There's a number of news articles on this but I'm linking the original paper instead.

Some news sites which picked it up:

http://www.information-age.com/technology/security/123459001/major-security-alert-40000-mongodb-databases-left-unsecured-internet

http://www.cso.com.au/article/566040/students-find-40k-unprotected-mongodb-databases-8-million-telco-customer-records-exposed/