Dead disk Forensics

Mike Cohen 2022-03-20

This article discusses a feature available since 0.6.4 release. This feature is still considered experimental and we are seeking feedback and wider testing.

Velociraptor’s killer feature is its VQL language making it possible to write powerful queries that triage and extract valuable forensic evidence from the running system. One of the most attractive features is the ability to write VQL artifacts encapsulating powerful VQL queries. Users have access to a library of packaged Artifacts that come with Velociraptor as well as a vibrant community and an Artifact Exchange.

Previously Velociraptor was most useful as a live analysis platform. Either deployed as an agent on the live endpoint, or via the Offline Collector collecting artifacts from the running system. However, many users are sometimes faced with analyzing a dead disk image - for example, when handed a clone of a cloud VM disk after a compromise.

It would be really nice to be able to leverage the same VQL artifacts developed and shared by the community on a disk image or VM clone without having to start the VM and install Velociraptor on it.

Dead disk analysis

When we want to analyze a disk image we mean that:

  1. A VQL query that looks at a disk (e.g. via the glob() plugin), should look inside the disk image instead of the real disk of the analysis machine.
  2. A VQL query that looks at system state (e.g. process listing) should fail - otherwise we will accidentally mix results from the analysis machine and the machine the image came from.

Consider the following scenario: I have a dead disk image (In a vmdk format) of a server on my analysis machine, and I want to run Velociraptor to triage this image.

The following query retrieves all event logs from a windows system:

SELECT OSPath
FROM glob(globs="C:/Windows/System32/WinEVT/Logs/*.evtx")

When I run this query, I want the results to come from the image and not from my analysis machine!

Of course I can always mount my dead image on a different drive (if my analysis machine is Windows) or a different directory (if my analysis machine is Linux). Then I can change the query accordingly to search for the event log files in the new location. But this is tedious and error prone - I have to carefully change all artifacts to point to the new drive, and if there are references in the dead image to a C: drive the artifact will look for files in the C: drive again.

What I really want is to remap the C: drive to the dead image - so whenever Velociraptor attempts to access a path beginning with C: drive, the data will come from the image! This way I can use all the artifacts as they are without modification, thereby leveraging all my existing favorite artifacts.

Remapping accessors

Velociraptor accesses files using filesystem accessors. You can think of an accessor as simply a driver that provides access to a file or directory.

There are a number of types of accessors available, in the following discussion the following accessors are important:

  • The auto accessor is the default accessor used when an accessor is not explicitly specified. The query SELECT * FROM glob(globs='/*') will use the auto accessor since an explicit accessor parameter is not provided.

    On Windows the auto accessor attempts to open files using the OS API and failing this, reverts to NTFS parsing (for locked files). This is the most commonly used accessor.

  • The file accessor uses the operating system APIs to open files and directories. It is used internally by the auto accessor but you can also use it explicitly.

  • The ntfs accessor is used to access files using the built in NTFS parser.

Mounting the image

The first step is to mount my dead disk image on my system so it can be accessed by Velociraptor. Since this is a vmdk image, I can use vmware-mount to mount a “flat” image easily:

$ sudo vmware-mount -f /vmware/TestVM/Windows\ 10\ x64.vmdk /mnt
$ ls -l /mnt/
total 62914560
-rw------- 1 mic mic 64424509440 Jan  5 16:42 flat

The entire disk image is now available as a classic dd style image within the /mnt/flat file.

Remounting configuration

Velociraptor normally interrogates the live machine it is running on. However in this case we want to emulate the system under investigation so that when Velociraptor attempts to access the system it is really parsing the dead disk image. This process of emulation is called remapping and it is controlled via remapping rules in the configuration file.

Although I can write these rules by hand, Velociraptor offers a quick tool that automates a lot of the remapping rule generation. Simply point velociraptor at the mounted flat image using the --add_windows_disk flag, and it will produce a new remapping yaml config:

$ velociraptor-v0.6.4-linux-amd64 -v deaddisk --add_windows_disk /mnt/flat /tmp/remapping.yaml
velociraptor: Enumerating partitions using Windows.Forensics.PartitionTable
velociraptor: Searching for a Windows directory at the top level
velociraptor: Adding windows partition at offset 122683392
velociraptor: Searching for a Windows directory at the top level

Velociraptor will enumerate the partitions in the disk image and attempt to mount each as an NTFS partition. It will then look for a /Windows directory at the top level to indicate a system drive and map it to the C: drive.

You can see the full generated configuration file here but in the next few sections we will examine some remapping rules in detail.

