CVE-2012-0158: RTF/OLE/CFBF/PE

Since support for the RTF file format has been added very recently with the version 0.9.4 of the Profiler, it’s a good idea to test it against real malware. I downloaded a pack of RTFs from contagiodump.blogspot.com and as I promised in the last post chose a more recent vulnerability: CVE-2012-0158. The reason why I picked a certain RTF from the pack is because most of the RTFs were automatically recognized and analyzed by the Profiler, while the following sample offers us a chance for some nice interactive analysis.

Unidentifed RTF

The first problem as you can see from the screenshot is that the RTF is not being automatically identified as such. That is because the signature is incomplete: the last two letters are missing. The next version of the Profiler will improve the detection in this regard. However, we can easily load it as RTF ourselves.

RTF foreign data

The RTF contains a lot of foreign data (meaning data which is not part of the RTF itself). Looking at the pattern an educate guess would be that it’s an encrypted payload.

The OLE stream contained in the RTF is flagged as containing possible shellcode. The Profiler detects it correctly. However, it’s actually the object embedded in the OLE stream which contains the shellcode. But wait, there’s no embedded object visible. This is because the extraction of the object failed, since the format of the OLE stream (which is undocumented) is different than usual. This is not a problem, we can just as easily load the object ourselves as the signature is easily recognizable.

Embedded CFBF

This last step was not strictly necessary, since we had already a detected shellcode in the OLE stream, but it increases the completeness of the analysis.

Since this is the header of the OLE stream:

Another educated guess would be that this is the component affected by the vulnerability. Let’s go back to the detected shellcode.

Detected shellcode

The initial instructions make sense and the following ones not. Let’s take a closer look.

This portion of code is easily recognizable as being a decryption loop for the code that follows. This is usually implemented to avoid detection. Didn’t work this time.

Let’s select the encrypted shellcode.

Encrypted shellcode

And decrypt it with the xor filter. We can confirm the correctness of the decryption by adding the ‘disasm/x86‘ filter.

Decrypted shellcode disasm

Back to the decrypted bytes, we use the script presented in the previous post to create an executable from the shellcode.

Shellcode to executable

A quick analysis with the help of the debugger.

The debugger was necessary only to check which APIs are retrieved by the shellcode and from there static analysis was easy. To sum up the shellcode decrypts two files, an executable and a doc file, executes the first directly and opens the second with the same program which is executing the shellcode.

From the shellcode we can retrieve the ranges of the encrypted payloads:

Embedded payloads

Now we can open the encrypted payloads and apply the simple decryption code.

Payload decryption

We save the decrypted payloads to disk. In the near future this won’t be necessary as such a filter will be easily created and used to load files inside the workspace of the Profiler itself.

We can use the safe text preview of Word Documents in the Profiler to view the text of the document opened by the shellcode.

DOC preview

From the text it seems to be directed at something gov: “My Esteemed Colleagues; Members of the Board of Governors of the Indian Business Chamber in Vietnam”.

The reason for opening the second document is clearly that the instance of the original program which ran the shellcode would’ve crashed and was therefore terminated cleanly with ExitProcess by the shellcode itself. Spawning a second instance with a clean document doesn’t make the user suspicious, from his point of view he just opened a document and a document has indeed been opened.

The executable is not protected by any means and so it’s just a matter of opening it with IDA Pro and spend a few hours understanding the whole code. But that’s beyond the scope of this demonstration.

Detect broken PE manifests

In the previous post we’ve seen a brief introduction of how hooks work. If you haven’t read that post, you’re encouraged to do so in order to understand this one. What we’re going to do in this post is something practical: verifying the XML correctness of PE manifests contained in executables in the Windows directory.

The hook INI entry:

And the python code:

That’s it!

What the code above does is to ask the PE object for a resource iterator. This class, as our customers can observe from the SDK documentation, is capable of both iterating and moving to a specific resource directory or item. Thus, first it moves to the RES_TYPE_CONFIGURATION_FILES directory and then goes through all its items. If the XML parsing does fail, then the file is included in our final report.

So let’s proceed and do the actual scan. First we need to activate the extension from the extensions view:

Then we need to specify the Windows directory as our scan directory and the kind of file format we’re interested scanning (PE).

Let’s wait for the scan to complete and we’ll get the final results.

