secrets-in-file-metadata

Secrets in file metadata

The fastest lead in an investigation is often already inside the file: GPS to six decimal places, a camera serial that ties four "unrelated" images to one body, a document author who is a real employee, a template path containing a corporate share name.

Two things beginners get wrong. Absent metadata is not suspicious — it is the normal state of anything that passed through a social platform. And present metadata is not proof: every tag is a claim written by whatever software touched the file last, and all of it is editable with one command.

Where to look first, given what you have

exiftool, properly

exiftool -G1 -a -u -g1 file.jpg        # everything, grouped into sections — best for reading
exiftool -G -a -u -s file.jpg

Why the defaults lose evidence:

• -G / -G1 prefixes each tag with its group ([EXIF], [XMP], [IPTC], [MakerNotes], [File], [Composite]). Without it you cannot distinguish a tag the device wrote from a Composite value exiftool calculated, or an EXIF timestamp from a filesystem one. -G1 gives the finer group.
• -a keeps duplicate tags instead of showing only the last. Inconsistencies live here: the same timestamp with two different values in two blocks means two programs disagreed, which means the file was edited.
• -u shows tags exiftool has no name for. Vendor blocks are often the point.
• -n disables pretty-printing — signed decimal degrees instead of 52 deg 22' 8.40" N.
• -s prints tag names rather than descriptions, so you can query them again.
• -ee extracts embedded data, including GPS tracks inside video streams.

Bulk triage, then sort the CSV looking for which files carry GPS at all, a recurring SerialNumber, timestamps outside the claimed window, and the one file whose Software differs:

Full command patterns: reference/exiftool-cookbook.md.

Reading the high-value fields

GPS. Position comes from GPSLatitude/GPSLongitude and their Ref tags, height from GPSAltitude plus GPSAltitudeRef. The forgotten ones matter more: GPSImgDirection is the compass bearing the camera was pointed, which places the photographer and orients the view — decisive when matching street-level imagery. GPSDestBearing is the bearing to the subject. GPSHPositioningError is the device's own accuracy estimate in metres and is the honest radius for your finding. GPSDateStamp/GPSTimeStamp are UTC, making them the only trustworthy clock in the file.

Device fingerprint. Make and Model give a device type. SerialNumber, BodySerialNumber, InternalSerialNumber or CameraSerialNumber identify one physical body — the highest-value tag in the file for link analysis, tying images across accounts, platforms and years to a single camera. LensSerialNumber does the same for glass, and a body/lens pair is tighter still. Vendor shutter-count and image-number tags let you order a device's output and estimate how much shooting happened between two frames. OwnerName, CameraOwnerName and Artist are user-set and frequently hold a real name.

Timestamps. DateTimeOriginal is when the shutter fired. CreateDate is when this digital file was created — identical on a camera, different on a scan, export or re-encode. ModifyDate is when it was last written; later than DateTimeOriginal means processing. The trap: EXIF datetimes carry no timezone. They are local device time unless OffsetTime, OffsetTimeOriginal or OffsetTimeDigitized is present, which many devices omit. So reconcile against the UTC GPSDateTime to derive the device's offset — which itself tells you what longitude band the device was set for. Never quote FileModifyDate as evidence about a photograph; it belongs to the filesystem you are looking at and changes on copy.

Thumbnail versus image. Sloppy editors update the main image and leave the embedded preview alone, so the preview can show the scene before the crop or retouch.

A different aspect ratio proves a crop. Different content is the whole case.

Editing chain. Software, ProcessingSoftware, HostComputer and XMP's CreatorTool name what touched the file. XMP media-management tags go further: DocumentID, OriginalDocumentID, InstanceID and DerivedFrom link an exported derivative back to a source file you have never seen, and to sibling derivatives of that same source.

IPTC/XMP on press images. By-line, Credit, Source, Caption-Abstract, Headline, DateCreated, City, Country-PrimaryLocationName, plus XMP's person-in-image and location-created structures. On a wire photo this is a complete human-written answer to who, where and when — written by an editor, so treat it as a sourced claim, not a sensor reading.

Documents. Author and LastModifiedBy are the two names. Company and Manager come from the Office install. Template can contain a full UNC path exposing an internal server and department. RevisionNumber and TotalEditTime show whether a document was worked on or produced in one pass to look official. LastPrinted proves a physical copy existed. Then open the container, because per-author identities in tracked changes are not exposed by exiftool:

Embedded images are the most-missed source of GPS in practice: the document was scrubbed, the photograph pasted into page four was not.

