NTFS Analysis

NTFS is the standard Windows filesystem. Velociraptor contains powerful NTFS analysis capabilities. This section describes Velociraptor’s NTFS capabilities and does not aim to be a complete description of NTFS itself. We will only introduce the basic and most relevant concepts of NTFS and examine how these can be used in a number of DFIR contexts.

The Master File Table

In NTFS, all files are represented in a Master File Table (MFT). The MFT is also a file within the filesystem with the special filename of $MFT. While this special file is normally hidden by the API, Velociraptor’s NTFS parser makes it available to view, read or upload.

The $MFT file contains a sequence of MFT Entries, each of a fixed size (usually 512 bytes). These entries contain metadata about files, called File Attributes. The different attributes contain different kinds of information about each file:

  • Filename (Long name/Short name)
  • Data attribute – contains file data runs.
  • I30 attribute (contains directory listing)
  • Security attributes such as ACLs

The MFT and NTFS
The MFT and NTFS

Data attributes may be compressed or sparse and contain a list of runs that comprize the content of the file. The data content is stored elsewhere on the disk, but the location is stored within the MFT entry.

In NTFS Each file may contain two different filenames, a long and a short filename. Filename attributes contain their own timestamps.

Although NTFS long and short filenames are usually closely related (e.g. the short filename is the first part of the long filename with a suffix such as %1), this is not a requirement.

It is very easy to create a file with a completely different short filename to its long filename. This can be problematic if you are looking for references to the long filename from e.g. registry keys.

In the below example, I set the shortname of the velociraptor.exe binary to runme.exe. I can then create a service that launches runme.exe instead. Tools that only show the long filename of the directory will fail to show the file and analysis may conclude that the service target is missing from the filesystem.

C:\Users\test>fsutil file setshortname velociraptor.exe runme.exe
C:\Users\test>dir /x *.exe
 Volume in drive C has no label.
 Volume Serial Number is 9459-F443

 Directory of C:\Users\test

08/19/2018  11:37 PM        12,521,472 RUNME.EXE    velociraptor.exe
               2 File(s)     16,140,732 bytes
               0 Dir(s)  11,783,704,576 bytes free
C:\Users\test>runme.exe -h
usage: velociraptor [<flags>] <command> [<args> ...]

The ntfs accessor

Velociraptor has a complete NTFS parser able to access files and directories by parsing the raw NTFS filesystem from the raw device. To make it easy to utilize this parser with VQL, Velociraptor implements the ntfs accessor (For a description of accessors, see here ).

The ntfs accessor makes it possible to see and access the normally hidden NTFS files such as $MFT. It also makes it possible to see Alternate Data Streams (ADS), which are additional data streams attached to the same MFT entry.

NTFS accessor
NTFS accessor

The NTFS accessor makes NTFS specific information available in the Data field. For regular files it includes the inode string, as well as the short filename.

When providing a path to the ntfs accessor, the first part of the path is interpreted as the drive letter or the device part.

For example providing a path starting with C: or D:, will be converted internally to Windows device notation, for example \\.\C: or \\.\D:. The ntfs accessor then uses this to open the raw logical device so it can be parsed.

This means that all paths returned from the ntfs accessor start with the device name, e.g. \\.\C:.

Since Velociraptor operated on the logical device it if not affected by full disk encryption such as Bitlocker. Velociraptor will be able to parse the raw NTFS filesystem regardless of the disk encryption status.

Volume Shadow Copies

NTFS allows for a special copy on write snapshot feature called Volume Shadow Copy or VSS. You can think of a VSS as a light weight snapshot of the current filesystem without needing to copy any data (future writes will simply be diverted to the current active snapshot).

