There are more than a few skills that make up the #DFIR "field", and just one of them is conducting DFIR analysis. Brett Shavers recently shared his thoughts on how to improve in this area, specifically by studying someone else's case work. In his article, Brett lists a number of different avenues for reviewing work conducted by others a means of improving your own skills.
Brett and I are both Marine veterans, and Marines have a long history of looking to the experience of others to expand, extend, and improve our own capabilities. In the case of war fighting, a great deal has been written, providing a wealth of information to dig into and study. Jim Mattis stated in his book, "Call Sign Chaos", that "...your personal experiences alone are not broad enough to sustain you." This is true not only for a Marine general, but also for a DFIR analyst. In fact, I would say even more so for an analyst.
Okay, so how do we apply this? One way is to follow Brett's advice, and seek out resources. There are numerous web sites available, and another resource is David Cowen's book, Computer Forensics InfoSec Pro Guide.
Another available resource is Investigating Windows Systems. What makes this book different from others that you might find is that when writing it, my goal was to demonstrate stitching together the analysis process, by explaining why certain decisions were made, and the data and thought processes led to various findings. Rather than simply presenting a finding, I wanted to illustrate the data that was laid out before me when I made each of the analysis decisions. As with all of my other books, I wrote IWS in large part due to the fact that I couldn't find any book (or other resource) that took this approach.
Another approach is participating in CTFs. However, if you don't feel confident in participating in the actual CTF itself, but still want to take a shot at the analysis and see how others went about answering the challenge questions, there are often options available. In 2018, DefCon had a forensic analysis CTF, and a bit after the conference, several (I found 3) folks posted their take on the challenges.
My "thing" with CTFs is that they very often aren't 'real world'. For example, in all of my time as an incident responder, I've never had someone ask me to identify a disk signature or volume serial number from an acquired image. Can I? Sure. But it's never been part of the analysis process, in providing services to a customer. As such, I posted something of my own take on a few of the questions (here, and here), so they're available for anyone to read, and because the images are available, anyone can walk through what I or the other folks did, following along using their own tools and their own analysis processes.
If you do decide to engage in developing your skills, one of the best ways to do so is when you have someone to help you get over the humps. I'll admit it...sometimes, I take time to research something and may come up with a solution that isn't at all elegant, and could perhaps be done better. Or maybe, due to the fact that I'm relying on my own experiences, I don't see or consider something that to someone else, is obvious. Having a mentor, someone you can go to with questions and bounce ideas off of can be very beneficial in both the long and short term.
What is "best"?
A lot of times I'll see a question in DFIR-related social media, along the lines of, "what is the best tool to do X?" I've seen this a couple of times recently, perhaps the most recent being, "what is the best carving tool?" Nothing was started with respect to what was being carved (files, records, etc.), what the operating or file system in question was, etc. Just, "what is the best tool?"I was recently searching online for a tire inflator. I live on a farm, and have a couple of tractors, a truck, and a horse trailer. I don't need a fully-functional air compressor, but I do need something portable and manageable for inflating tires, something both my wife and I can use not only around the farm, but also when we're on the road. As I began looking around at product reviews, I also started seeing those "best of" lists, where someone (marketing firm, editorial board, etc.) compiled a list of what they determined to be the "best" available of a particular product.
Understand that I have a pretty good idea of what I'm looking for, particularly with respect to features. I'm looking for something that can plug into the cigarette lighter in the truck or car, or to another power source, such as "house power" or a portable generator. I'm looking for something that can fill a tire to at least 100 psi (some tires go to 12 psi, others 90 psi), but I'm not super-concerned about the speed; my primary focus is ease of use, and durability. Being able to set the desired pressure and have it auto-stop would be very useful, but it's not a show-stopper.
Some of the inflators listed as "best" had to be connected directly to the vehicle battery. Yeah, I know...right? Not particularly convenient if my wife needs to add pressure to a tire, particularly when plugging into the cigarette lighter is much more convenient. I mean, really...how "convenient" is it to pull over to the side of the road, and have someone who hasn't used jumper cables to jump-start another vehicle connect an inflator to battery terminals? Some inflators not listed as "best" were considered to be "too expensive" (although no threshold for cost was provided as a basis for testing), and looking a bit more closely, those inflators allowed the user to connect to different power sources. Okay, so that sort of makes sense, but rather than say that the product is "too expensive", maybe list why. Another was described as "too heavy", although it weighed just 5 lbs (as opposed to the "best", which came in at just over 3 lbs).
