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Major Security Flaw in OS X Found (Patched By Everyone but Apple)

May 20, 2009

This is a Java vulnerability, or rather a class of Java vulnerabilities that allows to completely bypass the Java sandbox and execute arbitrary code remotely in Java enabled web browsers.
This was found by Sami Koivu. He reported the first instance of it (CVE-2008-5353) to Sun on August 1st 2008 and this instance has been fixed by Sun on December 3rd 2008. These vulnerabilities are both technically interesting and have a lot of impact.

Since they share core classes, OpenJDK, GIJ, icedtea and Sun’s JRE were all vulnerable at some point. And unfortunately, this vulnerability is still not fixed everywhere yet.

I’ve been wanting to talk about this for a while. I was holding off, while Apple was working to patch this vulnerability. Unfortunately, it is still not patched in their latest security update from just a few days ago. I believe that since this vulnerability has already been public for almost 6 months, making MacOS X users aware that Java needs to be disabled in their browser is the good thing to do.

As a side note, Sami Koivu and I paired at latest Pwn2own (his vulnerability, my exploit) and owned both Firefox and Safari on MacOS X on day one (Java is there and enabled by default on MacOS X). Unfortunately it fell out of the challenge criterions because the vulnerability had already been reported to Sun and I had already pinged Apple in January about it.

So let’s talk about the first reported instance of this class of vulnerabilities, the Calendar deserialization vulnerability.

For legacy reasons, the deserialization of the sun.util.calendar.ZoneInfo object in a java.util.Calendar has to be fine tuned, so the readObject() method in the Calendar class will handle it. However, an applet cannot access sun.util.calendar.ZoneInfo because it is inside “sun” and anything in “sun” has to be trusted for the Java Applet security model to hold.
For this reason the code responsible for the ZoneInfo deserialization has to run with privileges. The code in java.util is trusted and can get more privileges by using a doPrivileged block:

ZoneInfo zi = (ZoneInfo) AccessController.doPrivileged(
new PrivilegedExceptionAction() {
public Object run() throws Exception {
return input.readObject();
if (zi != null) {
zone = zi;
} catch (Exception e) {}

So what does this buy us ? We can craft an input and deserialize objects from it. By deserializing a Calendar, we can get a ZoneInfo object deserialized in a privileged context. Wait! How do they check this is a ZoneInfo object? They let Java’s type checking do this for them. So if we carefully craft our input, we can get an arbitrary object deserialized but it’ll not get affected to zi unless it’s a valid ZoneInfo.

To exploit this, let’s find a class that we would be forbidden to instantiate in an Applet because it would allow us to escape from the Java sandbox. The RuntimePermission class is a great source of inspiration. A ClassLoader seems to be exactly what we are looking for! Let’s make our own ClassLoader sub-class and override the readObject() method. This method will be called during deserialization. In this method we can affect ourselves (this) to a static variable so that our shiny new ClassLoader doesn’t get garbage collected and so that we can use it later.

With our own ClassLoader we can define classes with our own ProtectionDomain (with arbitrary privileges). That’s it!
There is more work to do. The overall exploit can be quite complex (mine is over 500 lines but you can make a simpler version) but you get the basic idea.
Also there is the problem of manually crafting the malicious serialized file. In a first version I did this manually by re-implementing the Serialization protocol. Later I found a nice trick: by overriding replaceObject(), you can let Java do all the work for you.

I’ve mentioned that this was a class of vulnerabilities: the reason is that with this design, every time Java code deserializes an attacker-controlled input in a privileged context, it’s a security vulnerability. Sun fixed the Calendar vulnerability (see this patch) by creating a new accessClassInPackage.sun.util.calendar privilege and restricting the doPrivileged block to this, so they didn’t fix the whole class of them (more on this in a later post).

That’s it for the technical part.

Now why do I think this client-side arbitrary remote code execution vulnerability is more interesting that most others?

First, according to Adobe and Sun, Java is available in 80% to 90% of all web browser, which makes it a nice target.

Secondly, for various reasons, Java is usually poorly updated:

  • The Sun Java update mechanism isn’t tied to the operating system update system on the Windows platform. Personal users and companies don’t update it often, some of them do have processes in place to deal with Microsoft’s patch Tuesdays but don’t for other software updates.
  • Many companies are using web applications or Java software that rely on a specific Java version. It may be tedious to update Java because it would break many things. This may be the reason why Apple’s Java updates are so infrequent.
  • Some Linux distributions don’t support Sun’s JRE (proprietary software) despite making it available. When I asked Ubuntu to fix this vulnerability, they fixed OpenJDK quickly but told me the Sun JRE was not supported (despite being available by default on the latest LTS Ubuntu release).

Third, and this is the important point: most other client-side vulnerabilities that can lead to arbitrary code execution, including other Java vulnerabilities are memory corruption vulnerabilities in a component written in native code. Exploiting those reliably can be hard. Especially if you have to deal with multiple operating system versions or with PaX-like protections such as DEP and ASLR.
This one is a pure Java vulnerability. This means you can write a 100% reliable exploit in pure Java. This exploit will work on all the platforms, all the architectures and all the browsers! Mine has been tested on Firefox, IE6, IE7, IE8, Safari and on MacOS X, Windows, Linux and OpenBSD and should work anywhere.

This is close to the holy grail of client-side vulnerabilities.

So MacOS X users, please disable Java in your web browser.
Others: make sure you have updated Java and still disable it in your web browser: it’s a huge attack surface and it suffers from many other security vulnerabilities.

Moreover, even without taking into consideration Java vulnerabilities themselves, since the Java plugin allocates all memory as RWX and doesn’t opt-in for randomization, a Java applet can be used to bypass ASLR and non executability (DEP on Windows) in browser exploits.

You can also get some information about this vulnerability on Sami Koivu’s blog, here and here and a time line for some of the bugs he reported to Sun here.

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