1 14th ACM Conference on Computer and Communications Security, Alexandria, VA Shuo Chen †, David Ross ‡, Yi-Min Wang † † Internet Services Research Center Microsoft Research ‡ Microsoft Security Technology Unit October 30 th, 2007

2 14th ACM Conference on Computer and Communications Security, Alexandria, VA A browser can visit pages from benign and malicious websites at the same time. Browser needs to provide an isolation mechanism so that pages from different domains cannot access each other. The policy of such a mechanism is commonly referred to as the same-origin policy (SOP) Otherwise, a foo.com page can do almost anything to a bank.com page Info leak: steal the user’s personal information in myBank.com Request forgery: transfer the user’s money to other places.

3 14th ACM Conference on Computer and Communications Security, Alexandria, VA Some SOPs are not clearly defined. The industry still needs to define some specific SOPs. However, even for well-defined SOPs, the current implementations of the isolation mechanisms are surprisingly error-prone. IE, Firefox, Netscape, Opera all had bugs in their implementations. Demos: attacks against IE 6 (on WinXP)

4 14th ACM Conference on Computer and Communications Security, Alexandria, VA Keep patching? Not a real solution, not effective for future bugs. Perform a thorough code review of the browser code base? Not realistic. The code base is huge, bugs are much trickier than buffer overruns. What kind of solution do we want? Comprehensive: solve this class of bugs Transparent: no need to change web applications Light-weight: low performance overhead Self-contained correctness: can be implemented correctly with only limited understanding of existing browser code base

5 14th ACM Conference on Computer and Communications Security, Alexandria, VA In human languages, accent is essentially an identifier of a person’s origin that is carried in communications Script accenting Each domain is associated with an “accent key”. Scripts and HTML object names are represented in their accented forms at the interface between the script engine and the HTML engine. Two frames cannot interfere if they have different accent keys (no need for an explicit check for the domain IDs)

6 14th ACM Conference on Computer and Communications Security, Alexandria, VA

7 Frame A’s domain is x, frame B’s domain is y. Isn’t it easy to simply check x==y? No, it’s much more complicated than this There are unexpected execution paths in the system to bypass the check or feed incorrect domain IDs to the check. Exploit scenarios take advantage of many complex mechanisms in the browser. Surprisingly smart ways of exploits!

8 14th ACM Conference on Computer and Communications Security, Alexandria, VA Frame2 = open(“ “frame2”); open(“file: javascript: doEvil”, “frame2”) Frame1: URL= file: javascript: doEvil javascript: doEvil Windows Shell Address Parser Frame2: URL= Salary=$1234 Direct deposit settings … Window Shell IE

9 14th ACM Conference on Computer and Communications Security, Alexandria, VA Frame1: URL= Frame2: URL= After 1 second, execute: “location.assign(‘ javascript:doEvil’)” (1) Set a timer in Frame2 to execute a statement after 1 second (2) Frame2.location.assign =window.location.assign (3) Navigate Frame1 to Frame1: URL=

10 14th ACM Conference on Computer and Communications Security, Alexandria, VA Frame1: URL= Frame2: URL= Frame0: URL= Frame0 executes a statement: Frame2.open(“javascript:doEvil”,Frame1)

11 14th ACM Conference on Computer and Communications Security, Alexandria, VA Frame1: URL= Frame0: URL= document.body.setCapture() onClick() { reference to the document in Frame1 by event.srcElement }

12 14th ACM Conference on Computer and Communications Security, Alexandria, VA The causes The SOP check is bypassed in some attack scenarios (the check may not be triggered) The SOP check is a single-point check buried deep in the call stack At the time of check, there are confusions of the domain-IDs. Developers cannot anticipate all these scenarios. Involving too many modules, too complex logic combinations

13 14th ACM Conference on Computer and Communications Security, Alexandria, VA

14 14th ACM Conference on Computer and Communications Security, Alexandria, VA Each domain D is assigned a random number as its accent key K D The current implementation uses  (i.e., XOR) To accent script S in domain D: S  K D Two basic and easy rules in the implementation Rule of script ownership A script is owned by the frame that supplies the source code of the script, and should be accented at the time when its source code is supplied. Rule of object ownership Every object is owned by the frame that hosts the DOM tree of the object, and is always referenced by its accented name.

15 14th ACM Conference on Computer and Communications Security, Alexandria, VA

16 14th ACM Conference on Computer and Communications Security, Alexandria, VA

17 14th ACM Conference on Computer and Communications Security, Alexandria, VA

18 14th ACM Conference on Computer and Communications Security, Alexandria, VA javascript Filename, not a javascript Frame2 = open(“ “frame2”); open(“file: javascript: doEvil”, “frame2”) Frame1: URL= file: javascript: doEvil javascript: doEvil Windows Shell Address Parser Frame2: URL= Window Shell IE Unrecognizable script code

19 14th ACM Conference on Computer and Communications Security, Alexandria, VA Frame1: URL= Frame2: URL= After 1 second, execute: “location.assign(‘ javascript:doEvil’)” (1) Set a timer in Frame2 to execute a statement after 1 second (2) Frame2.location.assign =window.location.assign (3) Navigate Frame1 to Frame1: URL= The script is accented using evil’s key, but deaccented using payroll’s key

20 14th ACM Conference on Computer and Communications Security, Alexandria, VA Frame1: URL= Frame2: URL= Frame0: URL= Frame0 executes a statement: Frame2.open(“javascript:doEvil”,Frame1) The script is accented using evil’s key (Frame0), but deaccented using payroll’s key (Frame1)

21 14th ACM Conference on Computer and Communications Security, Alexandria, VA Frame1: URL= Frame0: URL= document.body.setCapture() onClick() { reference to event.srcElement } Names of objects under srcElement are deaccented using payroll’s key.

22 14th ACM Conference on Computer and Communications Security, Alexandria, VA Compatibility Existing web applications do not need any changes. They can run normally without knowing the existence of the accenting mechanism. Performance The measurement about end-to-end browsing time did not show any noticeable slowdown. (despite a 3.16% worst-case performance overhead)

23 14th ACM Conference on Computer and Communications Security, Alexandria, VA We studied previous browser-isolation bugs, and identified key challenges in eliminating these bugs. We proposed the script accenting approach Easy to reason about its correctness without understanding the complex logic of existing browser code base. Evaluations show its comprehensive protection, compatibility with existing applications, and very small performance overhead.