What Is the Main Security Challenge Introduced by Sharding a Blockchain?

The main security challenge with sharding is the 'single-shard takeover' risk. By dividing the chain, the total security is also partitioned, meaning a malicious actor would only need to compromise a small fraction of the total network validators to control a single shard.

This allows them to validate invalid transactions on that shard. Solutions like random validator assignment and fraud proofs are implemented to mitigate this risk.