This script attempts to detect the Ebury ssh backdoor based on having base64 in the ssh client string.
Details about Ebury and original snort rules: https://www.welivesecurity.com/2014/02/21/an-in-depth-analysis-of-linuxebury/
More recent versions of Ebury will sometimes break up their base64 with a space to throw off more generic base64 regex. As a result we stick with just checking for a trailing '=' padding character. It's very simple and could miss some infections, but should result in a lot less false positives.