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Anchor tag error

When I was finishing my project I found the following error:

• Link 'rel' attribute should include 'noopener'.

The code was written as follows:

<footer>

  Challenge by <a  
    href="https://www.frontendmentor.io?ref=challenge"  
    target="_blank">Frontend Mentor</a>

  Coded by <a  
    href="https://github.com/caduovieira"  
    target="_blank">Cadu Vieira</a>

</footer>

If someone could help me explaining why this happened, I'd be thankful.