This is a serious vulnerability which allows an unauthenticated remote attacker to retrieve information from MongoDB's memory. A proof-of-concept is available to the public.
Similar to other heap disclosure vulnerabilities such as Heartbleed, the impact of exploitation will vary depending on the information an attacker is able to obtain from the heap. However, it is quite likely that the leaked memory will contain credentials or other sensitive information, especially as attackers learn more about the vulnerability and use it more effectively.
Regardless of patch status, MongoDB should not be exposed to the internet and access should be restricted by a firewall or similar controls. You should also apply the patch as soon as possible, to avoid the vulnerability being exploited internally.