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

#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 security@openzeppelin.com.