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/corev2 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.
Affected software
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| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| npm package | @prompty/core | >= 2.0.0-alpha.1, < 2.0.0-beta.3 | 2.0.0-beta.3 |
References
- CVSS vector
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N- Weakness (CWE)
CWE-94- Platform
- npm
- Package
@prompty/core- Disclosed
- 17.07.2026
- Updated
- 11.10.2026 10:22
Sources and licenses
OSV / GitHub Advisory Database - GHSA-c4gh-rv8h-q9vw - CC BY 4.0
Vulnerability history: @prompty/core
All published vulnerabilities of this package (npm package), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 3
- First → latest
- 07.2026 07.2026
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- Critical 1
- High 2
Disclosures per year
- 2026: 3
-
Prompty: Arbitrary code execution via JavaScript frontmatter in TypeScript loader this entry
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