The “mount” remapping rules

Let’s take a closer look at the following rule of type mount

- type: mount
  description: 'Mount the partition /mnt/flat (offset 122683392) on the C:
    drive (NTFS)'
  from:
    accessor: raw_ntfs
    prefix: |
      {
        "DelegateAccessor": "offset",
        "Delegate": {
          "DelegateAccessor": "file",
          "DelegatePath": "/mnt/flat",
          "Path":"122683392"
        },
        "Path": "/"
      }
  "on":
    accessor: ntfs
    prefix: '\\.\C:'
    path_type: ntfs

A mount rule tells Velociraptor to map all paths below a certain directory to a delegate accessor. When a VQL query attempts to open a file using the ntfs accessor, below the \\.\C: directory, Velociraptor will automatically map the request to raw_ntfs accessor with the above prefix.

For example, consider the request to list the \\.\C:\Windows directory. Since this directory is below the mount point of \\.\C:, Velociraptor will append the remainder (the Windows directory) to the mount point’s from prefix and use the raw_ntfs accessor to list the result.

So the following pathspec will be opened instead:

      {
        "DelegateAccessor": "offset",
        "Delegate": {
          "DelegateAccessor": "file",
          "DelegatePath": "/mnt/flat",
          "Path":"122683392"
        },
        "Path": "/Windows"
      }

The prefix is an OSPath object in the form of a complete pathspec object describing how the raw_ntfs accessor is to access files:

  1. The raw_ntfs accessor will first open it’s delegate container and then parse out the /Windows path within it.
  2. The delegate is the offset accessor - an accessor that maps an offset from it’s own delegate (in order to extract the partition on which the filesystem is written).
  3. The offset accessor in turn uses the file accessor to open the /mnt/flat image file. In this case the offset is 122683392 bytes into the image.

This remapping happens transparently - whenever Velociraptor accesses the ntfs accessor the data will be automatically taken from the remapped mount point.

Let’s see how this works in practice. I will start the GUI using:

$ velociraptor-v0.6.4-linux-amd64 --config_override /tmp/remapping.yaml gui -v

This simply starts the Velociraptor server and a single client talking to it. However, due to the --config_override flag, the remapping configuration will be applied to both client and server configurations.

Now when I interact with the client’s VFS view due to the remapping the result comes from the vmdk image.

Browsing the VFS with the remapped ntfs accessor
Browsing the VFS with the remapped ntfs accessor

Remapping the registry hives

The above default remapping rules also include the following rule

- type: mount
  description: Map the /Windows/System32/Config/SOFTWARE Registry hive on HKEY_LOCAL_MACHINE\Software
  from:
    accessor: raw_reg
    prefix: |-
      {
        "Path": "/",
        "DelegateAccessor": "raw_ntfs",
        "Delegate": {
          "DelegateAccessor":"offset",
          "Delegate": {
            "DelegateAccessor": "file",
            "DelegatePath": "/mnt/flat",
            "Path": "122683392"
          },
          "Path":"/Windows/System32/Config/SOFTWARE"
        }
      }
    path_type: registry
  "on":
    accessor: registry
    prefix: HKEY_LOCAL_MACHINE\Software
    path_type: registry

This rule mounts the registry accessor’s HKEY_LOCAL_MACHINE\Software path on the /Windows/System32/Config/SOFTWARE file found within the raw NTFS partition. Note how pathspec descriptors nest and can utilize multiple different accessors to achieve the final mount point (in this case, the file accessor, followed by offset followed by raw_ntfs followed by raw_registry)..

Browsing the VFS with registry accessors
Browsing the VFS with registry accessors

Normally, when interacting with a live Velociraptor client, the registry accessor refers to registry keys and values accessed through the OS API. However now we were able to mount a raw registry parser on top of the registry accessor.

By remapping the traditional accessors with emulated content, we are effectively allowing the same VQL queries to apply to very different scenarios without change. For example, an artifact that queries the registry using the API will now automatically query the raw registry parser which accesses the hive file as recovered from parsing the ntfs filesystem on a dead disk image.

We can apply the same artifacts to the dead disk image without any modification!

Remapping CurrentControlSet

In Windows there are virtual parts of the registry that get remounted at runtime. One such part is the HKEY_LOCAL_MACHINE\Software\CurrentControlSet key which is mounted from HKEY_LOCAL_MACHINE\Software\ControlSet001. Velociraptor can recreate this mapping using the following remapping rule:

- type: mount
  description: Map the /Windows/System32/Config/SYSTEM Registry hive on HKEY_LOCAL_MACHINE\System\CurrentControlSet
    (Prefixed at /ControlSet001)
  from:
    accessor: raw_reg
    prefix: |-
      {
        "Path": "/ControlSet001",
        "DelegateAccessor": "raw_ntfs",
        "Delegate": {
          "DelegateAccessor":"offset",
          "Delegate": {
            "DelegateAccessor": "file",
            "DelegatePath": "/mnt/flat",
            "Path": "122683392"
          },
          "Path":"/Windows/System32/Config/SYSTEM"
        }
      }
    path_type: registry
  "on":
    accessor: registry
    prefix: HKEY_LOCAL_MACHINE\System\CurrentControlSet
    path_type: registry

This is very important for queries that read sub-keys of CurrentControlSet.