Bringing this back to #DFIR, I ran across this article the other day, which reportedly provides a list of the "top 10 digital forensics consulting/services companies". A list of the companies is provided on the page, with a brief description of what each company does, but what really stood out for me is that the list is compiled by "a distinguished panel of prominent marketing specialists". This, of course, begs the question as to the criteria used to determine which companies were reviewed, and of those, which made the top 10.
In 2012, I attended a conference presentation where the speaker made comments about various tools, including RegRipper. One comment was, "RegRipper doesn't detect...", and that wasn't necessarily true. RegRipper was released in 2008 with the intention of being a community-driven tool. However, only a few have stepped up over the years to write and contribute plugins. RegRipper is capable of detecting a great deal (null bytes in key/value names, RLO char, etc.), and if your installation of RegRipper "doesn't detect" something, it's likely that (a) you haven't written the plugin, or (b) you haven't asked someone for help writing the plugin.
During that same presentation, the statement was made that "RegRipper does not scale to the enterprise". This is true. It is also true that it was never designed to do so. The use case for which RegRipper was written is still in active use today.
My point is simply this..."best" is relative. If you're asking the question (i.e., "..what is the best #DFIR tool to do X?"), then understand that, if you don't share your requirements, what you're going to get back is what's best for the respondent, if anything. No one wants to write an encyclopedia of all of the different approaches, and available tools. Although, I'm sure someone will be happy to link you to one. ;-)
When you're considering the best "tool", take a look at the process, and maybe consider the best approach. Sometimes it's not about the tool. Also, consider the what it is you're trying to accomplish (your goals), as well as other considerations, such as operating or file system, etc. If you're not comfortable with the command line, or would perhaps like to consider a GUI solution (because doing so makes for a good screen capture in a report), or if you require the use of a commercial (vs FOSS...some do) tool, be sure to take those details into consideration, and if you're asking a question online, share them, as well.
Artifact Clusters
Very often within the DFIR community, we see information shared (usually via blog posts) regarding a "new" artifact that has been recently unearthed, or simply being addressed for the first time. In some cases, the artifact is new to us, and in others, it may be the result of some new feature added to the Windows operating system or to an application. Sometimes when we see this new artifact discussed, a tool is shared to parse and make sense of the data afforded by that artifact. In some cases, we may find that multiple tools are available for parsing the same artifact, which is great, because it shows interest and diversity in the approach to accessing and making use of the data.
However, what we don't often see is how that artifact relates to other artifacts from the same system. Another way to look at it is, we don't often see how the tool, or more importantly, the data available in the source, can serve us to further an investigation. We may be left thinking, "Great, there's this data source, and a tool that allows me to extract and make some sense of the data within it, but how do I use it as part of an investigation?"
I shared an initial example of what this might look like recently in a recent blog post, and this was also the approach I took when I wrote about the investigations in Investigating Windows Systems. I hadn't seen any books available that covered the topic of digital forensic analysis (as opposed to just parsing data) from an investigation-wide perspective, completing an investigation using multiple artifacts to "tell the story". The idea was (and still is) that a single artifact, or a single entry derived from a data source, does NOT tell the whole story of the investigation. A single artifact may be a high fidelity indicator that provides a starting point for an investigation, but it does not tell the whole story. Rather than a single artifact, analysts should be looking at artifact clusters to provide the necessary context for an analyst to make a finding as to what happened.
Artifact clusters provide two things to the investigator; validation and context. Artifact clusters provide validation by reinforcing that an event occurred, or that the user took some action. That validation may be a duplicate event; in my previous blog post, we see the following events in our timeline:
Wed Nov 20 20:09:28 2019 Z
TIMELINE - Start Time:Program Files x86\Microsoft Office\Office14\WINWORD.EXE
REG - [Program Execution] UserAssist - {7C5A40EF-A0FB-4BFC-874A-C0F2E0B9FA8E}\Microsoft Office\Office14\WINWORD.EXE (5)
What we see here (above) are duplicate events that provide validation. We see via the UserAssist data that the user launched WinWord.exe, and we also see validation from the user's Activity Timeline database. Not only do we have validation, but we can also see what the artifact cluster should look like, and as such, have an understanding of what to look for in the face of anti-forensics efforts, be they intentional or the result of natural evidence decay.
In other instances, validation may come in the form of supporting events. For example, again from my previous blog post, we see the following two events side-by-side in the timeline:
Wed Nov 20 22:50:02 2019 Z
REG - RegEdit LastKey value -> Computer\HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\TaskFlow
TIMELINE - End Time:Windows\regedit.exe
In this example, we see that the user closed the Registry Editor, and that the user's LastKey value was set, illustrating the key that was in focus when the Registry Editor was closed. Rather than being duplicate events, these two events support each other.
