Affected: anomalyco opencode

OpenCode is an open source AI coding agent. The markdown renderer used for LLM responses will insert arbitrary HTML into the DOM. There is no sanitization with DOMPurify or even a CSP on the web interface to prevent JavaScript execution via HTML injection. This means controlling the LLM response for a chat session gets JavaScript execution on the http://localhost:4096 origin. This vulnerability is fixed in 1.1.10.

Affected: npm: opencode-ai

### Summary A malicious website can abuse the server URL override feature of the OpenCode web UI to achieve cross-site scripting on `http://localhost:4096`. From there, it is possible to run arbitrary commands on the local system using the `/pty/` endpoints provided by the OpenCode API.

### Code execution via OpenCode API

- The OpenCode API has `/pty/` endpoints that allow spawning arbitrary processes on the local machine. - When you run `opencode` in your terminal, OpenCode automatically starts an HTTP server on `localhost:4096` that exposes the API along with a web interface. - JavaScript can make arbitrary same-origin `fetch()` requests to the `/pty/` API endpoints. Therefore, JavaScript execution on `http://localhost:4096` gets you code execution on local the machine.

### JavaScript execution on localhost:4096

The markdown renderer used for LLM responses will insert arbitrary HTML into the DOM. There is no sanitization with DOMPurify or even a CSP on the web interface to prevent JavaScript execution via HTML injection.

This means controlling the LLM response for a chat session gets you JavaScript execution on the `http://localhost:4096` origin. This alone would not be enough for a 1-click exploit, but there's functionality in `packages/app/src/app.tsx` to allow specifying a custom server URL in a `?url=...` parameter:

```javascript // packages/app/src/app.tsx const defaultServerUrl = iife(() => { const param = new URLSearchParams(document.location.search).get(…