[Webkit-unassigned] [Bug 160960] New: WebKit blocks 'unsafe-inline' event handler upon execution, not upon setting

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=160960

            Bug ID: 160960
           Summary: WebKit blocks 'unsafe-inline' event handler upon
                    execution, not upon setting
    Classification: Unclassified
           Product: WebKit
           Version: Safari 9
          Hardware: All
                OS: All
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: WebCore JavaScript
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: april at pokeinthe.io

OS Version: OS X 10.11
URLs (if applicable) : http://output.jsbin.com/togusapobi / http://output.jsbin.com/wutojesugi
Other browsers tested:
  Add OK or FAIL after other browsers where you have tested this issue:
       Safari: FAIL
   Firefox 48: OK
     IE 7/8/9: N/A
         Edge: OK
    Chrome 52: FAIL

What steps will reproduce the problem?
1. Set Content-Security-Policy to block 'unsafe-inline'
2. Add an event handler to an attribute via setAttribute('onclick') or via onclick attribute added to DOM node

What is the expected result?

In Firefox and Edge, this code will be successfully blocked on being set:

document.querySelector('button').setAttribute('onclick', 'alert(1)')

As will this inline event handler:

<button onclick="alert(1);">yay</button>

What happens instead of that?

WebKit-based browsers block it upon execution, in violation of the CSP specification:

>  When an event handler content attribute is set, execute the following steps:
>  
>    If the Should element's inline behavior be blocked by Content Security Policy? algorithm returns "Blocked" when executed upon the attribute's element, "script attribute", and the attribute's value, then abort these steps. [CSP]

See also:
https://bugs.chromium.org/p/chromium/issues/detail?id=638957
https://bugzilla.mozilla.org/show_bug.cgi?id=1296027

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