@selianto pfSense 2.3.x is End of Life. As such, we do not offer older releases for download, nor do we support them.
If you have an urgent upgrade project, you should consider a Netgate Global Support subscription. The Support team may be able to assist with the upgrade in a way that does not require having to use an outdated image.
If you would like more information about a Netgate Global Support subscription, please email sales@netgate.com or if you need help now, you can find our different Netgate Global Support subscriptions here: https://netgate.com/support