The PayPal merchant SDK does not verify that the server hostname matches a domain name in the subject's Common Name (CN) or subjectAltName field of the X.509 certificate, which allows man-in-the-middle attackers to spoof SSL servers via an arbitrary valid certificate.
https://github.com/paypal/SDKs/commit/5f2d6dd77fb4211dcde34e36f1864234526c5d64
https://exchange.xforce.ibmcloud.com/vulnerabilities/79913