Microsoft Paint and Photos embed a server-issued GUID as an invisible pixel watermark in images generated locally on the device

Sources: primary, discussion
Summary: A reverse-engineering write-up traces a per-generation GUID from a moderation POST, through Watermarker.dll, into the pixels of images the local model produces, in Paint 11.2605.71.0 and Photos 2026.11060.2004.0. The author then finds the same value in the signed C2PA soft-binding assertion of a saved PNG, and obtained a live server response to confirm the identifier comes from the service rather than the device. The two applications differ on failure. Paint turns a watermarking failure into a failure of the whole generation, while Photos logs the error and returns the image unmarked.
Why it matters: Local generation still requires a remote moderation call that carries the prompt and returns the per-generation identifier written into the pixels, so the image itself links back to a server-side record.