A naive approach to stripping EXIF from a PNG would be to parse up to the start of the first eXIf chunk, discard the contents of that chunk, and then include the rest of the file verbatim without actually parsing anything.

But yes, a more sensibly coded EXIF stripper would deserialise and reserialise. Unfortunately I am no longer able to assume that programmers will behave sensibly.

Does anyone take this “naive” approach in practice? Any good image sanitization I’ve seen is equivalent to taking a screenshot of the image, re-serializing pixel contents but ignoring anything else. Any reputable service (e.g., Gmail) must take this approach to prevent proliferation of possible image-based malware.

As you noted above Discord doesn’t sanitize PNGs. This exposes a failing on their end as well, as large services taking input from users should sanitize images to protect both senders and recipients.

Discord strips EXIF but the author was still able to unredact the images they'd posted there

Some implementations of EXIF stripping might help, but it's not guarenteed

Discord doesn't strip EXIF from PNGs, only JPEGs

Seriously? What's the reasoning behind that?

It's rare to see PNGs in-the-wild containing EXIF data, it's a feature that's only been in the spec since ~2017. I'm actually looking for one to double-check my statement about discord, but I can't find any.

Edit: I made my own. I can confirm that the exif chunk was not stripped. https://cdn.discordapp.com/attachments/541730746805649476/10...

That's interesting. I've seen a couple of rotated PNGs before which I assumed were caused by Discord stripping the EXIF and orientation data. Found a PNG like that without EXIF from May 2022 so I wonder if Discord stopped stripping or if it was stripped on the person's device somewhere.