Unsecured MongoDB database servers allowed a "white-hat" hacker to access the account information for over 13 million MacKeeper users. MacKeeper is a program that supposedly optimizes your Mac so that it runs faster and safer.. This program, though, has a long and shady history of being labeled as a scareware or a potentially unwanted program due to how their affiliates promote the software using fake security alert advertisements and other scareware methods.
 

 
When Chris Vickery first discovered these open database servers, he downloaded the databases, and tried to contact Kromtech, the developer of MacKeeper. When he received no reply from them he posted about it on the Apple subreddit, which eventually caught the eye of Kromtech.  Since their initial contact, the databases were secured quickly and further information was released about the breach. 

It appears that due to a improper server configuration, these database servers were wide open to the world without any login required to access the data.  They also stated that the only person outside of their organization to access this data was the reporter, who did not share this information with anyone else.  Regardless, anyone who has an account with MacKeeper should immediately change their password at their site and anywhere else that they use the same credentials.