EXIF

What is in there

Camera make and model, lens, focal length, aperture, shutter speed, ISO, the exact capture timestamp, orientation, and often the device serial number. On a phone it usually also includes GPS coordinates accurate to a few metres.

Editing software adds more: the application name and version, and sometimes a full processing history. Some cameras additionally embed a small preview image inside the EXIF block.

The privacy problem

A holiday photograph posted publicly can carry the coordinates of the house it was taken in. A photograph of a document can carry the timestamp that contradicts the story around it. Neither is visible in the picture, and neither is obvious to the person sharing it.

The embedded thumbnail is the least-known risk and the sharpest. Some editors update the main image and leave the stale preview in place, which means a file whose full-size version is cropped or redacted can carry an original inside it. There are documented incidents of exactly this.

Stripping metadata removes the thumbnail along with everything else, which is a good reason to treat the step as mandatory rather than optional before publishing anything sensitive.

Two fields worth keeping

Orientation, unless your pipeline has already applied it. Cameras record which way up the sensor was rather than rotating the pixels, so stripping the tag from an unrotated file turns portrait photographs sideways. A good compressor bakes the rotation into the pixels before dropping the tag, which is why images come out the right way up with the metadata gone.

The colour profile, if the image is not sRGB. Strip a Display P3 profile and the colours shift towards flat and desaturated. Convert to sRGB first if you intend to strip it.

Everything else — device, lens, exposure, coordinates, software history — is of no use to a web image and some of it is a liability.