WordFence is a popular CAPTCHA used by 300,000 sites and recently a backdoor was added to it.
Evidently it was being used to benefit payroll load operations who tend to be paria businesses. With more back-links they get page-ranked higher.
The perpetrators have been identified and an updated version of WordFence should be installed immediately. Other plugins: Convert me Popup, Death To Comments, Human Captcha, Smart Recaptcha, and Social Exchange are also affected.
Soiza and Stacy Wellington and Charlotte Anne Wellington are implicated.