On server class Windows systems, you can create a VSS copy on your own machine using vssadmin create shadow, but on other Windows versions you will need to do this via WMI:

Creating shadow copy
Creating shadow copy

When a VSS copy is created, it is accessible via a special device. Velociraptor allows the VSS copies to be enumerated by listing them at the top level of the filesystem. At the top level, the accessor provides metadata about each device in the Data column, including its creation time. This is essentially the same output as vssadmin list shadows. In the below screenshot we can see the Data column of the fixed C: drive and the VSS device.

VSS info
VSS info

Operating on VSS

Because the ntfs accessor treats all devices at the first top level directory, it is possible to see the same file in all VSS copies at the same time. For example, the following finds all VSS copies of the event logs:

VSS globbing
VSS globbing

Simply use the VSS device name as a prefix to all paths and the ntfs accessor will parse it instead.

You can use it to analyze older versions of the drive!

Parsing the MFT

Since the ntfs accessor allows accessing the $MFT file as a regular file, you can download the entire $MFT file from the endpoint using the ntfs accessor, then process it offline. For example using the Windows.Search.FileFinder artifact with the ntfs accessor - or simply using the VQL:

SELECT upload(path="C:/$MFT", accessor="ntfs")
FROM scope()

However, in practice this is inefficient and does not scale. Typically we want to parse the MFT in order to answer some questions about the system, such as which files were modified within a timerange.

Velociraptor provides access to the $MFT parser using the parse_mft() plugin, so the MFT can be parsed directly on the endpoint using Velociraptor. The plugin emits a high level summary of each MFT entry, including its timestamps (for the $STANDARD_INFORMATION and $FILENAME streams) and MFT ID.

This plugin is most useful when you need to pass over all the files in the disk - it is more efficient than a recursive glob and might recover deleted files. For example to recover all the files with a .exe extension from the drive:

SELECT * FROM parse_mft(filename="C:/$MFT", accessor="ntfs")
WHERE FileName =~ ".exe$"

MFT Entries

An MFT Entry can have multiple attributes and streams. While parse_mft() plugin emits a high level summary for each entry, sometimes we need more information on each MFT entry. This information is provided by the parse_ntfs() VQL function which accepts and MFT ID:

Parse ntfs
Parse ntfs

The MFT ID can be take from the output of glob() or parse_mft().

In the above you will sometimes see the term inode referred to. This term traditionally comes from the Sleuthkit and is a string consisting of a triple of mft id, type id and stream id, e.g. 974-16-0 representing a stream of data

NTFS timestamps

A single MFT entry can have up to 16 timestamps, based on different attributes:

  • The $STANDARD_INFORMATION attribute contains 4 timestamps (Modified, Accessed, Inode Changed, Born)
  • There are often 2 $FILENAME attributes for a short name and a long name, each will have 4 further timestamps.
  • The $I30 stream of the parent directory also contains 4 timestamps for the file.

Timestamps are critical to forensic investigations as they help to establish a timeline of activity on the system.

Timestomping

Attackers sometimes change the timestamps of files to make them less obvious. E.g make malware look like it was installed many years ago. This makes timelines more difficult to establish and might cause you to miss important filesystem events.

For the next exercise we will stomp over some times. Use the following powershell to stomp over Velociraptor.exe’s timestamps.

$file = 'C:\Program Files\Velociraptor\Velociraptor.exe'
$stomp = Get-Date 2007-07-07
$(Get-Item $file).creationtime = $stomp
$(Get-Item $file).lastaccesstime = $stomp
$(Get-Item $file).lastwritetime = $stomp
Get-ChildItem $file | Select *, Fullname, *Time*

Before
Before
After
After

The above script uses the API to change the times of a file but this only changes the $STANDARD_INFORMATION stream. The real times are still present on the $FILENAME attributes. A common detection to this is to find files which have $STANDARD_INFORMATION times earlier then the $FILENAME times. When the file is created, $FILENAME times are set to the real times, then if the API is used to send the timestamps backwards the $STANDARD_INFORMATION timestamps will appear earlier than the $FILENAME times.

