The standard download page here:

https://get.adobe.com/flashplayer/

has a certificate valid until 2019. Where did you see a certificate expired message? Can you copy/paste the text of the error page?

From the Mozilla plugin check page. It says Flash Player is vulnerable. Adobe Flash Player Shockwave Flash 25.0 r0

v. 25.0.0.148 vulnerable

Update Now After I download the file and run I check the certificate and it returns this for the certificate expiration date:‎Sunday, ‎May ‎07, ‎2017 7:59:59 PM.

I just got to check this today and the installer I got was signed on May 19th, so it might have been updated just after you posted. (See attachment.)

You also can try the full installer from this address which seems to work year after year: https://download.macromedia.com/pub/flashplayer/latest/help/install_flash_player.exe