Bypassing MFA: A Guide to Session Cookie Hijacking

By ZedX

You did it. You finally enabled Multi-Factor Authentication (MFA) on your bank, your email, your corporate VPN. You get that little code on your phone and feel a warm sense of security. You think you’ve built a fortress. You think you’ve stopped people like me.

That’s cute.

You’ve built a bigger, stronger front door. But you’re leaving the windows wide open. You think MFA is a magic shield, but we’re not trying to break the shield. We’re just walking around it. And our favorite way to do that is by stealing the one thing you don’t even think about: your session cookie.

Your False Sense of Security: The MFA Illusion

Let’s be clear. MFA is a good thing. It stops the low-hanging fruit the script kiddies using credential stuffing attacks with old, breached passwords. When a login requires both a password (something you know) and a code from your phone (something you have), it becomes much harder to brute-force.

But here’s the secret you don’t understand: MFA is typically only enforced at the point of login. It’s a bouncer checking IDs at the door. Once you’re inside the club, you get a wristband that proves you belong. You can walk around freely without showing your ID again.

In the digital world, that wristband is your session cookie. And we don’t need to fight the bouncer if we can just steal your wristband.

The Real Golden Ticket: Understanding Session Cookies

When you successfully log in to a website, the server’s job is to remember you. It can’t ask for your password and MFA code every single time you click a new page; you’d go insane. So, after you prove who you are, the server gives your browser a small piece of data a session cookie.

Think of it as a temporary keycard. The cookie is just a string of text, something like this:

$$Set-Cookie: sessionid=a1b2c3d4e5f67890…; Secure; HttpOnly$$

For the rest of your visit, your browser sends that cookie back to the server with every request. The server sees the valid cookie and says, “Ah, yes, ZedX. I remember him. Access granted.”

This cookie is the proof that you’ve already authenticated. It’s the proof that you’ve already passed the MFA check. If I can get my hands on that cookie, I become you. The server won’t know the difference.

The Hunt: How We Steal Your Session

Stealing a cookie isn’t magic; it’s just a matter of finding the right vulnerability. We have several tried-and-true methods.

• Infostealer Malware: This is the most common and effective way. We trick you into running a malicious program maybe through a phishing email with a “harmless” attachment or a dodgy software download. This malware sits quietly on your system and does one thing very well: it exfiltrates data. It rummages through your browser’s storage and steals saved passwords, crypto wallets, and, most importantly, all your active session cookies. You log in, our malware grabs the cookie, and sends it directly to us. Game over.
• Man-in-the-Middle (MitM) Attacks: Are you on public Wi-Fi at a coffee shop or airport? I hope not. It’s child’s play for us to set up a rogue access point (e.g., “Free_Airport_WiFi”) and position ourselves between you and the internet. If you visit a website that isn’t properly secured with HTTPS, we can intercept the traffic and pluck your session cookie right out of the air. You’re checking your email, and we’re logging in as you from the table across the room.
• Cross-Site Scripting (XSS): This one is more elegant. We find a flaw on a website you trust a forum, a blog, an e-commerce site that allows us to inject a small piece of malicious JavaScript. When your browser loads the compromised page, our script runs and steals the cookie associated with that site, sending it to a server we control. You won’t see a thing.

The Takeover: Using the Stolen Cookie

Once we have your cookie, the hard part is over. We don’t need your password. We don’t need your phone. We don’t need to see your MFA prompt. That check has already been passed by you.

We simply take the stolen cookie and inject it into our own browser using a simple extension. Then, we navigate to the website. The server sees the valid session ID from our browser, assumes we are you, and grants us full access to your account. We can read your emails, transfer your money, access your company’s private data anything you could have done while logged in.

The entire MFA process is bypassed because, from the server’s perspective, we aren’t logging in at all. We are simply continuing a session that you so kindly started for us.

So, You Think You’re Safe?

MFA is not a silver bullet. It’s just one layer, and a bypassable one at that. If you want to make our lives harder, you need to protect the session itself.

1. Don’t Get Infected: The number one defense is not running malware in the first place. Be ruthlessly skeptical of email attachments and downloads. Keep your operating system and antivirus software updated. This protects you from infostealers.
2. Avoid Public Wi-Fi: If you absolutely must use it, use a reputable VPN. A VPN encrypts your traffic, making MitM attacks nearly impossible.
3. Log Out: When you’re done with a sensitive site, don’t just close the tab. Actively click “Log Out.” This invalidates the session cookie, making it useless even if it’s stolen.
4. Look for HTTPS: Never enter sensitive information on a site that doesn’t have a padlock icon in the address bar. It’s a basic check, but you’d be surprised how many people forget it.

Go ahead, layer up your security. We love a good challenge. Just remember, for every fortress you build, someone like me is already looking for the open window.