What happens if a sidechain network is insecure, and someone creates coins out of nowhere and integrates them back into the main bitcoin blockchain? Do sidechains increase the surface area for bitcoin vulnerabilities?
My understanding is that the worst two things that can happen are: 1) an attacker prevents a holder of bitcoins on a sidechain from reclaiming them on the bitcoin network (e.g. by preventing the relevant transaction getting into a block on the sidechain side)... this would be a net-plus for other bitcoin holders, I guess... since they would then own relatively more of them. 2) an attacker finds a way to release the coins on the bitcoin side. That would be bad for the rightful owner, of course, but it has no impact on anybody else on the bitcoin side.
If this happened, then the sidechain would effectively become "insolvent" (because the original bitcoin blockchain will never allow more coins to be returned than have been taken out of it).
There would then be a bank run from the sidechain, and some people would be left with unredeemable sidechain tokens.