omash: enabling secure web mashups via object abstractions steven crites, francis hsu, hao chen (uc...
TRANSCRIPT
OMash: Enabling Secure Web Mashups via Object
Abstractions
Steven Crites, Francis Hsu, Hao Chen (UC Davis)ACM Conference on Computer and Communications
Security (CCS), 2008
Presenter: Fu-Chi Ao
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Outline
• Introduction• The Same Origin Policy• Design• Usage Examples• Implementation• Related Work• Conclusion and Comments
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Introduction (1/2)
• Even before the rise of AJAX and mashups, at any given time a web browser likely contained pages from different domains
• Same Origin Policy (SOP) – Used by web browsers– Not support secure cross-domain communication
desired by web mashup developer– Supports only 2 states
• Full trust: third-party content runs with full privilege• No trust: no communication is allowed
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Introduction (2/2)
• OMash– New abstraction and access control model for
writing secure yet flexible mashup applications• Provide mashup developers with the ability to allow
safe, controlled communication and interaction between web sites, and allow for the various trust models they desire
– Does not rely on the SOP– Supports backwards compatibility– Tested on Mozilla Firefox 2.0
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The Same Origin Policy (1/2)
a.com
a.com a.com b.com
Server
Browser
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The Same Origin Policy (2/2)
• Applied to protect three browser resources:– Documents: Sites from one origin cannot access
documents from another origin via the Document Object Model
– Cookies: Sites can only set their own cookies and cookies are only sent to their originating site in HTTP requests.
– Access to remote services: Only permits XMLHttpRequest to issue requests to the origin of the containing document.• allows a script to issue an asynchronous HTTP request to a
remote server.
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Problems of SOP: DOM Access
• Extended to Domain Name System (DNS) domains and relies on it
• DNS Insecurity– Client vulnerabilities• DNS rebinding (Jackson et al, CCS 07)• Dynamic Pharming (Karlof et al, CCS 07)
– Server vulnerabilities• DNS cache poisoning (Kaminsky, BlackHat 08)
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Problems of SOP: Authentication Credentials
• Confused Deputy– When a request is made, cookies matching the
destination domain are added to the request• as well as any other form of HTTP Authentication (e.g.
Basic, Digest, NTLM) information for the domain
– Regardless of what page caused the browser to initiate this request
– Causes Cross-Site Request Forgery (CSRF) attacks
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Cross-Site Request Forgery
a.com
a.com b.com
Server
Browser
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Trust Levels
• Wang et al. in MashupOS enumerate all the possible trust levels available between integrators and providers– (1) isolated content that should be isolated from other domains– (2) access-controlled content that should be isolated but allows
for mediated access via, e.g. message passing– (3) (and (5)) open content that any domain can access and
integrate into itself 1– (4) unauthorized content that has no privileges of any domain.
Table 1: The Trust Model on the Web for a provider P and an integrator I as defined in MashupOS2009/7/17
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OMash Design
• Analog to OOP languages• Treat each web page as an object that declares
public and private data and methods – A web page can only access its own content and the
public content of another page– All data in the page are considered private by default
• To enable inter-page communication, a page may declare a public interface, getPublicInterface, which all pages can access
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Object Abstractions
• Java (analogy) • Web page object
public class FooObject {
public void publicMethod() { }
private int privateData;}
<html><script>function getPublicInterface() { function Interface() { this.publicMethod = function () {…} } return new Interface();}var privateData;</script></html>2009/7/17
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Page Objects
• A web page consists of – DOM tree– Scripts– Credentials (HTTP authentication token, e.g.
