[Webkit-unassigned] [Bug 231944] New: Use of window.alert is not allowed in different origin-domain frames despite `allow-same-origin` and `allow-modals`

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=231944

            Bug ID: 231944
           Summary: Use of window.alert is not allowed in different
                    origin-domain frames despite `allow-same-origin` and
                    `allow-modals`
           Product: WebKit
           Version: Safari 15
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Frames
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: yanahij531 at carpetd.com

Created attachment 441706

  --> https://bugs.webkit.org/attachment.cgi?id=441706&action=review

Video recording of error on safari 15

Please try visiting this URL in Safari 15:

https://safari-15-cross-domain-iframe-modal-bug.glitch.me/

The page has a cross-domain iframe in it:

```
<iframe src="https://safari-15-cross-domain-iframe-modal-bug-embed.glitch.me" sandbox="allow-scripts allow-same-origin allow-modals"></iframe>
```

And the source code for that iframe embed is just:

```
<script>alert(1)</script>
```

Since I've added `sandbox="allow-scripts allow-same-origin allow-modals"` to the iframe, I believe the modals should be allowed. Instead the following error is shown in the console:

```
Use of window.alert is not allowed in different origin-domain frames
```

The other major browsers correctly allow the modal with those attributes, and this behavior of allowing modals with those sandbox flags is discussed here:

https://github.com/whatwg/html/issues/5407#issuecomment-775621443

This bug does not exist on Safari 13 or 14. Please see attached video recording.

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