PDF. Producer names the library or driver that wrote the file and is a strong tell — a "scanned" document produced by a word processor was never scanned. Creator names the authoring application. PDF dates, unlike EXIF, do carry a timezone offset. PDFs support incremental updates, so earlier revisions of the content can still be inside the file, and they can carry attachments and images retaining their own metadata. Enumerate with pdfimages -list and pdfdetach -list, and expand the structure with qpdf --qdf before reading it.

Per-format tag catalogue: reference/tag-catalogue.md.

Where this goes wrong

• Stripping happens on the way in. The mechanism: a host that re-encodes to generate delivery renditions discards EXIF; a host that serves your original bytes keeps it. Large social platforms re-encode. Many forums, self-hosted CMSes, object-storage buckets, photo-community sites and mail attachments do not. CMSes are the interesting middle case — the resized image on the page is stripped while the original upload in the media directory is intact, so try to reach the original path.
• Absence proves nothing. Not that a file was scrubbed, not that it is fake, not that the uploader was careful. Write "no EXIF present", never "EXIF removed", unless you can show a copy that had it.
• Presence proves only that someone wrote a value. Corroborate location visually with geolocate-from-pixels and timing with sun position before relying on either.
• The clock is probably wrong. Camera clocks drift, stay on the wrong timezone after travel, and ignore DST. A bare EXIF datetime is ±hours until anchored against GPSDateTime or a dated event visible in frame.
• Composite tags are exiftool's arithmetic, not the file's content. GPSPosition, ImageSize and LensID are derived. Without -G you will quote a calculated value as though the device wrote it.
• Screenshots carry the screenshotting device's metadata, not the photograph's. Check whether Model is a phone before building a theory on it.
• Editing before extracting is unrecoverable. Rotating, cropping, or even opening in some editors rewrites tags. Hash and copy first.
• Online metadata viewers mean uploading your evidence to a stranger. Install exiftool; it is one Perl distribution and runs offline. A browser-local tool beats a server-side one, and neither is acceptable for material under a protective order, an NDA, or a live criminal matter. Assume anything uploaded is retained and may be indexed.

Confidence grading

• Confirmed — a metadata claim corroborated by non-metadata evidence: GPS that matches an independent visual geolocation of the same frame; a camera serial appearing in files from two unconnected sources; a document author matching a known employee found via find-anyone.
• Probable — internally consistent metadata from a plausible device, in a file from a host that does not strip, with timestamps agreeing across EXIF, XMP and the embedded thumbnail, and no editor in the chain.
• Unconfirmed — a single tag with nothing to check it against. Every GPS coordinate you have not visually verified sits here. That is fine, if labelled.
• Contradicted — timestamps that disagree, a thumbnail showing a different scene, GPS the visual evidence rules out, or an editing tool the source denied using. A contradiction is a finding in its own right, often better than the original tag would have been.

Worked example

Verify a PDF "site inspection report" attached to an insurance claim, allegedly written on site on a stated date.

exiftool -G1 -a -u -g1 report.pdf: Producer is a word-processor export, not a scanner driver. CreationDate and ModDate are eleven minutes apart, both with an offset three hours from the site's zone. Author is an initial and surname.

pdfimages -list shows four embedded photographs. Extract them and run exiftool — nothing. They were re-encoded on insertion. Dead end, and a common one.

The claim email also carried a DOCX. Unzip it: docProps/app.xml gives a TotalEditTime under four minutes and a Template UNC path whose hostname belongs to a third-party firm, not the insured. word/document.xml has tracked insertions under a second author. word/media/image2.jpeg still has intact EXIF — DateTimeOriginal two months before the claimed inspection, GPS present, Model a phone, GPSHPositioningError nine metres.

Author names probable, to be corroborated against the firm via x-ray-a-company. Photo capture date contradicted against the report's claimed date. Photo location unconfirmed, handed to geolocate-from-pixels with a nine-metre stated radius.

Pivots

Legal notes

Metadata in a file you lawfully hold is yours to read. Two constraints bite. Names, precise location and device identifiers in metadata are personal data under GDPR and comparable regimes whatever file they sit in, so minimisation, retention and purpose limits apply — collect the tags the objective needs, and do not keep a full dump of a subject's photo library because it was easy. And precise historical location data about an identifiable person is the most sensitive material in this skill: legitimate in due diligence, fraud and authorized investigation, and the raw material of stalking. If the only outcome of extracting a GPS tag is knowing where a private individual sleeps, stop. See ../../ETHICS.md.