cookies)• A page object can be contained in a– Window– Frame– Iframe
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Public and Private Members
• Public interface– Each object declares getPublicInterface()– Returns a closure of all public methods and data
• An expression (typically a function) that can have free variables together with an environment that binds those variables
• Using closures, pages can safely get and set information on other pages in a controlled manner
• Private data– DOM objects (document, etc.)– JavaScript objects and functions– Credentials (HTTP authentication token, e.g. cookies)
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Provider and Integrator Communicate via the Public Interface
• map.html • integrator.html<html>function getPublicInterface() { function Interface() { this.setCenter = function (lat,long){ … } } return new Interface();}</html>
<iframe id=“inner” src="map.html">...var win = getElementByID(“inner”).contentWindow;var map = win.getPublicInterface();...map.setCenter(lat, long);}
map.html
integrator.html
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Mediate DOM Access
• By using the getPublicInterface function, a page’s creator can specify what they want other pages to be able to access
• Using this approach, the mashup developer can model a variety of trust relationships– Isolated content– Access-controlled content– Open content– Unauthorized content
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• No access between provider and integrator
Isolated
function getPublicInterface() { function Interface() { } return new Interface();}
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• Limited access depending on caller• Provide methods for the returned interface that only allow
access to a site’s content based on the caller’s credentials
Access-controlled
function getPublicInterface() { function Interface() { this.auth = function(user,pass) { return token; }
this.do = function (token,...) { check(token); } } return new Interface();}
var api = win.getPublicInterface();
token =api.auth(user, pass);
api.do (token,...)
Provider Integrator
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Open
• Full access between provider and integrator function getPublicInterface() { function Interface() { this.getDocument = function () { return document; } } return new Interface();}
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Unauthorizedmap.html, which provides a map service outer.html, which uses the service provided
by the code in map.html
map.html
outer.html isolate the untrusted script library (provided content) within another page
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Mediate Authentication Credentials
• HTTP authentication (e.g. Basic, Digest, NTLM) – When authentication information (HTTP or a cookie) comes in, the
browser associates this information with the page that receives it, page P• Cookies
– The only cookies we want to treat this way are those that are used for authentication.
– Modification : Add extra attribute Authentication• Session cookies are associated with a page when they are set• Persistent cookies are associated with the first user-opened page
• XMLHttpRequest– Malicious sites can no longer leverage CSRF– Can logg in with 2 different accounts at the same time– Can accomplish safe cross-domain data exchange by lifting the restriction
of XHR
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Browser Sessions under OMash
• Each cookie– Belongs to a window– Is shared by subsequent pages from the same
domain in that window• Each window has an independent session– Desirable side effect:
Can log in to multiple accounts in different windows in the same browser
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Cross-window Sessions
• How to track a session across windows?• Cookie Inheritance– When page P1 loads P2, P2 inherits P1’s cookies– P1 and P2 now belong to the same session
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Preventing CSRF
a.com
a.com b.com
Server
Browser
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Preventing CSRF
a.com
a.com b.com
Server
Browser
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Preventing CSRF
a.com
a.com b.com
Server
Browser No cookie!
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Backward Compatibility with the Same Origin Policy
• If application prefers using SOP to allow inter-page communication:
• To implement this under OMash– Server embeds a shared secret in all pages– Pages authenticate each other using this secret
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Supporting SOP without DNS
secret = “1234”;function getPublicInterface() { function Interface() { this.foo=function (secret, … ) { check(secret); … } } return new Interface();}
<script>secret = “1234”api = win.getPublicInterface()api.foo(secret, …)</script>
Provider Integrator
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(b) outer.html. It authenticates by providing the argument secret in the call to the provider.
(a) inner.html. The function foo authenticates the caller by checking the parameter provided Secret against the embedded global variable secret.
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Implementation
• Implemented and tested on Firefox 2• Proof of concept as Firefox add-on– Use Mozilla’s Configurable Security Policies(CAPS)
system to allow the cross-domain access to the getPublicInterface• allow users to set up security policies for the browser, and
also have different security policies for different Internet sites
– Use Firefox 2’s Session store API to make each cookie private to a window
• No changes required on the server
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Related Work
• MashupOS (Wang et al, SOSP 07)– Rely on the Same Origin Policy for controlling DOM accesses or
cross-domain data exchange– Requires browser writers and application developers to support
and use several different abstractions• SMash (Keukelaere WWW 07)
– Isolates components using the iframe tag, and URL fragment identifiers allow the frames to establish communication links.
• Google’s Caja– Also allows web applications of different trust domains to
directly communicate with JavaScript function calls and reference passing
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Conclusion and Comments
• OMash is a new browser security model– Allows flexible trust relation– Simple– Familiar, easy to understand– Backward compatible
• Don’t rely on Same Origin Policy– Prevent CSRF attacks– Allows programmers to define “Same Origin”
flexibly based on shared secrets
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