In Serendipity before 2.0.5, an attacker can bypass SSRF protection by using a malformed IP address (e.g., http://127.1) or a 30x (aka Redirection) HTTP status code.
https://github.com/s9y/Serendipity/commit/fbdd50a448ed87ba34ea8c56446b8f1873eadd6f
https://blog.s9y.org/archives/271-Serendipity-2.0.5-and-2.1-beta3-released.html