Looking at different event sources during the same time period also helps us see the context of the events, as we get a better view of the overall artifact cluster. For example, consider the above timeline entries that pertain to the Registry Editor. With just the data from the Registry, we can see when the Registry Editor was closed, and the key that was in focus when it was closed. But that's about all we know.
However, if we add some more of the events from the overall artifact cluster, we can see not just when, but how the Registry Editor was opened, as illustrated below:
Wed Nov 20 22:49:11 2019 Z
TIMELINE - Start Time:Windows\regedit.exe
Wed Nov 20 22:49:07 2019 Z
REG - [Program Execution] UserAssist - {F38BF404-1D43-42F2-9305-67DE0B28FC23}\regedit.exe (1)
REG - [Program Execution] UserAssist - {0139D44E-6AFE-49F2-8690-3DAFCAE6FFB8}\Administrative Tools\Registry Editor.lnk (1)
From a more complete artifact cluster, we can see that the user had the Registry Editor open for approximately 51 seconds. Information such as this can provide a great deal of context to an investigation.
Evidence Oxidation
Artifact clusters give us a view into the data in the face of anti-forensics, either as dedicated, intentional, targeted efforts to remove artifacts, or as natural evidence decay or oxidation.
Wait...what? "Evidence oxidation"? What is that? I shared some DFIR thoughts on Twitter not long ago on this topic, and in short, what this refers to is the natural disappearance of items from artifact clusters due to the passage of time, as some of those artifacts are removed or overwritten as the system continues to function. This is markedly different from the purposeful removal of artifacts, such as Registry keys being specifically and intentionally modified or deleted.
This idea of "evidence decay" or "evidence oxidation" begins with the Order of Volatility, which lists different artifacts based on their "lifetime"; that is to say that different artifacts age out or expire at different rates. For example, a process executed in memory will complete (often with seconds, or sooner) and the memory used by that process will be freed for use by another process in fairly short order. That process may result in the operating system or application generating an entry into a log file, which itself may roll over or be overwritten at various rates (i.e., the entry itself is overwritten as newer entries are added), depending upon the logging mechanism. Or, a file may be created within the file system that exists until someone...a person...purposefully deletes it. Even then, the contents of the file may exist (NTFS resident file, etc.) for a time after the file is marked as "not in use", something that may be dependent upon the file system in use, the level of activity on the system, whether a backup mechanism (backup, Volume Shadow Copy, etc.) occurred between the file creation and deletion times, etc.
In short, some artifacts may have the life span of a snowflake, or a fruit fly, or a tortoise. The life span of an artifact can depend upon a great deal; the operating system (and version) employed, the file system structure, the auditing infrastructure, the volume of usage of the system, etc. Consider this...I was once looking at a USN Change Journal from an image acquired from a Windows 7 system, and the time span of the available data was maybe a day and a half. Right around that time, a friend of mine contacted me about a Windows 2003 system he was examining, for which the USN Change Journal contained 90 days worth of data.
Windows systems can be very active, even when they appear to be sitting idle with no one actively typing at the keyboard. The operating system itself may reach out for updates, during which files are downloaded, processes are executed, and files are deleted. The same is true for a number of applications. Once a user becomes active on the system, the volume of activity and changes may increase dramatically. I use several applications (Notepad++, UltraEdit, VirtualBox, etc.) that all reach out to the Internet to look for updates when they're launched. Just surfing the web causes browser history to be generated and cached.
However, what we don't often see is how that artifact relates to other artifacts from the same system. Another way to look at it is, we don't often see how the tool, or more importantly, the data available in the source, can serve us to further an investigation. We may be left thinking, "Great, there's this data source, and a tool that allows me to extract and make some sense of the data within it, but how do I use it as part of an investigation?"
I shared an initial example of what this might look like recently in a recent blog post, and this was also the approach I took when I wrote about the investigations in Investigating Windows Systems. I hadn't seen any books available that covered the topic of digital forensic analysis (as opposed to just parsing data) from an investigation-wide perspective, completing an investigation using multiple artifacts to "tell the story". The idea was (and still is) that a single artifact, or a single entry derived from a data source, does NOT tell the whole story of the investigation. A single artifact may be a high fidelity indicator that provides a starting point for an investigation, but it does not tell the whole story. Rather than a single artifact, analysts should be looking at artifact clusters to provide the necessary context for an analyst to make a finding as to what happened.
