Shadow copies become less visible

<offtopic>You may find this article interesting: Measured Boot and Malware Signatures: exploring two vulnerabilities found in the Windows loader. It’s about two registry-related vulnerabilities found in the Windows loader.</offtopic>

Many examiners use tools like Arsenal Image Mounter to access shadow copies on disk images. I don’t recommend this method, because you won’t see offline shadow copies, but many of us still rely on it.

And it seems there will be more caveats…

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$STANDARD_INFORMATION vs. $FILE_NAME

There are two common misconceptions about NTFS:

  1. A typical file has 8 timestamps.
  2. Windows Explorer displays $STANDARD_INFORMATION timestamps.

A file with a single name has 12 timestamps: 4 timestamps come from the $STANDARD_INFORMATION attribute in a file record, 4 timestamps come from the $FILE_NAME attribute in the same file record, and 4 timestamps come from the $FILE_NAME attribute in an index record ($I30) of a parent directory.

If there is a short file name together with a long one, the number of timestamps is 20 (8 more timestamps come from two additional $FILE_NAME attributes in a file record and in an index record of a parent directory respectively).

You can also add an UUIDv1 timestamp from the $OBJECT_ID attribute, timestamps recorded in the USN journal and in the $LogFile journal. But these aren’t always present.

Things are more complicated with timestamps displayed by Windows Explorer.

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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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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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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>“).

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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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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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BAM internals

This is a reply to the Sunday Funday 4/5/20 challenge. The goal of this post is to document the process, not just the results. You have been warned.

The Background Activity Moderator (BAM) is a Windows 10 thing that does… something! Because we don’t know much about it.

We know that this thing provides evidence of execution by listing executables under the following registry key:

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\bam\State\UserSettings\<User SID>

Each piece of evidence is stored as a registry value (REG_BINARY), its name is set to an executable path and its data is set to a binary structure with a FILETIME timestamp inside (this is believed to be the last execution timestamp).

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The “\$Extend\$Deleted” directory

In the Linux world, a deleted file which is still open isn’t actually removed from a disk. Instead, it’s just unlinked from the directory structure. This is why a system call used to remove files is named “unlink”.

unlink() deletes a name from the filesystem. If that name was the last link to a file and no processes have the file open, the file is deleted and the space it was using is made available for reuse.

If the name was the last link to a file but any processes still have the file open, the file will remain in existence until the last file descriptor referring to it is closed.

(Source.)

The same behavior can be observed in other Unix-like operating systems.

But in Windows 10, similar behavior can be seen too!

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