Introduction to Forensics

A break-in can happen to any system administrator. Find out how to use Autopsy and Sleuthkit to hit the ground running on your first forensics project.
Start the Investigation

Now that you have created the case, added a host and selected any disk images, you are ready to start the analysis. On the Host Manager page, you will see all the partitions available to analyze. The root (/) partition is a good place to start, so select it, and click Analyze. The Analyze page lists a number of different ways to investigate the filesystem, but click the File Analysis button along the top of the screen to enter one of the main pages you will use for this analysis.

Figure 3. File Analysis

The File Analysis page gives you a complete view of the filesystem, starting at its root. The top-right frame lists all the files in the current directory, along with additional information each in its own field, including MAC times, permissions and file size. MAC (Modified, Accessed and Changed times), refers to three different changes the filesystem keeps track of for each file. The modified time is the last time the file or directory actually was written to. For instance, if you open a text file, edit it and save the changes, this updates the modified time. The access time is the last time the file or directory was accessed at all. Reading a file updates its access time, and listing the contents of a directory also updates its access time. The changed time keeps track of the last time the file's metadata (such as file permissions and owner) were changed. It's possible, in some cases, for some or all of these times to match.

Each of the files or directories in the File Analysis page are hyperlinked. If you click a directory, the page changes to list the contents of that directory. If you click a file, the bottom-right frame changes to list the contents of the file (even if it's binary) along with a number of functions you can perform on that file. You can display the ASCII or Hex versions of a file or have Autopsy scan the file and display only the ASCII strings inside. This feature is particularly handy to try on suspected trojan files. Often the ASCII strings inside a trojan binary list strange IRC channels or other remote servers or passwords the attacker is using. You also can export a copy of the file to your local disk for further examination.

Attackers often like to delete files to cover their tracks, but Autopsy can attempt to recover them from the free space on the filesystem. Go to the File Analysis page, click the All Deleted Files button on the bottom of the left-hand frame, and Autopsy lists all the deleted files it finds on the system. If Autopsy can recover that much information, you also can see the MAC times and may even be able to click on the file and recover its original contents!

All of these features are handy, but one of the most useful is the Add Note feature. If, for instance, you notice a system binary in your /bin directory that has a strange recent modified date and you notice some suspicious ASCII strings inside, you could click Add Note and list your findings. On the Add Note page, you also can add a sequencer event based on MAC time. If you thought the modified time was suspicious, you might select M-Time on the Add Note page. When you add notes like this for a number of files or directories, you end up with a large series of notes on what you have found along with interesting times. From the Host Manager window (the window that lists the host's partitions), click View Notes to see the list. This is an invaluable feature when you are trying to piece together the sequence of events from an attacker—particularly when you want to share your findings with others.

If you find a piece of information, such as an IP address or a particular server name as you scan files, you also can click Keyword Search at the top of the Analysis page to scan the entire filesystem for that keyword. You might find log entries or additional files the attacker uploaded that reference that keyword in unlikely places with this tool.

One thing you will discover is that the sequence of events is very important when figuring out an attacker's steps. The File Analysis window lets you sort by any of the headers, including the MAC times. An attacker often will replace a system binary under /bin or /sbin with a trojan, and because that will update the modified time for a file, if you sort the /bin and /sbin directories by modified time in the File Analysis window, you quickly can see suspicious file changes, such as a series of core programs, like ls, vi and echo, all modified a few days ago at a time when you know you didn't update any programs.

Where to Search

If you are new to forensics, you might not be sure of exactly where to start looking in your filesystem. A few directories often contain evidence of an attack that will at least give you a starting point. I've already mentioned the /bin and /sbin directories, as attackers often replace core system binaries in these directories with trojans. The /tmp and /var/tmp directories also are favorite locations, as any user on the system can write to them, so attackers often start their attacks in these directories and download rootkits and other tools here. Pay particular attention for hidden directories (directories that start with a .) in /var/tmp, as that's one way for attackers to cover their tracks from a casual observer. Finally, scan under /home and /root for suspicious files or strange commands in each users' .bash_history file.

What you hope to find is some idea of when attackers were active on your system. Once you have an idea of when the attackers were there, you can check file access and modify times during that period to track down where the attackers were on your system and which files they touched. Although you certainly could browse through the File Analysis window directory by directory, Autopsy provides an easier way via its File Activity Time Line. If you are currently in the File Analysis window, click Close to return to the main Host Manager window that lists the images you have added for your host. From there, click the File Activity Time Line button. Next, click Create Data File, click the check box next to all of the images it lists, and then click OK. This job will take some time, depending on the size and speed of your disk and your CPU.

Once the data file is created, click OK to proceed to the Create Timeline window. In this window, you can narrow down your timeline so that it lists only a particular time period; however, just leave all the options as they are for now and click OK. As you never exactly know where an investigation will lead, you don't want to rule out periods of time that might have valuable clues. When the timeline has been created, click OK to view the Web-based timeline viewer, but a note on that page gives a valuable tip—the timeline is easier to view via a text editor than from the Web interface. Find the raw timeline text file under /var/lib/autopsy/case/host/output/timeline.txt. If you named your case Investigation1 and your host Gonzo, you can find the file under /var/lib/autopsy/Investigation1/Gonzo/output/timeline.txt.

Figure 4. Sample timeline.txt File

The timeline.txt file lists every file on your image sorted by MAC time. This file contains a lot of information, but once you figure out what each field stands for, it's easier to decipher. The first column lists the time in question for a file followed by the file size. The next field denotes whether this time was a time the file was modified, accessed, changed or any combination of the three. If a file was both modified and accessed at this time, but its metadata was not changed, you would see “ma.” in this field. The next field lists the file permissions, followed by the user and group that owned the file. The final two fields list the filesystem inode and the full path to the file or directory. Note that if a group of files has the same time, only the first time field is filled.

If you have found one of the attackers' files, try to locate it in the timeline and see what other files were accessed and especially modified during that time period. With this method, you often can see a list of accessed files that show someone compiling or executing a program. If you notice that the attackers used a particular account on the system, use the File Analysis window to check the /home/username/.bash_history for that user and see any other commands the attackers might have run.

In addition, look at the login history, which often is found under /var/log/messages, for other times that user has logged in and try to correlate those times with any other file activity on the system inside the timeline.txt file. Remember to add notes for each clue you find—as you dig further and further into the filesystem, it can be difficult to keep track of all the different files and how they correlate, but the notes page makes it easy to see. The ultimate goal is to try to locate the earliest time attackers left tracks on the system and use that information to figure out how they got in.

As you might gather, thorough forensics analysis can be a time-consuming process. Even with a tool like Autopsy, it still takes time and experience to make sense of all of the data it presents so you can piece together an attack. One easy way to gain experience is to image your personal system and view it through Autopsy. Create a timeline and see whether you can track down some of the commands you last ran or files you last edited. You might possibly even want to attack your own machine and see if you can use Autopsy to retrace your steps. Although nothing can replace real data, this sort of practice goes a long way toward understanding forensics so you're prepared when a real attack occurs.

Kyle Rankin is a Senior Systems Administrator in the San Francisco Bay Area and the author of a number of books, including Knoppix Hacks and Ubuntu Hacks for O'Reilly Media. He is currently the president of the North Bay Linux Users' Group.


Kyle Rankin is Chief Security Officer at Purism, a company focused on computers that respect your privacy, security, and freedom. He is the author of many books including Linux Hardening in Hostile Networks, DevOps Troubleshooting and The Official Ubuntu