The default authentication service misses to invalidate empty strings as password. Therefore it is possible to authenticate backend and frontend users without password set in the database.
Note: TYPO3 does not allow to create user accounts without a password. Your TYPO3 installation might only be affected if there is a third party component creating user accounts without password by directly manipulating the database.

References

• https://github.com/FriendsOfPHP/security-advisories/blob/master/typo3/cms/2016-04-12-3.yaml
• https://typo3.org/teams/security/security-bulletins/typo3-core/typo3-core-sa-2016-011