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 In some cases they give a false sense of security because the algorithm is wrong

HTTP background GET le.html Client sends request to server ple.html Sample file This is a sample Server locates and sends back file Sample file This is a sample Client displays file Basic HTTP operation

HTTP background GET /sample.php Client sends request Sample.php: Sample file ’; echo ‘ This is a sample ’; ?> Server executes script Sample file This is a sample Server returns generated file Sample file This is a sample Client displays file Server side scripting

HTTP background POST /sample.php?name =Mr. Dummy&soc= &credit= Client sends data to server Sample.php: <?php # save data somewhere …. echo ‘ Now I own you. ’ ?> Server executes script Please send me your important financial information: Name: Mr. Dummy__ Soc: Credit card number: SUBMIT User fills in fields and presses ‘Submit’ Form management Now I own you. Server sends response page to client

HTTP background Client side scripting My First Web Page document.write(" " + Date() + " ");

HTTP background Client side scripting My First Web Page Tue Feb :28:07 GMT-0500 (EST)

HTTP background Client side scripting My First Web Page Tue Feb :28:07 GMT-0500 (EST)

XSS attack Server side code prints text entered by a user from an earlier session. Consider this code: <?php echo ‘ Note from ‘.$user.’ ’ echo ‘ ’.$note.’ ’ ?> Suppose $note contains document.write(" ") The sky is falling.

XSS attack The result is that the following is sent to your browser: Note from Mr. Apocalypse document.write(" ") The sky is falling.

XSS attack Your browser displays the following: Note from Mr. Apocalypse [img] The sky is falling. And the attacker has gotten your cookie.

XSS attack The 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: document.write(" ") The sky is falling._______ Any website that echoes back a user input can be used for an XSS attack.

XSS attack The following can be used to obtain the cookie for your bank account: document.location=' me= document.write(" ") '

Sanitization One solution is to escape out sensitive characters document.write(" ") becomes <script>document.write(“<img src= + document.cookie + “>”)</script> Problem: sanitization needs to be done in a context sensitive manner and the rules are very complex

Web page parsing

Challenge 1: context sensitivity Consider this code: echo ‘ ’.$note.’ ’ Here one can replace ‘ with > to block attacks. However consider: echo ‘ ’ Consider the following url: picture.jpg’ onLoad=‘document.location=…”

Challenge 2: Sanitizing nested contexts Consider this piece of php code: echo ‘ var x = ‘.$UNTRUSTED_DATA.’... ’ One needs to block both the possibility of a and that of a ‘ to prevent attacks

Challenge 3: Browser transductions Consider:... hidden comment... Even if all the “ characters are replaced with &quot, HTML 5 removes the encoding before passing the text to Javascript.

Challenge 4: Dynamic code Consider this program: function foo(untrusted) { document.write(" "); } 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 This page changes itself Original content document.anchors[0].innerHTML=“New content”;

DOM based errors Javascript can actually reference the content of a web page This page changes itself New content document.anchors[0].innerHTML=“New content”;

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 KLOC 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 Look for $_GET and $_POST variables for user input Use MAMP to run