The Past, Present and Future of XSS Defense Jim Manico 2011 OWASP Paris, May 24

Jim Manico Managing Partner, Infrared Security Web Developer, 15+ Years OWASP Connections Committee Chair OWASP ESAPI Project Manager OWASP Podcast Series Producer/Host Kauai/Hawaii Resident with wife Tracey

XSS Defense, Past Exploitable Defenses Input Validation Alone Sometimes applications needs to support < ' " & … and other “dangerous” characters Can be very difficult File upload input HTML inputs HTML Entity Encoding Alone Works well for untrusted data placed in HTML “normal” contexts Does not stop XSS in unquoted HTML attribute and other contexts

XSS Defense Today: Quite Challenging 1. All untrusted data must first be canonicalized Reduced to simplest form 2. All untrusted data must be validated Positive Regular Expression Rule Blacklist Validation 3. Untrusted data must be contextually sanitized/encoded - HTML Body - HTML Attribute - URI Resource Locator - Style Tag - Event handler - Within Script tag AuthN, AuthZ

Danger: Multiple Contexts Browsers have multiple contexts that must be considered! HTML Body HTML Attributes <STYLE> Context <SCRIPT> Context URL Context 4 Copyright 2010 - AppSec Training LLC Version:2010-03-10.001

Danger: DOM Based XSS 1. Untrusted data should only be treated as displayable text. 2. Always JavaScript encode and delimit untrusted data as quoted strings 3. Use document.createElement(“…”), element.setAttribute(“…”,”value”), element.appendChild(…), etc. to build dynamic interfaces. Avoid use of HTML rendering methods. 4. Understand the dataflow of untrusted data through your JavaScript code. If you do have to use the methods above remember to HTML and then JavaScript encode the untrusted data 5. Make sure that any untrusted data passed to eval() methods is delimited with string delimiters and enclosed within a closure or JavaScript encoded to N-levels based on usage, and wrapped in a custom function. 6. Limit the usage of dynamic untrusted data to right side operations. And be aware of data which may be passed to the application which look like code (eg. location, eval()). 7. When URL encoding in DOM be aware of character set issues as the character set in JavaScript DOM is not clearly defined. 8. Limit access to properties objects when using object[x] accessors 9. Don’t eval() JSON to convert it to native JavaScript objects. Instead use JSON.toJSON() and JSON.parse()

(1) Auto-Escaping Template Technologies XHP from Facebook Makes PHP understand XML document fragments similar to what E4X does for ECMAScript Context-Sensitive Auto-Sanitization (CSAS) from Google Runs during the compilation stage of the Google Closure Templates to add proper sanitization and runtime checks to ensure the correct sanitization. Java XML Templates (JXT) from OWASP Fast and secure XHTML-compliant context-aware auto-encoding template language that runs on a model similar to JSP. Copyright 2010 - AppSec Training LLC

Context-aware Auto-escaping Tradeoffs Developers need to write highly compliant templates No “free and loose” coding like JSP Requires extra time, but increased quality These technologies often do not support complex contexts Some choose to let developers disable auto-escaping on a case-by-case basis (really bad decision) Some choose to encode wrong (bad decision) Some choose to reject the template (better decision)

(2) Javascript Sandboxing Capabilities JavaScript (CAJA) from Google Applies an advanced security concept, capabilities, to define a version of JavaScript that can be safer than the sandbox JSReg by Gareth Heyes Javascript sandbox which converts code using regular expressions The goal is to produce safe Javascript from a untrusted source ECMAScript 5 Object.seal( obj ) Object.isSealed( obj ) Sealing an object prevents other code from deleting, or changing the descriptors of, any of the object's properties Allows highly selective use of powers that would have been forbidden by the sandbox For the truly non-technical, you might think of capabilties as a 'magic word' system. The permission to do a particular operation on a particular piece of data is represented by a magic word. Moreover, the magic word itself contains information about when it can be said, and by whom. And it can be different every time, so you can't just write it down and reuse it later. So rather than putting everyone in a sandbox, we get smart about what magic words are created, and what they can do, and who gets them. They can be passed along, but if someone tries to say them in the wrong context, nothing happens. For instance, the JavaScript embedded within a Webmail could be given the magic word that allows it to display something, but never given the magic words that allow read or write access to the mail directory. For the slightly more technical, Caja is taking advantage of a general characteristic of object oriented (OO) languages, JavaScript being one. A true OO language never allows direct programmatic access to a piece of data. Reading or modifying the data requires invoking a 'method' (a small program) that hides the details of the operation. Data is never naked in an OO system, instead it is 'encapsulated' by an enclosing object and the associated methods. This is obviously fairly close to the 'magic word' notion - if you don't know the name of the particular object, and the right method to call, nothing happens. But few OO languages have any notion of program or user identity at a fundamental level. Most also have built-in methods (often related to reflection) that could be used to circumvent a capabilities model if left exposed. Copyright 2010 - AppSec Training LLC

