This is an unfortunate example of why you need a good maintenance plan for your website, such as our Maintenance +Security plans we offer.

Security issues that could degrade your site like this pop up for all types of websites powered by content management systems.  Not to scare anyone into spending money, but need to be aware and be informed of how your website may be at risk, regardless of what web company you happen to work with.

The backdoor code in all three plugins works in a very similar way by calling to a remote server and inserting content and links on the affected sites. Experts believe the backdoor code is used to inject hidden SEO spam (cloaked links) on affected sites that help improve the search engine ranking of other sites.

https://www.bleepingcomputer.com/news/security/three-more-wordpress-plugins-found-hiding-a-backdoor/