Artifact clusters provide two things to the investigator; validation and context. Artifact clusters provide validation by reinforcing that an event occurred, or that the user took some action. That validation may be a duplicate event; in my previous blog post, we see the following events in our timeline:
Wed Nov 20 20:09:28 2019 Z
TIMELINE - Start Time:Program Files x86\Microsoft Office\Office14\WINWORD.EXE
REG - [Program Execution] UserAssist - {7C5A40EF-A0FB-4BFC-874A-C0F2E0B9FA8E}\Microsoft Office\Office14\WINWORD.EXE (5)
What we see here (above) are duplicate events that provide validation. We see via the UserAssist data that the user launched WinWord.exe, and we also see validation from the user's Activity Timeline database. Not only do we have validation, but we can also see what the artifact cluster should look like, and as such, have an understanding of what to look for in the face of anti-forensics efforts, be they intentional or the result of natural evidence decay.
In other instances, validation may come in the form of supporting events. For example, again from my previous blog post, we see the following two events side-by-side in the timeline:
Wed Nov 20 22:50:02 2019 Z
REG - RegEdit LastKey value -> Computer\HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\TaskFlow
TIMELINE - End Time:Windows\regedit.exe
In this example, we see that the user closed the Registry Editor, and that the user's LastKey value was set, illustrating the key that was in focus when the Registry Editor was closed. Rather than being duplicate events, these two events support each other.
Looking at different event sources during the same time period also helps us see the context of the events, as we get a better view of the overall artifact cluster. For example, consider the above timeline entries that pertain to the Registry Editor. With just the data from the Registry, we can see when the Registry Editor was closed, and the key that was in focus when it was closed. But that's about all we know.
However, if we add some more of the events from the overall artifact cluster, we can see not just when, but how the Registry Editor was opened, as illustrated below:
Wed Nov 20 22:49:11 2019 Z
TIMELINE - Start Time:Windows\regedit.exe
Wed Nov 20 22:49:07 2019 Z
REG - [Program Execution] UserAssist - {F38BF404-1D43-42F2-9305-67DE0B28FC23}\regedit.exe (1)
REG - [Program Execution] UserAssist - {0139D44E-6AFE-49F2-8690-3DAFCAE6FFB8}\Administrative Tools\Registry Editor.lnk (1)
From a more complete artifact cluster, we can see that the user had the Registry Editor open for approximately 51 seconds. Information such as this can provide a great deal of context to an investigation.
Evidence Oxidation
Artifact clusters give us a view into the data in the face of anti-forensics, either as dedicated, intentional, targeted efforts to remove artifacts, or as natural evidence decay or oxidation.
Wait...what? "Evidence oxidation"? What is that? I shared some DFIR thoughts on Twitter not long ago on this topic, and in short, what this refers to is the natural disappearance of items from artifact clusters due to the passage of time, as some of those artifacts are removed or overwritten as the system continues to function. This is markedly different from the purposeful removal of artifacts, such as Registry keys being specifically and intentionally modified or deleted.
This idea of "evidence decay" or "evidence oxidation" begins with the Order of Volatility, which lists different artifacts based on their "lifetime"; that is to say that different artifacts age out or expire at different rates. For example, a process executed in memory will complete (often with seconds, or sooner) and the memory used by that process will be freed for use by another process in fairly short order. That process may result in the operating system or application generating an entry into a log file, which itself may roll over or be overwritten at various rates (i.e., the entry itself is overwritten as newer entries are added), depending upon the logging mechanism. Or, a file may be created within the file system that exists until someone...a person...purposefully deletes it. Even then, the contents of the file may exist (NTFS resident file, etc.) for a time after the file is marked as "not in use", something that may be dependent upon the file system in use, the level of activity on the system, whether a backup mechanism (backup, Volume Shadow Copy, etc.) occurred between the file creation and deletion times, etc.
In short, some artifacts may have the life span of a snowflake, or a fruit fly, or a tortoise. The life span of an artifact can depend upon a great deal; the operating system (and version) employed, the file system structure, the auditing infrastructure, the volume of usage of the system, etc. Consider this...I was once looking at a USN Change Journal from an image acquired from a Windows 7 system, and the time span of the available data was maybe a day and a half. Right around that time, a friend of mine contacted me about a Windows 2003 system he was examining, for which the USN Change Journal contained 90 days worth of data.
Windows systems can be very active, even when they appear to be sitting idle with no one actively typing at the keyboard. The operating system itself may reach out for updates, during which files are downloaded, processes are executed, and files are deleted. The same is true for a number of applications. Once a user becomes active on the system, the volume of activity and changes may increase dramatically. I use several applications (Notepad++, UltraEdit, VirtualBox, etc.) that all reach out to the Internet to look for updates when they're launched. Just surfing the web causes browser history to be generated and cached.
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