A Blank Screen Is a Finding: Viewing File Formats for Forensic Clues
An attacker trying to sneak a file past a network administrator doesn't need sophisticated malware if a simpler trick works just as well: rename the file, change its extension, and let the system's own assumptions about file types do the hiding.An attacker trying to sneak a file past a network administrator doesn't need sophisticated malware if a simpler trick works just as well: rename the file, change its extension, and let the system's own assumptions about file types do the hiding.
An attacker trying to sneak a file past a network administrator doesn't need sophisticated malware if a simpler trick works just as well: rename the file, change its extension, and let the system's own assumptions about file types do the hiding. A file called
report.jpg that isn't actually a JPEG will often sail past a casual glance, and even an automated filter that trusts extensions over content. This lab worked through the tool built specifically to catch that trick, one that doesn't trust what a file claims to be, and shows what happens when a file fails to deliver on its own name.The Scenario This Tool Is Built For
The lab's framing was specific: a network administrator had flagged unknown files moving across the company's network after a security breach, and investigators discovered the attacker had deliberately disguised a file's format to avoid detection. The tool used to cut through that disguise was File Viewer, a dedicated application for opening and inspecting files across a wide range of formats, built for exactly this kind of verification rather than just casual viewing.
Opening a File That Behaves as Expected
After installing File Viewer, I opened a sample image file,
Friends2.jpg , through the application's standard File → Open dialog, deliberately selecting "All files" as the file type filter rather than limiting the dialog to a specific expected format. That choice matters on its own: filtering a file-open dialog to only show .jpg files, for instance, would silently hide any file that had been renamed with a different extension, exactly the kind of file an investigator most needs to be able to select and inspect.
The image opened and displayed correctly, exactly as expected for a genuine JPEG. At the bottom of the multiviewer window, File Viewer displayed the file's name, its extension, and its size in bytes. I also checked File → File Properties, which surfaced further metadata about the selected file.
That straightforward result, a JPEG file that opens and displays as a JPEG, is the baseline case: the file's claimed format and its actual content agree with each other. Nothing here flags for further investigation, but establishing what "normal" looks like in the tool is exactly what makes the next result stand out as clearly abnormal.
Opening a File That Doesn't Behave as Expected
The second file told a different story entirely. I opened
520px-Biohazard_symbol_(blue).mp4 , a file whose name and extension both claimed it was a video file. File Viewer attempted to process it, returned an error (an "LTMM" error specifically), and ultimately failed to render anything at all, leaving a blank screen where video playback should have appeared.The lab's explanation for this result is the actual forensic insight of the entire exercise: a blank screen or failed render happens specifically when a file is either genuinely corrupted, or, more relevantly to the scenario, when its file extension has been forcefully changed to something that doesn't match its actual underlying content. In both cases, the file's claimed format and its real structure have come apart, the extension is making a promise the actual bytes inside the file don't keep.
Why a Failed Open Is the Actual Evidence, Not a Dead End
This is the detail that makes the whole exercise worth understanding rather than just clicking through. A less careful investigator might treat File Viewer's failure to open the mp4 as simply a broken or useless file, not worth further attention, move on to the next item. The lab framed it as exactly the opposite: a file that fails to open as its claimed type isn't a dead end, it's the actual flag. That failure is what distinguishes a file worth investigating further from one that's exactly what it appears to be.
The lab was explicit that actually identifying what such a file really is, determining its true underlying format once the claimed extension has been shown to be unreliable, is a separate skill covered in a later module on Windows forensics. This lab's job wasn't to solve that deeper question. It was to teach the first, essential step: noticing that something doesn't match its label in the first place. You can't investigate a disguised file if you never notice the disguise.
What This Demonstrates About Forensic Tooling in General
A few things stood out about why this specific approach, and this specific tool's behavior, actually matters.
File extensions are metadata, not proof. An extension is just a label the operating system (and most casual tools) use to decide how to handle a file, it's trivially easy to change and carries no inherent guarantee that it accurately describes what's actually inside. A forensic tool that simply trusts the extension and tries to render accordingly is only useful for confirming files that are already exactly what they claim to be, it won't catch deception on its own. What made this exercise useful was watching the mismatch actually surface as a visible failure, rather than assuming compatibility.
"All files" as a filter isn't a minor UI detail. Deliberately not restricting the file-open dialog to an expected extension is what allows an investigator to actually select and attempt to open a file regardless of what it claims to be. A workflow that filters by extension first has already limited what it's capable of discovering before the actual inspection even begins.
A tool's failure mode can itself be diagnostic. File Viewer didn't produce a clean error message explaining "this file's content doesn't match its extension." It produced a generic processing error and a blank screen. Recognizing that specific failure pattern, rather than dismissing it as the tool simply not working, is what turns a frustrating dead end into an actual forensic signal.
This connects directly to the hashing and integrity-checking labs covered earlier in the same module. A hash mismatch flags that a file's content has changed from a known baseline. A failed or mismatched file-type render flags that a file's claimed format doesn't match its actual content. Both are the same underlying forensic principle applied in different ways: don't trust a file's surface-level claims about itself, verify them directly, and treat any disagreement between what a file says it is and what it actually is as something worth investigating rather than explaining away.
This was a short, almost deceptively simple lab, open two files, watch one work and one fail. But the actual lesson is about where an investigation's attention should go. A working image file that opens correctly confirms nothing suspicious and needs no further action. A video file that returns an error and shows a blank screen isn't a broken tool or a corrupted download to shrug off, it's a legitimate forensic finding: this file is not what its name claims it to be, and that mismatch is exactly the kind of signal an attacker trying to disguise a malicious payload would produce.
The real skill here isn't operating File Viewer. It's recognizing that a failure to open as expected is itself the evidence, and knowing to stop and flag it rather than moving on.
Enjoyed reading this content? Let the author know!
Your likes, comments, shares, and saves help creators reach more builders.
Loading recommendations
Loading article