A simple browser extension which checks for window.opener vulnerabilities as you browse. When it finds one, it shows a warning page in the vulnerable tab, so that you can report or fix the vulnerability. (Use the back button to return to the page.) Individual pages and whole domains can be added to an ignore list, and by default, vulnerabilities between pages of the same origin (e.g. https://example.com/foo.html and https://example.com/bar.html) are not reported, though this can be changed in the settings.
window.opener vulnerabilities allow Web pages to control the tab which opened them. They can be fixed on many browsers simply by adding rel=noopener to your links. For more details, check out Mathias Bynens' article.
Please use responsibly. Disclose vulnerabilities you find, or fix them. Keep your hat white.
Contributions welcome! Check out the GitHub issues if you're looking for something to do.
Install it through the Chrome Web Store or addons.mozilla.org.