An introduction to Filters

While filters have always been a part of the Profiler, they are now partly exposed only with the upcoming 0.8.7 version. Shortly explained: filters are algorithms of different nature which can be applied to portions of data. The starting point to interact with filters is the filter view. This view is triggered with the Ctrl+T shortcut. Let’s start with some step by step screenshots.

This is a Zip file and the gray area marks a compressed file. Let’s select this area with Ctrl+Alt+A.

Zipped file

Now let’s press Ctrl+T and take a look at the filter view.

Filter View

The tree at the top-left displays the available filters. The property editor next to it contains the available parameters for the currently selected filter and the list beneath the filters is the stack of filters which need to be applied to the input data. The hex view on the left contains the data which we have previously selected in the hex view (meaning the compressed file) and represents the input data.

Now if we want to decompress the data, let’s add the unpack/zlib filter (with the raw parameter checked) to the stack by clicking on “Add”.

Filter stack

And at this point we can click on the “Preview” button.

Unzipped file

But what if we want to retrieve the SHA1 of the decompressed data? Just add the hash/sha1 filter to the stack.

Filter stack

This time we’ll have only the SHA1 hash in the output view.


The filter stack can be imported and exported. Just right-click inside the stack list. The exported filters of the current stack will look like this:

If we had opened the filter view without a selection we would have had an editable hex view on the left, which we could fill with our custom data. Also, the hex views in the filter view have all the benefits of a regular hex view (that is if the filter view is not triggered while loading a file, as we’ll see later); this means that it’s possible to trigger a new filter view by selecting something in the hex view containing the filtered output data.

What I’ve showed until now isn’t an every-day task, but it serves the purpose of introducing the topic. We’ll see some real-word cases after I have introduced the misc/basic filter. But before of that let me shortly mention text output filters.

Right now the Profiler features two filters with text output: disasm/x86 and disasm/x64. When a filter outputs text, we’ll be shown by default a text view instead of the hex view on the right.

x86 filter

This can come handy when decrypting shellcode. What we also most probably need is the misc/basic filter, which I think will be one of the most commonly used filters. Let’s look at its parameters.

basic filter params

operation specifies what is going to be performed on the input data. Available operations are: set(fill), add, sub, mul, div, mod, and, or, xor, shl, shr, rol, ror. bits specifies the size in bits of each element the operation has to be performed on (values up to 64 bit are supported). endianness applies only to elements greater than 8 bits. radix is related to the input format of value. value contains the data used for the operation.

If I now inserted “FF” as value, then I would be xoring every byte of the input data with that 1-byte value. If, however, I inserted “FF 00”, then I would be xoring every even byte with “FF” and every uneven one with “00”. I may even only modify every third byte by using wildcards and leaving the first two unchanged, I only have to insert “* * FF”. Wildcards and arrays can be used even in the context of values bigger than 1 byte of course.

Now that I’ve introduced the basic filter, let’s see our first real-world case. What you see is a PDF containing an encrypted malware.

Malware PDF

At the caret position an encrypted Portable Executable starts, it’s quite easy to figure out because of the repetitive sequence of bytes (a PE header is full of zeroes). I select the data with the go to dialog (Ctrl+G).

Select encrypted malware

Now instead of triggering the filter view directly, I’m going to load the embedded file with Ctrl+E. This triggers the load file dialog.

Load malware

As you can see I specified the format of the file to load (PE) and inserted an optional name for it, but now it’s necessary to tell the Profiler how to decrypt the file, otherwise it won’t be able to load it. So I’ll just click on the “Filters” button at the bottom-right and a filter view dialog will pop up.

Malware filters

The decryption sequence is “EB FF FD FC EB FF 23 95”. It can be expressed like I did as an array.

But since it amounts to a 64-bit value, it can also be expressed like this.

Ok, back to the malware. After having added the decryption filter to the stack I just close the dialog and confirm the load file dialog. We’re now able to navigate the decrypted PE.

Decrypted malware

Now I can just save the project (which is under 1KB) and send it to a colleague who will be able to inspect the decrypted file.

The last real-world case I’m going to present I made it myself, because it was easier than to start looking for one. I just took a cursor resource from a PE and appended another RC4-encrypted PE to it. Then replaced the original resource and opened the host file with the Profiler.

Fake resource

The Profiler tells us that the are problems in the resource and also shows us the file. In fact, we can see where the foreign data begins. I just select the foreign data (I can do this both from the resource directory or from the DIBCUR embedded file), trigger the load file dialog and apply the decryption filter.

In upcoming updates there will be many improvements to filters: many more will be added and the concept will be extended and made customizable. Clearly this post doesn’t cover all of that, but I think for now this is enough to keep the introduction simple.

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