Impersonating an operating system

We discussed how accessors can be remapped using the remapping rules in order to make VQL plugins that access files emulate running on the target system. However, many artifacts need to examine more than just the filesystem. For example, most artifacts have a precondition such as SELECT * FROM info() WHERE OS =~ "Windows". If we were to run on a Linux system these artifacts will not work since they are intended to work on windows - despite the remapping rules emulating a Windows system.

We therefore need to impersonate a windows system - even when we are really running on a Linux machine. The impersonation rule looks like:

- type: impersonation
  os: windows
  hostname: VirtualHostname
  env:
  - key: SystemRoot
    value: C:\Windows
  - key: WinDir
    value: C:\Windows
  disabled_functions:
  - amsi
  - lookupSID
  - token
  disabled_plugins:
  - users
  - certificates
  - handles
  - pslist

This rule has a number of functions

  1. The OS type is set to Windows- This affects the output from SELECT * FROM info() - this query controls most of the artifact preconditions.

  2. A specific hostname is set to “VirtualHostname”. When the client interrogates, this hostname will appear in the Velociraptor GUI.

  3. We specify a number of environment variables. This affects the expand() function which expands paths using environment variables. Many artifacts use environment variables to locate files within the filesystem.

  4. Disabled functions and plugins: Many artifacts use plugins and functions that query non-disk system state in order to enrich the collected data. I.e. their output does not depend just on the disk. Using this impersonation rule we can disable those plugins and functions (essentially return nothing from them) so the query can complete successfully.

Impersonation aims to make it appear that the VQL artifacts are being collected from the target system as if it were running live.

Here is an example of collecting some common Windows artifacts from my flat image above - running on a Linux analysis machine. We can see some of our favorite artifacts, such as Windows.Forensics.Usn, Windows.Timeline.Prefetch, Windows.Forensics.Bam and many more.

Collecting some common artifacts
Collecting some common artifacts

Analysis of non windows disk images

In the previous example, we exported the vmdk image as a flat file and simply relied on Velociraptor to parse the filesystem using its inbuilt NTFS parser.

For other operating systems, Velociraptor does not currently have a native parser (for example for Linux or MacOS). Instead, Velociraptor relies on another tool mounting the image filesystem as a directory.

We can still perform the analysis as before however, by remapping the mounted directory instead of a raw image.

To demonstrate this process I will mount the flat image using the Linux loopback driver and the built in Linux NTFS filesystem support.

$ sudo mount -o loop,offset=122683392 /mnt/flat /tmp/mnt/

I can now generate a second remapping configuration:

$ ./velociraptor-v0.6.4-linux-amd64 deaddisk --add_windows_directory /tmp/mnt/ /tmp/remapping2.yaml
velociraptor: Adding windows mounted directory at /tmp/mnt/
velociraptor: Checking for hive at /tmp/mnt/Windows/System32/Config/SOFTWARE
velociraptor: Checking for hive at /tmp/mnt/Windows/System32/Config/SYSTEM
velociraptor: Checking for hive at /tmp/mnt/Windows/System32/Config/SYSTEM

Velociraptor will inspect the directory and determine it is a Windows image, then attempt to map the raw registry hives at the correct place as before. The C: drive remapping rule is:

- type: mount
  description: 'Mount the directory /tmp/mnt/ on the C: drive (NTFS)'
  from:
    accessor: file
    prefix: /tmp/mnt/
  "on":
    accessor: file
    prefix: 'C:'
    path_type: windows

Conclusions

Velociraptor is an extremely capable triage and analysis tool which works best when running live on the endpoint - where it can correlate information from disk, memory and volatile system state. Velociraptor has a vibrant community with powerful user contributed artifacts designed for use in this context.

However, sometimes we do not have the luxury of running directly on the running endpoint, but have to rely instead on dead disk images of the target system. The latest Velociraptor release makes it possible to impersonate a live system based on information from the dead disk. While this is not perfect (because a lot of the enrichment information obtained from the live system is missing) for basic disk focused forensic analysis, we are able to use most artifacts directly without change.

This feature opens Velociraptor to more traditional image based forensic analysis use cases - these users are now able to leverage the same artifacts we all use in live triage to quickly triage dead disk images.

Since this is such a new feature it is still considered experimental - we value your feedback, bug reports and discussions. If you would like to try out these features in Velociraptor, It is available on GitHub under an open source license. As always, please file issues on the bug tracker or ask questions on our mailing list velociraptor-discuss@googlegroups.com. You can also chat with us directly on discord at https://www.velocidex.com/discord.

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