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
.
Avoid untrusted external calls during initialization.
https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3006
This issue was identified and reported by @chaitinblockchain through our bug bounty on Immunefi.
If you have any questions or comments about this advisory, or need assistance executing the mitigation, email us at [email protected].