OpenZeppelin Contracts initializer reentrancy may lead to double initialization

Impact

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.

Patches

A fix is included in the version v4.4.1 of @openzeppelin/contracts and @openzeppelin/contracts-upgradeable.

Workarounds

Avoid untrusted external calls during initialization.

References

https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3006

Credits

This issue was identified and reported by @chaitinblockchain through our bug bounty on Immunefi.

For more information

If you have any questions or comments about this advisory, or need assistance executing the mitigation, email us at [email protected].