A federal court appointed me Special Master, tasked to, in part, search the file slack space of a party’s computers and storage devices. The assignment prompted me to reconsider the value of this once-important forensic artifact.
Slack space is the area between the end of a stored file and the end of its concluding cluster: the difference between a file’s logical and physical size. It’s wasted space from the standpoint of the computer’s file system, but it has forensic significance by virtue of its potential to hold remnants of data previously stored there. Slack space is often confused with unallocated clusters or free space, terms describing areas of a drive not currently used for file storage (i.e., not allocated to a file) but which retain previously stored, deleted files.
A key distinction between unallocated clusters and slack space is that unallocated clusters can hold the complete contents of a deleted file whereas slack space cannot. Data recovered (“carved”) from unallocated clusters can be quite large—spanning thousands of clusters—where data recovered from a stored file’s slack space can never be larger than one cluster minus one byte. Crucially, unallocated clusters often retain a deleted file’s binary header signature serving to identify the file type and reveal the proper way to decode the data, whereas binary header signatures in slack space are typically overwritten.
A little more background in file storage may prove useful before I describe the dwindling value of slack space in forensics.
Electronic storage media are physically subdivided into millions, billions or trillions of sectors of fixed storage capacity. Historically, disk sectors on electromagnetic hard drives were 512 bytes in size. Today, sectors may be much larger (e.g., 4,096 bytes). A sector is the smallest physical storage unit on a disk drive, but not the smallest accessible storage unit. That distinction belongs to a larger unit called the cluster, a logical grouping of sectors and the smallest storage unit a computer can read from or write to. On Windows machines, clusters are 4,096 bytes (4kb) by default for drives up to 16 terabytes. So, when a computer stores or retrieves data, it must do so in four kilobyte clusters.
File storage entails allocation of enough whole clusters to hold a file. Thus, a 2kb file will only fill half a 4kb cluster–the balance being slack space. A 13kb file will tie up four clusters, although just a fraction of the final, fourth cluster is occupied is occupied by the file. The balance is slack space and it could hold fragments of whatever was stored there before. Because it’s rare for files to be perfectly divisible by 4 kilobytes and many files stored are tiny, much drive space is lost to slack space. Using smaller clusters would mean less slack space, but any efficiencies gained would come at the cost of unwieldy file tracking and retrieval.
So, slack space holds forensic artifacts and those artifacts tend to hang around a long time. Unallocated clusters may be called into service at any time and their legacy content overwritten. But data lodged in slack space endures until the file allocated to the cluster is deleted–on conventional “spinning” hard drives at any rate.
When I started studying computer forensics in the MS-DOS era, slack space loomed large as a source of forensic intelligence. Yet, apart from training exercises where something was always hidden in slack, I can’t recall a matter I’ve investigated this century which turned on evidence found in slack space. The potential is there, so when it makes sense to do it, examiners search slack using unique phrases unlikely to throw off countless false positives.
But how often does it make sense to search slack nowadays?
I’ve lately grappled with that question because it seems to me that the shopworn notions respecting slack space must be re-calibrated.
Keep in mind that slack space holds just a shard of data with its leading bytes overwritten. It may be overwritten minimally or overwritten extensively, but some part is obliterated, always. Too, slack space may hold the remnants of multiple deleted files; that is, as overlapping artifacts: files written, deleted overwritten by new data, deleted again, then overwritten again (just less extensively so). Slack can be a real mess.
Fifteen years ago, when programs stored text in ASCII (i.e., encoded using the American Standard Code for Information Interchange or simply “plain text”), you could find intelligible snippets in slack space. But since 2007, when Microsoft changed the format of Office productivity files like Word, PowerPoint and Excel files to Zip-compressed XML formats, there’s been a sea change in how Office applications and other programs store text. Today, if a forensic examiner looks at a Microsoft Office file as it’s written on the media, the content is compressed. You won’t see any plain text. The file’s contents resemble encrypted data. The “PK” binary header signature identifying it as compressed content is gone, so how will you recognize zipped content? What’s more, the parts of the Zip file required to decompress the snippet have likely been obliterated, too. How do you decode fragments if you don’t know the file type or the encoding schema?
