Read the full announcement in the blog or check out the changelog.
Security Fixes
ERC777: fix potential reentrancy issues for custom extensions toERC777. (#2483)
If you're using our implementation of ERC777 from version 3.3.0 or earlier, and you define a custom _beforeTokenTransfer function that writes to a storage variable, you may be vulnerable to a reentrancy attack. If you're affected and would like assistance please write to security@openzeppelin.com. Read more in the pull request.