The NT kernel can ignore your hardware clock during the boot

This started as an attempt to solve a puzzle:

Some virtualization software allows a user to launch a virtual machine with its real-time clock starting ticking from custom base time. For example, a user can launch a virtual machine with base time set to 2020-12-31 23:59:59 UTC, the real-time clock inside this virtual machine will start ticking from that value, regardless of the current date and time set on a host. I use this feature to test artifacts without telling Windows to move the clock.

However, if you decide to go back and restart the same virtual machine without defining custom base time (also without Internet access and without changing the date and time settings in the running operating system), the guest operating system won’t necessary use the current date and time (as set on a host). In some cases, it will continue to run using the previously defined (future) date.

How is that possible?

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Exporting registry hives from a live system

When triaging a live system or performing live forensic acquisition, we often need to copy registry hives from a disk. Currently, there are five common ways to do this:

  1. execute the “reg save <hive> <file>” command;
  2. call the RegSaveKeyEx/RegSaveKey routine from an acquisition tool;
  3. copy a hive file from an existing shadow copy;
  4. copy a hive file from a newly created shadow copy;
  5. directly read a hive file from an NTFS volume.

Are there any pros and cons of each way?

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Containerized registry hives in Windows

If you read my Windows registry file format specification, you might already know about layered keys. Today, let’s talk about them in more detail.

Some editions of Windows 10 are capable of running Windows containers using Docker. Each Docker container is based on an immutable image with all modified data stored in an overlay. When a Windows container is used, the system has to record modifications affecting both the file system and the registry.

In 2016, Microsoft implemented new functionality called layered keys to allow programs access a merged view of keys and values from two or more registry hives! Now, this functionality is utilized by Docker…

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Offline shadow copies

This was already described here, but let’s revisit the topic.

Let’s install the Windows Server 2016 operating system on a machine, install all available updates, configure the machine as a domain controller and an RDP server, create several domain user accounts. Then, create a shadow copy and delete it. After some time, create a new shadow copy and keep the machine running for a while, then create another shadow copy. How many shadow copies are there? Two (the oldest one was deleted, thus not counted).

Let’s simulate a remote attack against this domain controller. The attack involves dumping the ntds.dit file. In order to copy that file, I will use an approach outlined in this guide: create a shadow copy, copy the ntds.dit file from it, then delete this shadow copy to remove my tracks (all these actions are performed over an RDP connection, just like a real attack).

Finally, let the system run for some time and occasionally create two more shadow copies. How many shadow copies are there now?

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Storage Reserve blocks some tools from thoroughly wiping unallocated space

Storage Reserve is a relatively new feature that keeps some disk space in a system volume available for downloading and installing Windows updates. Its implementation is simple – the current amount of free space visible to applications is decreased, so the “no space left” condition occurs before the space is really exhausted.

Take a look at these screenshots:

81-f
Windows 8.1

20h1-f
Windows 10

Both of them illustrate the same drive. On the first screenshot, this drive is attached to a Windows 8.1 installation. On the second one, the same drive (actually, exactly the same image of a virtual drive) is attached to a Windows 10 “20H1” installation. And the amount of free space reported by these operating systems is different!

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Extracting unallocated clusters from a shadow copy

Yes, shadow copies may contain a relatively small number of unallocated clusters. In this post, I will describe a new way to extract such clusters for further analysis.

The problem with unallocated clusters in shadow copies is that the volsnap driver doesn’t care about them. This driver can snapshot some unallocated ranges, but most of them are out-of-scope.

When reading unallocated clusters from a shadow copy, data from the current state of a volume can be returned. Obviously, this data has nothing to do with the shadow copy:

However unexpectedly when I ran the Encase Recover Folders feature across the HarddiskShadowcopy5 volume it found traces of the Sony folder and in fact many other files post dating the creation of the shadow copy.
<…>
The Encase Recover Folders feature parses unallocated clusters looking for folder metadata. It seems that it found data in unallocated clusters relating to the current volume. Therefore I believe that any deleted but recoverable data within the shadow copies needs to be treated with caution.

Null bytes instead of real data can be returned as well.

There is no way to distinguish between “real” and “fake” unallocated data when reading a shadow copy using the device exposed by the volsnap driver (“HarddiskVolumeShadowCopy<N>“).

Continue reading “Extracting unallocated clusters from a shadow copy”

Trim and unallocated space

Have you ever heard that solid-state drives destroy evidence? Let’s revisit the facts before going further.

When first solid-state drives appeared, there was no Trim command. There was no easy way for a drive to reclaim unused blocks of user data (i.e., data exposed to a host as drive contents) for the wear-leveling process.

To mitigate this problem, manufacturers did a clever trick: they began producing file-system-aware solid state drives!

Some articles deny the existence of such a trick, but the truth is that some ancient solid-state drives were capable of parsing a partition table and an NTFS file system to locate unallocated (free) clusters and reclaim their blocks for the wear-leveling process (thus, wiping remnant data in these clusters).

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OneDrive and NTFS last access timestamps

You might already know that the NTFS “Last Access” updates will be back by default in Windows 10 “20H1”. Previously, there were back for installations with small system volumes only. What is the reason behind this? Why do we need last access timestamps?

If you visit the “Configure Storage Sense or run it now” page in the “Settings” window of Windows 10 “19H2”, you may notice the “Delete files in my Downloads folder if they have been there for over” option. The same option in “20H1” reads: “Delete files in my Downloads folder if they haven’t been opened for more than“.

So, this old new NTFS feature has something to do with Storage Sense. It’s a component used to delete unneeded files “to keep your storage optimized”. And the “Last Access” updates are a good way to detect such unneeded files (and the “StorageUsage.dll” library actually uses last access timestamps to find “cold” files).

But there is something you might not notice. Look at the same settings page in Windows 10 “19H2” and read:

"OneDrive
Content will become online-only if not opened for more than"

Wait a minute! The “Last Access” updates are on for a relatively small subset of Windows 10 “19H2” installations only… Does this option really work for systems with large system volumes?

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Deceptive NTFS short file names

Are you aware of DLL hijacking? If yes, let’s suppose there is a program that executes the following line of code:

LoadLibrary('riched32.dll');

Its executable has the following name: “i_use_riched32.exe” (just as an example).

Now, take a look at the following contents of a directory containing this executable, the screenshots were taken of three tools: Explorer, FTK Imager Lite, The Sleuth Kit (each one points to the same directory).

svl-explorer
Explorer

svl-ftki
FTK Imager Lite

svl-tsk
The Sleuth Kit

Is the “riched32.dll” library hijacked for the “i_use_riched32.exe” executable? Let’s assume that no attempts to hijack the library have been made outside of the directory shown above.

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Prepopulated artifacts

Have you ever encountered on-disk artifacts originating from another system?

Typically, this is something you see when a custom operating system image had been deployed to multiple computers by IT staff (on-disk artifacts appeared before the image is captured become a part of that image).

But there are some minor artifacts existing in installation images coming from Microsoft!

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