Microsoft Paint Cocreator and Photos Image Creator send the prompt and style to an Azure Front Door moderation endpoint before local Stable Diffusion runs on a Copilot+ NPU. The reply includes a 16-byte watermarkId GUID. Watermarker.dll’s WmkWriteWatermark then writes that GUID into pixels as InvisMark, separate from the optional Copilot-logo badge. Dual-use is provenance under EU AI Act Article 50 versus a server-issued mark that survives metadata stripping.

The GUID is a per-request identifier, not a proven user-ID field in the pixels. Paint also returns lastPromptGenerationId on the next call, so the server can chain sessions if it stores prompt, account, and device data it already says it collects for abuse monitoring. C2PA manifests name algorithm com.microsoft.invismark.1 and repeat the same GUID. Paint aborts if the write fails; Photos may log and still return the file. Exports are limited to C2PA-preserving PNG, JPEG, GIF, and .paint.

Xusheng Li of Vector 35 published the reverse-engineering on 21 August 2026. Tom’s Hardware and CyberInsider confirmed the path on 25–26 August. Microsoft Learn already documents C2PA plus invisible watermarks for Foundry systems. Support pages mention remote filtering and C2PA, not that the pixel payload is the moderation GUID.

Net risk is silent persistence of a Microsoft-issued mark on images users treat as local. Oversight gap: no in-app notice that the visible-watermark toggle does not control the pixel write, and no public decoder for third-party audit. Watch whether Microsoft documents the GUID and lastPromptGenerationId chain.

Sources

Microsoft Paint and Photos Embed Server-Issued GUIDs as Invisible Watermarks in Locally-Generated Images

https://xusheng.dev/posts/reversing/mspaint\_invisible\_watermark/main/

Xusheng Li reverse-engineering of Paint and Photos, watermarkId, WmkWriteWatermark, C2PA binding, and Copilot+ remote moderation.

Microsoft Paint and Photos apps add invisible watermark to AI-generated content — developer reverse engineers GUID embedding

https://www.tomshardware.com/tech-industry/artificial-intelligence/microsoft-paint-and-photos-apps-add-invisible-watermark-to-ai-generated-content-developer-reverse-engineers-guid-embedding

26 August 2026 Mark Tyson report confirming mandatory pixel GUID on local Copilot+ generation.

Microsoft Paint embeds hidden IDs in locally generated AI images

https://cyberinsider.com/microsoft-paint-embeds-hidden-ids-in-locally-generated-ai-images/

25 August 2026 summary of the 16-byte GUID, independent visible watermark, and C2PA-preserving export formats.

Content provenance - Azure AI services

https://learn.microsoft.com/en-us/azure/foundry/responsible-ai/content-safety/provenance-disclosure

Microsoft documentation that Foundry systems use both signed C2PA credentials and invisible watermarks.

Quick Facts: Transparency rules for AI systems

https://digital-strategy.ec.europa.eu/en/factpages/quick-facts-transparency-rules-ai-systems

EU page stating Article 50 machine-readable marking of synthetic content applies from 2 August 2026.

Paint and Photos Embed Server GUIDs in Local AI Images After Remote Prompt Checks