Unprotected function

Missing (or incorrectly used) modifier on a function allows an attacker to use sensitive functionality in the contract.

Attack Scenario

A contract with a changeOwner function does not label it as private and therefore allows anyone to become the contract owner.

Always specify a modifier for functions.

An onlyOwner modifier is defined but not used, allowing anyone to become the owner
April 2016: Rubixi allows anyone to become owner
July 2017: Parity Wallet. For code, see initWallet
BitGo Wallet v2 allows anyone to call tryInsertSequenceId. If you try close to MAXINT, no further transactions would be allowed. Fix: make tryInsertSequenceId private.
Feb 2020: Nexus Mutual's Oraclize callback was unprotected—allowing anyone to call it. Oraclize triggers a rebalance to occur via Uniswap.