Summary tinacms uses the gray-matter package in an insecure way allowing attackers that can control the content of the processed markdown files, e.g., blog posts, to execute arbitrary code.

Details The gray-matter package executes by default the code in the markdown file's front matter. tinacms does not change this behavior when process markdown file, e.g., by passing a custom engine property for js/javascript in the options object.

PoC 1. Create a tinacms app using the cli/documentation: npx create-tina-app@latest 2. Modify one of the blog posts to contain the following front matter: js ---js { "title": "Pawned" + console.log(require("fs").readFileSync("/etc/passwd").toString()) } --- 3. Start the tinacms server, e.g., with npm run dev 4. Observe the console of the server printing the password file, showing that attackers can execute arbitrary commands.

Impact RCE: attackers can execute arbitrary JavaScript code on the server hosting tinacms.

Feasibility Potential attack scenarios can be executed like this: Companies often have technical writers as contractors. These contractors produce md files, which they send over email or upload in a shared cloud folder. Developers download these files and upload them in tinacms's content folder. While this example might appear speculative or contrived, a general observation is that developers would be very surprised to find out that processing untrusted markdown files via tinacms = server-side code execution = complete machine take over. That is, tinacms users might not expect markdown files to contain anything else than data and gray-matter violates that assumption.