So seems these file have a problem with their manifests. Let’s open one and go to its manifest resources:

(if the XML is missing new-lines, just hit “Run action (Ctrl+R)->XML indenter”)

As you can see some attributes in assemblyIdentity contain double quotes. I don’t know whether this DLL has been created with Visual C++, but I do remember that this could happen when specifying manifests fields in the project configuration dialog.

Previews

The upcoming version 0.9.2 of the Profiler adds previews for various things: images (all supported formats), several Portable Executable resources and Office Word Documents (text-only).

PE resources preview

Since media elements are rendered through third-party code, the Profiler displays a warning box before actually rendering a media element.

Preview warning

The ‘Allow all’ button allows media elements for the current session only. If the Profiler is running in a safe environment (like a VM), the user can decide to permanently disable the warning box and allow all media elements.

Preview settings

Last but not least, text-only preview of Office Word Documents has been introduced. This allows users to safely inspect the text content of a document without processing the file with an official viewer which could be the target of exploits.

Office document preview

While there are already enough new features to release, some smaller additions will be squeezed into 0.9.2 during the next days. Stay tuned!

.NET support

Although there haven’t been customer requests for this, the upcoming 0.9.0 version of the Profiler adds support for .NET, which includes format, layout ranges and an MSIL disassembler.

As usual, let’s begin with the format itself. Since some users probably have used CFF Explorer to inspect the .NET format in the past, I have kept the same format view in the Profiler as well.

So how is it better than CFF Explorer? First of all, it’s very fast. There’s absolutely no wait time in opening a large metadata set as the following.

And then one handy feature which was not available in CFF Explorer, is the capability to display a second set of metadata. For instance, .NET native images (meaning those files in the assembly cache created with ngen.exe) contain two sets of metadata. The Profiler lets you inspect both sets.

Layout ranges cover all PE parts, so it was normal to add them for .NET as well. These are the ranges available for .NET:

Let’s see them in the hex editor.

If you’re asking yourself why it’s all gray, it’s because more ranges are blended together, meaning that all .NET metadata is contained in the .text section of the PE and that section is marked as executable even if the assembly contains only MSIL code. While it doesn’t make much sense to mark as executable a region of data containing strings and tables, my best guess is that old versions of Windows had no in-built support for .NET in the loader, which is why assemblies contain a single ‘mscoree.dll’ import descriptor and the entry point is just a jmp to the only IAT thunk (_CorExeMain for executables, _CorDllMain for dlls). That API then loads the .NET framework if necessary. Since that single native jmp instruction requires a section in the PE marked as executable and because the granularity of sections is the same as virtual memory pages, it was probably considered a waste to use an entire memory page just for a jmp instruction and so the result is that everything is contained into a single executable section. This could probably be changed as new versions of the framework do not even run on older systems.

However, for our inspection purposes it suffice to get rid of the Code range by pressing Ctrl+Alt+F.

We can now inspect .NET layout ranges without the conflict. One nice aspect about it is that the code of IL methods is easy to distinguish between its header and extra sections.

Included is also an IL disassembler. Its purpose is to let our customers quickly browse the contents of an assembly. As such readability was a priority and the output has been grouped for classes: I have always found it cumbersome in ILDasm to open every single method to inspect an assembly.

Here’s an output example:

And the original code:

There’s one last thing worth mentioning. .NET manifest resources are displayed as sub-files.

However, the parsing of ‘.resources’ files was still too partial and thus won’t be included in 0.9.0.

From the last two posts you may have guessed the topic of the upcoming release. So stay tuned as there’s yet more to come.

Microsoft Authenticode

Based on RSA’s PKCS7 standard, Authenticode is the technology developed by Microsoft to digitally certify programs and drivers on Windows. Trusted signatures guarantee that the certificate owner is indeed the author of the signed executable, and also that the data itself has not been tampered with by anyone else.

In a default configuration scenario, the operating system considers these signatures during all but three events:

  1. When kernel-mode drivers are loaded
  2. When executable images that derive directly from content downloaded using Internet Explorer (or any other third-party browser which supports it) are written to disk.
  3. When an application requires admin privileges.