Timestomp detection
Timestomp detection

Although it might appear to be a solid detection to timestomping, generally timestomping detections are not very reliable in practice. It turns out that a lot of programs set file timestamps after creating them into the past by design - mostly archiving utilities like 7zip or cab will reset the file time to the times stored in the archive.

Conversely it might appear that the $FILENAME times are the most reliable and should be mostly relied upon in an investigation since they are not directly modifiable by the Win32 APIs.

Unfortunately this is not the case - the $FILENAME attributes can be easily modified by simply renaming the file (after timestomping) and rename it back. Windows will copy the timestamps from the $STANDARD_INFORMATION attribute to the $FILENAME when renaming the file.

Timeline analysis

Timelines in forensic analysis are very important as they place events in chronological order and may reveal causal relationships. We can get a timeline by sorting the table on the modified or birth timestamps.

SELECT * FROM parse_mft(filename="C:/$MFT", accessor="ntfs")
WHERE Created0x30 > "2020-01-02"
ORDER BY Created0x30

It is more efficient to narrow the time of interest first.

The $I30 INDX stream

In NTFS a directory is simply an MFT entry with $I30 streams. The streams contains a B+ tree of the MFT entries in the directory.

Since INDX streams are a B+ tree when a record is deleted, the tree will be reordered. Sometimes this leaves old entries in the slack space. INDX stream is allocated in 4096 byte blocks which leaves a lot of slack space to potentially hold residual data.

I30 slack
I30 slack

Velociraptor can report on the $I30 streams and carve out headers from slack using the parse_ntfs_i30() function as duscussed in this article. An example query:

SELECT * FROM foreach(
   row={
     SELECT FullPath, Data.mft AS MFT
     FROM glob(globs=DirectoryGlobs, accessor="ntfs")
     WHERE IsDir
   },
   query={
     SELECT FullPath, Name, NameType, Size, AllocatedSize,
            IsSlack, SlackOffset, Mtime, Atime, Ctime, Btime, MFTId
     FROM parse_ntfs_i30(device=FullPath, inode=MFT)
})

The USN journal

Update Sequence Number Journal or Change journal is maintained by NTFS to record filesystem changes. Primarily designed to support backup programs, the USN journal records metadata about filesystem changes.

The journal resides in the path $Extend\$UsnJrnl:$J and is normally a hidden NTFS internal file (so it can only be accessed via the ntfs accessor).

Windows appends USN records to the end of the file. However, the file is sparse - periodically NTFS will remove the range at the start of the file to make it sparse and preserve disk space.

Typically the file will report a huge size but will actually only take about 30-40mb on disk since the first part of the file is sparse.

The USN journal
The USN journal

When collecting the journal file, Velociraptor will collect the sparse file only (Velociraptor is aware of sparse files and preserves their sparse ranges by adding an additional .idx file to the collection with the ranges containing real data. You can see this in the Uploaded Files tab of the collection - the file size is reported to be very large, however only about 30mb was actually collected.

The USN journal collected
The USN journal collected

Downloading the file from the Uploaded Files tab will pad the sparse regions and produce a large file with ranges of 0 in it. On the other hand, exporting the zip file from the Overview tab will store the collected file and the idx range file into the zip file so will only store about 30mb.

Parsing USN journal

Velociraptor can parse each entry in the USN journal directly on the endpoint. This allows for queries to target specific files or times of interest on the endpoint.

Since the beginning of the file is sparse, we start parsing from the first valid range.

The USN journal may record interactions with files that have been removed. Many files represent evidence of system interaction (such as lnk files or prefetch files) and the USN journal can therefore uncover the “smoking gun” when the system was initially compromised.

The USN journal
The USN journal

You can collect the USN journal using the Windows.Forensics.Usn artifact.

The USN journal contains so much valuable evidence that it might be worth carving for USN records from the raw disk. Although this is a slow process it can yield very good results if your are lucky - see this blog post for more information.