Ethereum Will Be the Backbone of the New Internet
medium.com
medium.com
That might mean there aren't any, or that there aren't many people looking, or any exploit is really hard to find and hasn't been figured out yet, or that a malicious attacker has found one but hasn't used it yet because they're waiting to maximise their gain.
The fact there aren't any published exploits is not a sign something is secure (or not secure).
And anyway, if someone hacks a small contract s/he will get away and keep the money, but if someone hacks the main net then Vitalik will undo^H^H^H^H fork and make it worthless.