[pcthrowaway]

This is exactly what happened with Ethereum in the early days when a bad actor was able to exploit a third-party contract to the tune of 5% of all ETH.

The Ethereum everyone talks about today is the fork, due to the Ethereum Foundation which owns the trademark leading the fork.

The Ethereum blockchain with the unaltered history is called Ethereum Classic

https://en.wikipedia.org/wiki/Ethereum_Classic#The_DAO_bailo...

[dyndos]

Note that the way this fork was pulled off was very ad-hoc.

Ethereum devs were unable to create a legitimate transaction reverting the DAO funds because they do not have access to the hackers' private key. The reversion was done with a "surgical state change" hardcoded into the client itself.