Originally published on rohitraj.tech
Microsoft Copilot Cowork was exfiltrating SharePoint and OneDrive files via a 5-line prompt injection hidden inside an 81-line skill file — and it worked on 5 of 5 trials against Claude Opus 4.7, model-agnostic. Most "build an MCP server" tutorials ship code that has the same hole. This is the secure TypeScript build, end to end, with the defense layer none of the top tutorials include.
Read the full version with code samples, diagrams, and architecture details: Build a Secure MCP Server in TypeScript: The Post-Copilot Defense Playbook (2026)
More engineering notes: rohitraj.tech/en/notes
