hacktricks/pentesting-web/postmessage-vulnerabilities.md

PostMessage Vulnerabilities

Tips/Bypasses in PostMessage vulnerabilities

Copied from https://jlajara.gitlab.io/web/2020/07/17/Dom_XSS_PostMessage_2.html

• If indexOf() is used to check the origin of the PostMessage event, remember that it can be bypassed if the origin is contained in the string as seen in The Bypass
• @filedescriptor: Using search() to validate the origin could be insecure. According to the docs of String.prototype.search(), the method takes a regular repression object instead of a string. If anything other than regexp is passed, it will get implicitly converted into a regexp.

"https://www.safedomain.com".search(t.origin)

In regular expression, a dot . is treated as a wildcard. In other words, any character of the origin can be replaced with a dot. An attacker can take advantage of it and use a special domain instead of the official one to bypass the validation, such as www.s.afedomain.com.

• @bored-engineer: If escapeHtml function is used, the function does not create a new escaped object, instead it over-writes properties of the existing object. This means that if we are able to create an object with a controlled property that does not respond to hasOwnProperty it will not be escaped.

// Expected to fail:
result = u({
  message: "'\"<b>\\"
});
result.message // "&#39;&quot;&lt;b&gt;\"
// Bypassed:
result = u(new Error("'\"<b>\\"));
result.message; // "'"<b>\"

File object is perfect for this exploit as it has a read-only name property which is used by our template and will bypass escapeHtml function.