I don't know how definitive this is (so take it with a grain of salt), but one of the hosting companies I use sent out an email indicating that non-updated clients will not be able to connect to an updated server (v12), and vice-versa. They'll both have to be updated.
P.S. Not sure why you wouldn't go ahead and update Go 12 to Go 13. Is there something special about holding onto the older version?
Thanks for the reply, I might set up a test server to verify this before I hit the button on our production server.
The reason we have not updated some clients to 13 is that they are using iPod Touch Gen 3 devices which only support iOS6 which in turn, only supports FMGo v12. To make matters worse, these are actual clients buying in to a solution so forcing them to upgrade devices they bought just 2 - 3 years ago isn't something we want to do right now!
If we are forced to adopt this Security patch on our FM12 server because FMGo v13.0.9 will not connect to un-patched servers, we will be in something of a catch-22 situation.
Following up on this, I performed a test installation of FMSA188.8.131.520 and then applied the 4/21/15 update and it would appear Mike's info was correct.
FileMaker Go Clients with a version < 13.0.9 are NOT able to connect to a FileMaker 12 Server which has the 4/21/15 Security patch applied if SSL is enabled.
Also, FileMaker Go Clients v13.0.9 (or later presumably) are NOT able to connect to a FileMaker 12 Server WITHOUT the 4/21/15 Security patch applied if SSL is enabled.