am assuming a Windows client here).

01-6. Are there any default browser holes?

Mosaic 2.2, and all previous version of the NCSA Mosaic for the X Window System had a serious security hole that would allow telnet URLs to execute an arbitrary UNIX command. In Mosaic 2.3 this bug was fixed.

There is a way (involving reconfiguration of both client and server) to have Mosaic execute any arbitrary shell script that is passed over the network. This is a documented feature that cannot be activated accidentally. It must be configured.

An entry is placed in the user or system mailcap that looks like this:

        application/x-csh; csh -f %s

The client then accesses a document on a local or remote HTTP server that is typed application/x-csh.

Obviously, csh -f will be used as the "viewer" for the document, which means the shell script will be executed on the client's host.

Since Mosaic is not shipped with support for application/x-csh or anything similar in the default settings, this is not a security hole unless you specifically modify your config files to make it so. The same goes for Netscape. You can alter the netscape.ini file and match up certain file types to certain applications, including csh.

I will not go into details regarding what entries should be in the shell script, although mailing the passwd file does seem to be the obvious.

01-7. What about Internet Explorer?

Okay, first off, IE v3 had a problem with .LNK and .URL files. These extensions on web pages can cause commands to be issued on your local workstation by viewing the pages. LNK is the extension used for shortcuts in Windows 95, and URL is used in both 95 and NT.

These extensions, coupled with Server Side Includes on the evil page's home server, could allow for a customized attack against Windows platforms. For details, do a web search for the "cybersnot" IE bug.

Another similiar bug was discovered that allows remote program execution for IE 3.0, by using a directory as a part of a <IFRAME> tag, a Windows Explorer window sized small enough to look like an icon can be double clicked on that can run a remote program. But even more interesting was the IE 3.0 ISP bug, reported at

All of these bugs have been patched with the latest patches.

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