Bypassing Microsoft’s Patch for the Sandworm Zero Day, the Root Cause

On October 21, we warned the public that a new exploitation method could bypass Microsoft’s official patch (MS14-060, KB3000869) for the infamous Sandworm zero-day vulnerability. As Microsoft has finally fixed the problem today via Security Bulletin MS14-064, it’s time to uncover our findings and address some confusion. This is the first of two posts on this issue. (Read part 2 here.) McAfee has already delivered various protections against this threat to our customers.

Sandworm background

This zero-day attack was disclosed at almost the same time that the patch was available on the last “Patch Tuesday” (October 14). We found that this is a very serious zero-day attack not only because the attack targeted many sensitive organizations (such as NATO), but also from the technical properties of the vulnerability and exploitation.

  • This vulnerability is a logic fault. It’s not related to memory corruption (such as a heap-based overflow or use-after-free) so proven-effective exploitation mitigations such as ASLR and DEP on Windows 7 or later will fail to block the exploit. Nor can Microsoft’s enhanced security tool Enhanced Mitigation Experience Toolkit (EMET) block the attack by default.
  • Though the in-the-wild samples are organized as PowerPoint Show (.ppsx) files, this is due to a vulnerability in the Windows Packager COM object (packager.dll). Considering that COM objects are OS-wide function providers, any applications installed on the system can invoke them, which means that other formats can be attacks paths as well. This indicates that all Windows users, not only Office users, are at risk.

The attack has been going on for quite a long time. For example, an exploit generator found on VirusTotal suggests that the vulnerability was discovered in June 2013.

Microsoft’s patch and two bypasses

On October 17, three days after its release, we found that Microsoft’s patch could be bypassed with some tricks. We reported our findings to Microsoft on the same day, which lead to an emergency Security Advisory 3010060, released October 21, with a temporary “Fix It.”

We created a proof of concept (PoC) demonstrating the bypass. We later learned that some other parties, including the Google Security Team, have detected in-the-wild samples that are said to bypass the patch. We analyzed some samples in the wild, and found that they will trigger a user account control (UAC) warning when one logs in with a standard nonadministrator account. However, users on an administrator account or who have disabled the UAC will not see the warning, and the malicious code will execute automatically.

Our PoC takes another path and does not trigger the UAC at all. Thus our PoC is a full bypass while the in-the-wild samples are a partial bypass.

At the root

The vulnerability exists in the Packager object. In fact, there are two issues rather than one.

The first issue allows an attacker to drop an arbitrary file into the temp folder. (We warned the public about this security issue in a July post. Anyone who followed our advice at that time, preventing Office from invoking the Packager object, is immune to the Sandworm attack.)

The second issue is the core of the matter. While the former allows only the writing of a file into the temp folder, the latter allows an attacker to “execute” the file from the temp folder. Let’s take a closer look at how it works.

Looking at the slide definition XML file inside the .ppsx sample, we find something interesting at the following lines:

The “verb” definition in slide1.xml.

The Packager is an OLE object that supports embedding one file into another container application. As described on this MSDN page, OLE objects that provide embedding functions must expose the interface IOleObject. For the preceding XML definition, this calls the DoVerb() method of this IOleObject. Another MSDN page provides the prototype of this method:

Prototype of the IOleObject::DoVerb() function.

And the following shows the location of the IOleObject and the DoVerb() function in the packager.dll:

The IOleObject interface and the DoVerb() function in packager.dll.

The string “cmd=3” in the slide1.xml suggests that the value of the first parameter (iVerb) is 3. Depending on different values of iVerb, we see a switch to different code in IOleObject::DoVerb(). Following we have the REed code (source code generated through reverse engineering) when iVerb equals 3.

The REed code for handling iVerb=3 in the IOleObject::DoVerb() function.

With further research and testing, we realized that this code performs the same action as clicking the second item on the following menu after right-clicking the filename, as shown here. (The print in red is our addition.)

The “right-click” menu for .inf file.

Reading the whole code of IOleObject::DoVerb(), we see that depending on different values of iVerb, the code will switch to different code paths. We split them into two situations.

  • For iVerb values greater than or equal to 3, the code will perform the same action as clicking on the pop-up menu. As we see in the REed code, it subtracts the fixed value 2 from the iVerb value 3, with the result 1, which represents the second item on the right-click menu. We can also invoke any command below “Install” on the menu by supplying a larger iVerb value. For example, if we want to click the third item on the preceding menu, we can set iVerb=4 (“cmd=4”) in the slide definition file.
  • For an iVerb value less than 3, the program will follow other code that we have not shown. These actions, such as performing the default action (iVerb=2) or renaming the display name of the Packager object (iVerb=1), are handled well from a security point of view.

We are focusing on the first situation: When the iVerb value is greater than or equal to 3, it will effectively click “Install” or a lower choice from the pop-up menu for the specific file.

For a .inf file, the right-click menu will appear exactly as in our image for a default Windows setup. Thus, in this example “InfDefaultInstall.exe” will execute and various bad thing will happen.

In this post, we have introduced the case and explained the essence of the vulnerability. In a second part, we discuss the MS14-060 patch, how to bypass it, and more.

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