Warning: Hackers could take over your email account by stealing cookies, even if you have MFA

The Federal Bureau of Investigation (FBI) has issued a warning that cybercriminals are taking over email accounts via stolen session cookies, allowing them to bypass the multi-factor authentication (MFA) a user has set up.

Here’s how it works.

Most of us don’t think twice about checking the “Remember me” box when we log in. When you log in and the server has verified your authentication—straight away or after using MFA–the server creates a session and generates a unique session ID. This session ID is stored in a session cookie (or a “Remember-Me cookie” as the FBI calls it) on your browser, which is typically valid for 30 days.

Every time you return to that website within the time frame, you don’t need to log in. That’s really convenient… unless someone manages to steal that cookie from your system.

If someone steals the session cookie, they can log in as you—even if you have MFA enabled.

This is particularly relevant for email handlers that have an online—webmail—component. This includes major players like Gmail, Outlook, Yahoo, and AOL.

With access to your email account, a cybercriminal can find a lot of useful information about you, such as where you bank, your account numbers, your favorite shops, and more. This information could then be used for targeted cyberattacks that mention information that’s relevant to you only, leaving you more likely to fall for them.

Cybercriminals could use your account to spread spam and phishing emails to your contacts. And perhaps most worrying of all, once an attacker is in your email account they can reset your passwords to your other accounts and login as you there too.

How do these criminals get their hands on your session cookies? There are several ways.

On very rare occasions, session cookies can be stolen by you visiting a malicious website, or via a Machine-in-the-Middle (MitM) attack where a cybercriminal can intercept traffic and steal cookies if they’re not protected by HTTPS on an unsecured network.

However, session cookies are usually stolen by malware on the your device. Modern information-stealing malware is capable of, and even focuses on, stealing session cookies as part of its activity.  

How to keep your email account safe

There are a few things you can do to stay safe from the cookie thieves:

Use security software on every device you use. Keep your devices and the software on them up to date, so there aren’t any known vulnerabilities on them. Decide whether you think it’s worth using the Remember me option. Is convenience worth the risk in this situation? Delete cookies, or—even better—log out when you are done. That should also remove or invalidate the session ID from the server, so nobody can use it to log in, even if they have the session cookie. Only visit sites with a secure connection (HTTPS) to protect your data from being intercepted during transmission. For important accounts regularly check the log in history where you can see which devices logged in when and from where. You should be able to find this option in your account settings.

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