CVE-2026-53597: Prompty: Arbitrary code execution via JavaScript frontmatter in TypeScript loader
Summary The TypeScript Prompty loader used gray-matter without overriding executable frontmatter engines. gray-matter supports JavaScript frontmatter blocks such as ---js and evaluates them while parsing. An attacker-controlled .prompty file could therefore execute arbitrary JavaScript during prompt loading.
Affected package - npm @prompty/core v2 prerelease line: >= 2.0.0-alpha.1 < 2.0.0-beta.3 - Fixed in @prompty/core@2.0.0-beta.3
The legacy v1 JavaScript runtime had a historical hardening change for this issue. During the v2 TypeScript runtime rebuild, the loader again called gray-matter directly and the vulnerable behavior was present in the v2 prerelease packages until 2.0.0-beta.3.
Impact Applications that load untrusted .prompty files, user-provided prompt paths, or prompt bundles from less-trusted locations could execute arbitrary JavaScript in the host Node.js process during frontmatter parsing.
Remediation Upgrade @prompty/core to 2.0.0-beta.3 or later.
The fix explicitly overrides the js and javascript gray-matter engines and rejects JavaScript frontmatter in .prompty files. Prompty frontmatter is YAML-only; executable frontmatter is unsupported. A regression test now verifies that ---js frontmatter is rejected and not evaluated.
Fix details Fixed by commit c27402da2487075be577f06aa79df627fb9d6853 and released via typescript/2.0.0-beta.3.
Other sources
Prompty is a markdown file format (.prompty) for LLM prompts. From 2.0.0-alpha.1 until 2.0.0-beta.3, the @prompty/core TypeScript loader in runtime/typescript/packages/core/src/core/loader.ts used gray-matter without overriding executable js and javascript frontmatter engines, allowing an attacker-controlled .prompty file with ---js frontmatter to execute arbitrary JavaScript during prompt loading. This issue is fixed in version 2.0.0-beta.3.
— MITRE
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
npm/@prompty/coreto a version that resolves this vulnerability.Fixed in 2.0.0-beta.3 - Upgrade
Upgrade
@prompty/coreto a version that resolves this vulnerability.Fixed in 2.0.0-beta.3Patch c27402da2487075be577f06aa79df627fb9d6853
Event History
Frequently Asked Questions
What is the severity of CVE-2026-53597?
CVE-2026-53597 has a high severity rating of 8.7 according to the CVSS score.
How does CVE-2026-53597 allow for arbitrary code execution?
CVE-2026-53597 allows arbitrary code execution due to the improper handling of JavaScript frontmatter blocks in the TypeScript Prompty loader.
What software is affected by CVE-2026-53597?
The affected software is the Prompty TypeScript loader, specifically @prompty/core from npm.
What type of security risk does CVE-2026-53597 represent?
CVE-2026-53597 represents a code injection risk due to executing attacker-controlled JavaScript in certain files.
How can I mitigate the risk associated with CVE-2026-53597?
To mitigate CVE-2026-53597, ensure to update the Prompty package to the latest version that addresses this vulnerability.