The best answer I have is you throw common encodings against the slack and hope something matches up with the search terms. More-and-more, nothing matches, even when what you seek really is in the slack space. Searches fail because the data’s encoded and invisible to the search tool. I don’t know how searching slack stacks up against the odds of winning the lottery, but a lottery ticket is cheap; a forensic examiner’s time isn’t.
That’s just the software. Storage hardware has evolved, too. Drives are routinely encrypted, and some oddball encryption methods make it difficult or impossible to explore the contents of file slack. The ultimate nail in the coffin for slack space will be solid state storage devices and features, like wear leveling and TRIM that routinely reposition data and promise to relegate slack space and unallocated clusters to the digital dung heap of history.
Taking a fresh look at file slack persuades me that it still belongs in a forensic examiner’s bag of tricks when it can be accomplished programmatically and with little associated cost. But, before an expert characterizes it as essential or a requesting party offers it as primary justification for an independent forensic examination, I’d urge the parties and the Court to weigh cost versus benefit; that is, to undertake a proportionality analysis in the argot of electronic discovery. Where searching slack space was once a go-to for forensic examination, it’s an also-ran now. Do it, when it’s an incidental feature of a thoughtfully composed examination protocol; but don’t bet the farm on finding the smoking gun because the old gray mare, she ain’t what she used to be!
See? I never metaphor I didn’t like.
Postscript: A question came up elsewhere about solid state drive forensics. Here was my reply:
The paradigm-changing issue with SSD forensic analysis versus conventional magnetic hard drives is the relentless movement of data by wear leveling protocols and a fundamentally different data storage mechanism. Solid state cells have a finite life measured in the number of write-rewrite cycles.
To extend their useful life, solid state drives move data around to insure that all cells are written with roughly equal frequency. This is called “wear leveling,” and it works. A consequence of wear leveling is that unallocated cells are constantly being overwritten, so SSDs do not retain deleted data as electromagnetic drives do. Wear leveling (and the requisite remapping of data) is handled by an SSD drive’s onboard electronics and isn’t something users or the operating system control or access.
Another technology, an ATA command called TRIM, is controllable by the operating system and serves to optimize drive performance by disposing of the contents of storage cell groups called “pages” that are no longer in use. Oversimplified, it’s faster to write to an empty memory page than to initiate an erasure first; so, TRIM speeds the write process by clearing contents before they are needed, in contrast to an electromagnetic hard drive which overwrites clusters without need to clear contents beforehand.
The upshot is that resurrecting deleted files by identifying their binary file signatures and “carving” their remnant contents from unallocated clusters isn’t feasible on SSD media. Don’t confuse this with forensically-sound preservation and collection. You can still image a solid state drive, but you’re not going to get unallocated clusters. Too, you won’t be interfacing with the physical media grabbing a bitstream image. Everything is mediated by the drive electronics.
Dear Reader, Sorry I’ve been remiss in posting here during the COVID crisis. I am healthy, happy and cherishing the peace and quiet of the pause, hunkered down in my circa-1880 double shotgun home in New Orleans, enjoying my own cooking far too much. Thanks to Zoom, I completed my Spring Digital Evidence class at the University of Texas School of Law, so now one day just bubbles into the next, and I’m left wondering, Where did the day go?. Every event where I was scheduled to speak or teach cratered, with no face-to-face events sensibly in sight for 2020. One possible exception: I’ve just joined the faculty of the Tulane School of Law ten minutes upriver for the Fall semester, and plan to be back in Austin teaching in the Spring. But, who knows, right? Man plans and gods laugh.
We of a certain age may all be Zooming and distancing for many months. As one who’s bounced around the world peripatetically for decades, not being constantly on airplanes and in hotels is strange…and stress-relieving. While I miss family, friends and colleagues and mourn the suffering others are enduring, I’ve benefited from the reboot, ticking off household projects and kicking the tires on a less-driven day-to-day. It hasn’t hurt that it’s been the best two months of good weather I’ve ever seen, here or anywhere. The prospect of no world travel this summer–and no break from the soon-to-be balmy Big Easy heat–is disheartening, but small potatoes in the larger scheme of things.
Be well, be safe, be kind to yourself. This, too, shall pass and as my personal theme song says,
There's a Great Big Beautiful Tomorrow. Just a Dream Away.