Can the CEI Pattern Prevent All Types of Reentrancy?

The Checks-Effects-Interactions (CEI) pattern is highly effective against the classic, direct reentrancy attack that drains funds. However, it cannot prevent all types.

It does not prevent "read-only" reentrancy, where the attacker calls back to read a stale state variable before the Effects step is complete. It also may not prevent cross-contract reentrancy in complex systems where Contract A calls B, which calls C, and C then calls back to A in an unexpected way.

For full protection, CEI is often paired with a reentrancy guard.