Point two is a weak security measure for the following reasons:

  1. Even if the right browser is used, there’s still no guarantee that the verification request (because it is by no means mandatory) is honored by the operating system unless the UAC privilege elevation dialog is invoked (either via manifest or using the “Run as Administrator” menu item). It’s important to note that if the Authenticode signature can’t be verified, the dialog being shown to the user is the same as the one for unsigned executables. At this point, there is no way to distinguish from unsigned (no code signature) and untrusted programs (invalid code signature) using the UAC dialog alone.
  2. Once an application has been authorized by a user and his certificate store, no checks are performed when it is moved to another system (even if the new certificate store can’t validate the Authenticode signature)
  3. The whole mechanism heavily relies on the file system being used; copying unauthorized files to a non-NTFS file system (which happens quite a lot, considering the vast majority of USB drives are using FAT32) doesn’t preserve the alternate data streams created by browser.

Also the verification is not self evident if done manually using Windows Explorer, as the properties dialog doesn’t show the validity of the certificate until the user clicks the “Details” button. This is highly misleading, because the user might get a false sense of security by just checking whether the executable contains a digital signature.

Windows certificate dialog

The upcoming version 0.8.6 of the Profiler introduces support for this technology, allowing users to very quickly access and verify code signing information.

The following screenshot shows a perfectly valid digital signature; all the certificates taken from the Authenticode data have been successfully used to build a trust chain that validates the PKCS7 using the system store, which means that those at the root of the tree have been directly validated by the Windows Certificate Store.

Microsoft Authenticode – A valid digital signature

As you can see, the risk factor is set to zero, since the validity of the publisher has been determined. This behavior can be changed from the Risk panel of the options: it is not on by default!

For comparison, the following screenshot shows how an invalid digital certificate is displayed:

Microsoft Authenticode – An invalid digital signature

In this case the hash is no longer what expected, issuing both a digest and an invalid certificate errors.
Countersignatures are of course supported and I think you’ll be pleased with how fast our implementation is.

Additional resources:

  1. Windows NTFS Alternate Data Streams, from Symantec
  2. Mark of the Web, from MSDN

Validation of Portable Executable resources

One of the new features of the upcoming 0.8.6 version of the Profiler is the validation of resources. This means the Profiler verifies the integrity of resources and lets the user inspect problems, making it easy to discover things like appended files or fake resources. This feature comes handy since very often malware is hidden in resources and droppers often use resources to store their payload.

All the most important resource types are supported:

  • Version info
  • Bitmaps
  • Icons
  • Cursors
  • Icon groups
  • Cursor groups
  • Configuration files
  • Accelerators
  • Menus
  • Dialogs
  • String tables
  • Message tables
  • Any other supported file format

So let’s see a simple test case. What I did is to append a DLL to a bitmap and then replace one of the bitmaps in explorer.exe with my modified one.

I could’ve used any other resource type, or even a PNG or GIF, it wouldn’t have mattered.

The simplified resource tree highlights problems with their risk color, while unsupported types are highlighted in gray.

One can jump to problems with the F2 shortcut, no need to scroll the tree ourselves in search of problems.

In the screenshot above the analysis shown is for my fake bitmap. As it’s possible to see, the bitmap ends where the red-marked data begins.

In this context it is very easy to just load the embedded PE with the “Load as…” (Ctrl+W) command.

And this gives me the opportunity to mention briefly another nice improvement to the hierarchy view.

As you can see files are now grouped according to their type. This makes it much easier to go over the files or to look for specific types. This behavior is optional and can be changed from the settings.

I think I could’ve presented this new feature with a more interesting real-world case. However, there are still some things to do in order for the new version to come out and there wasn’t enough time. I hope, nevertheless, that you enjoyed the post. 🙂

PE analysis (part 1)

This is the first of a series of posts which will be dedicated to PE analysis features. In previous posts we have seen how the Profiler has started supporting PE as a format and while it still lacks support for a few directories (and .NET), it supports enough of them for x86 PE analysis.

PE Analysis 1

While the upcoming version 0.8.4 of the Profiler also features analysis checks as CRC, recursion, metadata, etc., this post will be about the in-depth range analysis for PE files. As the screenshot above previews, in-depth ranges show PE data structures in a hex view and the distribution of data in a PE file.

Let’s take as first sample “kernel32.dll”. After having it opened in the Profiler, let’s execute the “PE->Display ranges” action.

