CVE-2026-73299: Microsoft Prompty

Critical severity, CVSS 10.0. EPSS: 1.8% chance of exploitation in the next 30 days.

Prompty is a markdown file format (.prompty) for LLM prompts. Prior to 0.1.5 and 2.0.0-beta.5, the TypeScript Nunjucks renderer evaluated untrusted .prompty template bodies with unrestricted JavaScript member access. An attacker-controlled template could traverse constructor and prototype properties to execute JavaScript in the host Node.js process. This issue is fixed in versions 0.1.5 and 2.0.0-beta.5.

Affected products

• Microsoft Prompty: before 0.1.5 (fixed in 0.1.5); from 2.0.0-alpha.1, before 2.0.0-beta.5 (fixed in 2.0.0-beta.5)

Published 2026-08-12. Last modified 2026-09-09.