"The vulnerability itself was found within a test bed for BillMeLater’s SOAP API, which allowed for queries to be made to any given host URL." researcher explained in his blog post.
An attacker is able to send request to any internal network through the API and get the response. Some internal admin pages allowed him to query internal databases without asking any login credentials.
Researcher says that a successful exploitation may result in compromising the customers data.
The bug was reported to Paypal on October 2013 and he got reward from them on Jan. 2014.
Paypal has partially fixed the bug by restricting the SOAP API to access the internal servers. However, researcher says that it still act as proxy to view other hosts.
If you would like to know more details about SSRF vulnerability and how it can be exploited for port scanning or internal network finding, you can refer the Riyaz Waliker blog post and this document.