Microsoft's npm Packages Got Backdoored. Again. And AI Agents Pulled the Trigger.

73 cryptographically signed npm packages from Microsoft were compromised last week with advanced credential-stealing malware that fires the moment a developer opens one in an AI coding agent. Claude Code, Gemini CLI, Cursor, VS Code — all trigger it. It's the second supply-chain attack in two months against the same Microsoft account. "The genius of this Miasma worm lies in how it adhered to legitimate workflows. It does not exploit any software vulnerability in GitHub or npm. Instead, it exploits the underlying trust model of the modern engineering ecosystem." — Cloudsmith What actually changed 73 official Microsoft npm packages were poisoned with the Miasma worm — a clone of TeamPCP's open-sourced Mini Shai-Hulud toolkit Malware executes automatically when any of the 73 packages are opened inside an AI coding agent The payload (28 KB) harvests credentials from AWS, Azure, GCP, Kubernetes, 90+ dev tool configs, and password managers , then spreads laterally through cloud infrastructure Attack vector: stolen Microsoft publisher credentials → bypasses the build pipeline entirely → malicious build published with valid SLSA provenance attestation Each infection gets a uniquely encrypted payload — meaning hash-based IOCs are useless for detection GitHub initially flagged packages as "terms of service violations" rather than malware; Microsoft only acknowledged possible malicious content 48 hours later The same Microsoft account was compromised in May 2026 (durabletask Python SDK on PyPI, 400k downloads/month) — and apparently wasn't fully remediated Why this one stings The supply-chain attack playbook has levelled up. SLSA provenance — the framework designed to give you cryptographic confidence that a package came from a legitimate build — was used against you here. Attackers stole a legitimate Microsoft OIDC token, published a malicious build with real provenance, and conventional scanners waved it through as a routine trusted update. The AI agent angle makes it worse.