openzeppelin/contracts and openzeppelin/contracts-upgradeable are vulnerable to Initializer reentrancy that may lead to double initialization. Initializer functions that are invoked separate from contract creation (the most prominent example being minimal proxies) may be reentered if they make an untrusted non-view external call. Once an initializer has finished running it can never be re-executed. However, an exception put in place to support multiple inheritance made reentrancy possible in the scenario described above, breaking the expectation that there is a single execution. Note that upgradeable proxies are commonly initialized together with contract creation, where reentrancy is not feasible, so the impact of this issue is believed to be minor. A fix is included in the version v4.4.1 of @openzeppelin/contracts and @openzeppelin/contracts-upgradeable. Vulnerable versions are 3.2.0 through 4.4.0.