Tina is a headless content management system. Prior to 2.5.2, the TinaCMS CLI package's Vite dev server packages/@tinacms/cli/src/next/vite/cors.ts origin callback returns false for a disallowed origin but does not reject the request, and packages/@tinacms/cli/src/next/vite/plugins.ts still routes POST /media/upload/* to mediaRouter.handlePost. The upload code in packages/@tinacms/cli/src/next/commands/dev-command/server/media.ts writes attacker-controlled multipart contents inside the configured media root. A remote attacker can cause a developer's browser to submit this state-changing request by inducing the developer to visit an attacker-controlled page while tinacms dev is running. This issue is fixed in version 2.5.2.

Affected: npm: @tinacms/cli

### Summary A browser-based cross-origin request flaw in the Tina dev server allows an attacker-controlled website to cause arbitrary file creation inside the configured media upload directory on a developer machine running `tinacms dev`. No manual file upload is required. The attacker page builds the multipart request in JavaScript and sends it directly to the local Tina server. The browser blocks access to the response because of CORS, but the server still processes the request and writes the file.

### Details The issue is in the dev server path of `@tinacms/cli`.

The Vite dev server installs CORS middleware:

```ts // packages/@tinacms/cli/src/next/vite/plugins.ts server.middlewares.use( cors({ origin: corsOriginCheck, methods: ['GET', 'HEAD', 'PUT', 'PATCH', 'POST', 'DELETE'], }) ); ```

For disallowed origins, the origin callback only returns `false` to the CORS library:

```ts // packages/@tinacms/cli/src/next/vite/cors.ts return (origin, callback) => { ... callback(null, false); }; ```

That does not reject the request server-side. The same request is still routed into the upload handler:

```ts // packages/@tinacms/cli/src/next/vite/plugins.ts if (req.url.startsWith('/media/upload')) { await mediaRouter.handlePost(req, res); return; } ```

The upload handler then accepts attacker-controlled path and body data and writes the file:

```ts // packages/@tinacms/cli/src/next/commands/dev-command/server/media.ts bb.on('file', async (_name, file, _info…