In order to ensure that you also get those replies, the attacker can set up a forwarding rule in their own email account and automatically forward those replies to your address.
3. Two-factor authentication everywhere
Turn on two-factor authentication -- this is sometimes called two-step verification -- for any account that supports it, including Yahoo. This will prompt the online service to ask for a one-time-use code sent via text message, phone call, email or generated by a smartphone app when you try to access the account from a new device. This code is required in addition to your regular password, but Yahoo also has a feature called Account Key that does away with regular passwords completely and instead requires sign-in approval via phone notifications.
Two-factor authentication is an important security feature that could keep your account secure even if hackers steal your password.
4. Never reuse passwords
There are many secure password management solutions available today that work across different platforms. There's really no excuse for not having unique, complex passwords for every single account that you own. If you do want memorable passwords for a few critical accounts use passphrases instead: sentences made up of words, numbers and even punctuation marks.
According to Yahoo, this breach happened in August 2013, at a time when the company hadn't yet switched to the more secure bcrypt password hashing algorithm. As a result, most passwords that were stolen are in the form of MD5 hashes, which are highly vulnerable to cracking.
If you made the mistake of using your Yahoo password elsewhere and haven't changed it yet, you should do so immediately and review the security settings of those accounts too. It's very likely that hackers have already cracked your password and had three years to abuse it.
5. Phishing follows breaches
Large data breaches are typically followed by email phishing attempts, as cybercriminals try to take advantage of the public interest in such incidents. These emails can masquerade as security notifications, can contain instructions to download malicious programs that are passed as security tools or can direct users to websites that ask for additional information under the guise of "verifying" accounts.
Be on the lookout for such emails and make sure that any instructions you decide to follow in response to a security incident came from the affected service provider or a trusted source. Official Yahoo emails are easily recognizable in the Yahoo Mail interface because they are marked with a purple Y icon.
In the future, be selective in what personal information you choose to share and which websites you choose to share it with, even when those websites are legitimate. There's no guarantee that they won't be hacked in the future and you simply don't know how securely they store your details.
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