md-to-pdf vulnerable to arbitrary JavaScript code execution when parsing front matter

Summary

A Markdown front-matter block that contains JavaScript delimiter causes the JS engine in gray-matter library to execute arbitrary code in the Markdown to PDF converter process of md-to-pdf library, resulting in remote code execution.

Details

md-to-pdf uses the gray-matter library to parse front-matter. Gray-matter exposes a JavaScript engine that, when enabled or triggered by certain front-matter delimiters (e.g. ---js or ---javascript), will evaluate the front-matter contents as JavaScript. If user-supplied Markdown is fed to md-to-pdf and the front-matter contains malicious JS, the converter process will execute that code.

PoC

const { mdToPdf } = require('md-to-pdf');

var payload = '---javascript\n((require("child_process")).execSync("calc.exe"))\n---RCE';

(async () => {
	await mdToPdf({ content: payload }, { dest: './output.pdf'});
})();

Running the PoC on Windows launches the calculator application, demonstrating arbitrary code execution.

Impact

• Remote code execution in the process that performs Markdown->PDF conversion.
• If the converter is run in a web app or cloud service, an attacker uploading malicious Markdown can execute arbitrary commands on the