---
title: Defender for Office 365: prompt-injection protection quarantines high-confidence email before Copilot sees it | ctxsec
url: https://ctxsec.com/items/mdo-prompt-injection-protection
description: ""
status: "confirmed (vendor)"
published: 2026-10-13
---

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[← Context & Harness Attacks](/context) · [markdown](/items/mdo-prompt-injection-protection.md)

confirmed (vendor)

2026-10-13

### Defender for Office 365: prompt-injection protection quarantines high-confidence email before Copilot sees it

Microsoft documents a mail-flow layer that detects prompt-injection content in inbound email before delivery to users or AI assistants. Detections land as High confidence phishing with technology "Prompt injection protection" (MDO Plan 2 / E5-class). Focus: instructions that try to exfiltrate via URL, reveal system prompts, or enumerate write-capable tools — including hidden/white-on-white text. Runtime Copilot safeguards remain a second layer. This is harness/context defense at the email channel, not a new CVE.

Sources: [Microsoft Learn: Prompt injection protection](https://learn.microsoft.com/en-us/defender-office-365/step-by-step-guides/prompt-injection-protection-defender-for-office-365) · [Microsoft Learn: Copilot prompt defense in depth](https://learn.microsoft.com/en-us/microsoft-365/copilot/copilot-prompt-defense-in-depth)