I don't recall it ever being hacked but it did suffer from the ROCA vulnerability few years ago. https://en.wikipedia.org/wiki/ROCA_vulnerability

On Apr 12, 2023, at 15:59, D. Hugh Redelmeier via talk <talk@gtalug.org> wrote:

I seem to remember that Estonia's system was
hacked.  Of course the current non-system is regularly hacked.