The physical memory of computers can be imaged and analyzed using a variety of tools. Because the procedure for accessing physical memory varies between operating systems, these tools are listed by operating system. Once memory has been imaged, it is subjected to memory analysis to ascertain the state of the system, extract artifacts, and so on.
One of the most vexing problems for memory imaging is verifying that the image has been created correctly. That is, verifying that it reflects the actual contents of memory at the time of its creation. Because the contents of memory are constantly changing on a running system, the process can be repeated but the results will never--to a high degree of probability--be the same. Thus, repeating the acquisition and comparing the results is not a feasible means of validating correct image creation. Memory analysis can reveal whether the image's contents are consistent with the known layout and structure of a given operating system, as well as answering other questions, but it cannot answer the question as to whether the image accurately reflects the system from which it was taken at the time it was taken.
Memory Imaging Techniques
- Crash Dumps
- When configured to create a full memory dump, Windows operating systems will automatically save an image of physical memory when a bugcheck (aka blue screen or kernel panic) occurs. Andreas Schuster has a blog post describing this technique.
- LiveKd Dumps
- The Sysinternals tool LiveKd can be used to create an image of physical memory on a live machine in crash dump format. Once livekd is started, use the command ".dump -f [output file]"
- Hibernation Files
- Windows 98, 2000, XP, 2003, and Vista support a feature called hibernation that saves the machine's state to the disk when the computer is powered off. When the machine is turned on again, the state is restored and the user can return to the exact point where they left off. The machine's state, including a compressed image of physical memory, is written to the disk on the system drive, usually C:, as hiberfil.sys. This file can be parsed and decompressed to obtain the memory image. Once hiberfil.sys has been obtained, Sandman can be used to convert it to a dd image.
- Mac OS X very kindly creates a file called /var/vm/sleepimage on any laptop that is suspended. This file is NOT erased when the machine starts up. It is unencrypted even if the user turns on File Vault and enables Secure Virtual Memory. .
- It is possible for Firewire or IEEE1394 devices to directly access the memory of a computer. Using this capability has been suggested as a method for acquiring memory images for forensic analysis. Unfortunately, the method is not safe enough to be widely used yet. There are some published papers and tools, listed below, but they are not yet forensically sound. These tools do not work with all Firewire controllers and on other can cause system crashes. The technology holds promise for future development, in general should be avoided for now.
- At CanSec West 05, Michael Becher, Maximillian Dornseif, and Christian N. Klein discussed an exploit which uses DMA to read arbitrary memory locations of a firewire-enabled system. The paper lists more details. The exploit is run on an iPod running Linux. This can be used to grab screen contents.
- This technique has been turned into a tool that you can download from: http://www.storm.net.nz/projects/16
- The Goldfish tool automates this exploit for investigators needing to analyze the memory of a Mac.
- Virtual Machine Imaging
- There are numerous popular virtual machines that are in wide use such as xen, qemu or vmware. If the memory image is for a machine running in this kind of virtual environment, there are usually two methods for obtaining a memory image. The common method is to pause/suspend/stop the system and then collect the resulting memory image file, this has the disadvantage of taking the machine offline during the suspend time. Alternatively most of these systems support live dumping of a memory image. Qemu supports the pmemsave function, Xen has the xm dump-core command.
Memory Imaging Tools
- Tribble PCI Card (research project)
- CoPilot by Komoku
- Komoku was acquired by Microsoft and the card was not made publicly available.
- Forensic RAM Extraction Device (FRED) by BBN
- Not publicly available. http://www.ir.bbn.com/~vkawadia/
- winen.exe (Guidance Software - included with Encase 6.11 and higher)
- included on Helix 2.0
- included on Helix 2.0
- F-Response with FTK imager, dd, Encase, WinHex, etc
- Beta 2.03 provides remote access to memory that can be acquired using practically any standard imaging tool
- MANDIANT Memoryze
- Can capture and analyze memory. Supports reading dumps (raw/dd format) from other tools.
- On Microsoft Windows systems, dd can be used by an Administrator user to image memory using the \Device\Physicalmemory object. Userland access to this object is denied starting in Windows 2003 Service Pack 1 and Windows Vista.
- Windows Memory Forensic Toolkit (WMFT)
- Fastdump and Fastdump Pro
- Fastdump (free with registration) Can acquire physical memory on Windows 2000 through Windows XP 32 bit but not Windows 2003 or Vista.
- Fastdump Pro Can acquire physical memory on Windows 2000 through Windows 2008, all service packs. Additionally, Fastdump Pro supports:
- -32 bit and 64 bit architectures
- -Acquisitions of greater than 4GB
- -Fast acquisitions through the use of larger page sizes (1024KB) but also supports a strict mode that enforces 4KB page sizes.
- -Process probing which allows for a more complete memory image of a process of interest.
- -Acquisition of the system page file during physical memory acquisition. This allows for a more complete memory analysis.
- On Unix systems, the program dd can be used to capture the contents of physical memory using a device file (e.g. /dev/mem and /dev/kmem). In recent Linux kernels, /dev/kmem is no longer available. In even more recent kernels, /dev/mem has additional restrictions. And in the most recent, /dev/mem is no longer available by default, either. Throughout the 2.6 kernel series the trend has been to reduce direct access to memory via pseudo-device files. See, for example, the message accompanying this patch: http://lwn.net/Articles/267427/. On Red Hat systems (and derived distros such as CentOS), the crash driver can be loaded to create a pseudo-device for memory access ("modprobe crash").
- Second Look
- This commercial memory analysis product has the ability to acquire memory from Linux systems, either locally or from a remote target via DMA or over the network. It comes with pre-compiled Physical Memory Access Driver (PMAD) modules for hundreds of kernels from the most commonly used Linux distributions.
- Idetect (Linux)
- fmem (Linux)
- fmem is kernel module, that creates device /dev/fmem, similar to /dev/mem but without limitations. This device (physical RAM) can be copied using dd or other tool. Works on 2.6 Linux kernels. Under GNU GPL.
- Goldfish is a Mac OS X live forensic tool for use only by law enforcement. Its main purpose is to provide an easy to use interface to dump the system RAM of a target machine via a Firewire connection. It then automatically extracts the current user login password and any open AOL Instant Messenger conversation fragments that may be available. Law Enforcement may contact goldfish.ae for download information.
- Windows Memory Analysis
- Linux Memory Analysis