PE ranges action

We get the PE ranges for kernel32.

Kernel32 ranges

The big region of data marked as fluorescent green represents executable code. As you can see, it is interrupted by a gray region of data which the tooltip tells us being a combination of “Code” and “Export Name Data”. If we move the cursor, we can see that it’s not only Export data, but also Import data. Which means that the Export and Import directory are contained in the executable part of the file (the IAT is in the thin gray area at the beginning of the code section). But we may not be interested in having the code section covering other data regions. This is why we can filter what we want to see (Ctrl+B).

PE ranges filter

I unmarked the “Code” range. Thus, we now get all the ranges except the unmarked one.

Kernel32 ranges without code

We can also jump to regions of data, but before seeing that, I want to briefly mention that the hex view can be printed to file/PDF or captured.

Hex View caputre

Not a big feature, but it may come handy when generating reports.

Now let’s look at a file I have especially crafted for the occasion, although it reflects a very common real-world case.

PE high entropy

We’ve got a PE with an extremely high quantity (50%) of foreign data and the entropy level of that data is also extremely high.

So let’s jump to the first occurrence of foreign data (Ctrl+J).

Ranges jump

What we see is that right there where the analyzed PE files finishes, another one has been appended.

Appended PE

So let’s select the contiguous range of data (Ctrl+Alt+A: this will select the foreign range of data) and “Load selection as…” (Ctrl+E) will asks us to select the file type to load (it is automatically identified as being a PE).

Load appended PE

We are now able to analyze the embedded PE file.

Loaded appended PE

While this procedure doesn’t highlight anything new, since loading of embedded files has been featured by the Profiler from its earliest versions, I wanted to show a practical use of it in connection with ranges.

It has to be noted that this particular case is so simple that it can be detected automatically without interaction of the user. In fact, detection of appended files in PEs will be added most probably in version 0.8.5.

Hope you enjoyed this post and stay tuned for the next parts!

PS: take advantage of our promotional offer in time. Prices will be updated in August!

Resource & Load Config Directory

The upcoming 0.8.3 version of the Profiler features two new directories. Most of the work went into implementing an efficient model view controller for the Resource Directory tree.

Resource Directory

Last year I was notified by Ange Albertini that his resourceloop.exe sample crashed the CFF Explorer (by exhausting the stack). Recursion is one of the things to look after when parsing a file as mentioned in my speech about the security of non-executable files. The screenshot below shows Ange’s sample in the Profiler.

Recursive Resource Directory

The red marked Resource Directory Entry points back to the top-level Resource Directory and thus creates a recursion in the tree.

The Profiler is intended to offer complete support for the PE file format, this is why all directories will be supported. One of the directories missing in the CFF Explorer, for instance, is the Load Config (alias SafeSEH).

Load Config Directory

Another small addition is the smart address converter (VA/RVA/Offset). This action guesses the address which needs to be converted from the context. Jumping to a location in the hex view is usually a matter of Ctrl+R and twice Ctrl+Enter.

Address converter

Five directories are still missing: Security, Exception, Bound Import, Delay Import and .NET. The analysis of x86 PEs can already be implemented by adding basic support for the Security Directory. x64 needs Exception as well. Bound and Delay Import support will follow. .NET is the one which has least priority, but it won’t take much time to impelement.

The coming posts about Portable Executable will involve analysis and will be more interesting than this one. So stay tuned. 😉

Portable Executable: coming soon

In the upcoming 0.8.1 release of the Profiler initial support for PE files has been introduced. 🙂

Optional Header

Most of the work went into optimizing the UI and allowing for complex custom views to be built easily, while maintaining great speed. Even the grid control you can see here displayed is a custom control.

In the following screenshot you can see a complex view displaying the section headers.

Section Headers

And here’s a more basic view for the import directory.

Import Directory

Another eye candy screenshot of the section headers with entropy computation for one section.

Section Entropy

What will be present in this first PE edition is mainly about the file format itself. What is going to be missing is a viewer for the resources and one for the .NET directory, because we need first to implement an efficient and customizable tree control. Also ranges won’t be supported as long as the whole file format isn’t supported. This is due to the fact that the PE is one of the most studied and documented file formats around, hence the necessity to be very precise when calculating data ranges.

Also, soon we’ll release a demo of the Profiler. Stay tuned!