JSReg: Protecting JavaScript with JavaScript JavaScript re-writing Parses untrusted HTML and returns trusted HTML Utilizes the browser JS engine and regular expressions No third-party code First layer is an iframe used as a safe throw away box The entire JavaScript objects/properties list was whitelisted by forcing all methods to use suffix/prefix of “$” Each variable assignment was then localized using var to force local variables Each object was also checked to ensure it didn’t contain a window reference first layer was an iframe, the iframe was created each time of execution enabling fresh prototypes and a throw away box whitelisted the entire JavaScript objects/properties, this was done by forcing all methods to use suffix/prefix of “$” Each variable assignment was then localized using var to force local variables. Each object was also checked to ensure it didn’t contain a window reference.

Google CAJA: Subset of JavaScript Caja sanitizes JavaScript into Cajoled JavaScript Caja uses multiple sanitization techniques Caja uses STATIC ANALYSIS when it can Caja modifies JavaScript to include additional run-time checks for additional defense Weak static analysis. Although Caja is less dynamic than JavaScript, we still assume that it is impractical to perform any interesting analysis, such as type inference, both statically and safely. As a result, Caja's static restrictions can only enforce simple syntactic rules. Remaining restrictions must be enforced by runtime checks. 10 Copyright 2010 - AppSec Training LLC 10

CAJA workflow The web app loads the Caja runtime library, which is written in JavaScript All un-trusted scripts must be provided as Caja source code, to be statically verified and cajoled by the Caja sanitizer The sanitizer's output is either included directly in the containing web page or loaded by the Caja runtime engine

CAJA Compliant Applications A Caja-compliant JavaScript program is one which is statically accepted by the Caja sanitizer does not provoke Caja-induced failures when run cajoled Such a program should have the same semantics whether run cajoled or not 12 Copyright 2010 - AppSec Training LLC

#@$( This Most of Caja’s complexity is needed to defend against JavaScript's rules regarding the binding of “this". JavaScript's rules for binding “this“ depends on whether a function is invoked by construction by method call by function call or by reflection If a function written to be called in one way is instead called in another way, its “this" might be rebound to a different object or even to the global environment. The subset of Caja without “this" is a perfectly reasonable and expressive programming language. Caja supports “this" in order to ease the porting of old code. For new code, we recommend sticking to the this subset of Caja, which we refer to as Cajita, the diminutive, meaning “small box". The Caja runtime will provide a safe caja.eval operation. For caja.eval to accept Caja code, the Caja sanitizer would need to be written in JavaScript and included in the Caja download. To minimize download size, caja.eval will instead accept only Cajita code. 13 Copyright 2010 - AppSec Training LLC

(3) Browser Protections Content Security Policy JavaScript policy standard Reflective Defense XSS in Chrome IE 8 Cross-Site Scripting Filter Blacklist browser-based URL filters Early versions of IE8’s browser-based filter actually made XSS possible on sites that did not even have XSS vulns due to errors in MS’s filter CSP: Note: event-handling is still enabled in CSP without using HTML attributes. Copyright 2010 - AppSec Training LLC

Awesomeness: Content Security Policy Externalize all JavaScript within web pages No inline script tag No inline JavaScript for onclick or other handling events Push all JavaScript to formal .js files using event binding Define the policy for your site and whitelist the allowed domains where the externalized JavaScript is located Add the X-Content-Security-Policy response header to instruct the browser that CSP is in use Will take 3-5 years for wide adoption and support

THANK YOU! jim@owasp.org 2011 OWASP Paris