Microsoft Paint Sends Your Prompt to a Moderation Server and Bakes the Returned GUID Into the Image Pixels

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Xusheng Li of Vector 35 reverse engineered Watermarker.dll and found that Paint and Photos build an 18-byte message of 0x4c plus a 16-byte server-issued GUID plus a checksum, expand it to 144 bits, and place each bit at least three times using content-adaptive block-domain steganography. The same GUID appears in signed C2PA metadata under the assertion name com.microsoft.invismark.1, so the pixel watermark and the file manifest are two layers of one identifier that traces back to the prompt Microsoft moderated. The Register picked it up on 2026-08-25 with the sharper framing that the marks are linked to user IDs; the watermark applies to Cocreator and Image Creator output, not to ordinary hand-drawn images.