GHSA-c83v-7274-4vgp: Malicious website can execute commands on the local system through XSS in the OpenCode web UI
SummaryA 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 APIThe 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:4096The 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:Using this custom server URL functionality, you can make the web UI connect to and load chat sessions from an OpenCode instance on another URL. For example, tricking a user into opening http://localhost:4096/Lw/session/ses_45d2d9723ffeHN2DLrTYMz4mHn?url=https://opencode.attacker.example in their browser would load and display `ses_45d2d9723ffeHN2DLrTYMz4mHn` from the attacker-controlled server at https://opencode.attacker.example.Note on exploitabilityBecause the localhost web UI proxies static resources from a remote location, the OpenCode team was able to prevent exploitation of this issue by making a server-side change to no longer respect the `?url=` parameter. This means the specific vulnerability used to achieve XSS on the localhost web UI no longer works as of `Fri, 09 Jan 2026 21:36:31 GMT`. Users are still strongly encouraged to upgrade to version 1.1.10 or later, as this disables the web UI/OpenCode API to reduce the attack surface of the application. Any future XSS vulnerabilities in the web UI would still impact users on OpenCode versions before 1.10.0.Proof of ConceptA simple way to serve a malicious chat session is by setting up mitmproxy in front of a real OpenCode instance. This is necessary because the OpenCode web UI must load a bunch of resources before it loads and displays the chat session.Spawn an OpenCode instance in a Docker containerCreate a file called `plugin.py` with the contents belowStart mitmproxy with the plugin in reverse proxy modeStart OpenCode in your terminal as the victimVisit the following URL in a browser on the same machine running OpenCode: http://localhost:4096/Lw/session/ses_45d2d9723ffeHN2DLrTYMz4mHn?url=http://localhost:12345Confirm the file `albert-was-here.txt` was created in the `/tmp/` directory
CVSS v4.0: 9.4