Multi-factor authentication is a strong front-door lock…

But it is not the only thing that determines whether someone can get into your accounts.

Once you sign in, your browser keeps you logged in using a session token, often stored as a cookie. Think of it like a wristband at an event: once you have been checked, the wristband shows you belong there. If an attacker manages to steal that wristband, they may not need to beat your MFA prompt at all.

That is the core problem with session cookie hijacking. The attacker is not “cracking” MFA. They are stepping around it by replaying an already authenticated session. This is not a reason to stop using MFA. It is a reason to stop treating MFA as the finish line.

If sessions can be stolen, the practical defence becomes a layered one: phishing-resistant sign-ins, healthy and well-managed devices, tighter session policies, and monitoring that spots suspicious access early.

Why MFA Isn’t a “Game Over” Control

MFA is still one of the best security improvements most businesses can make, but it does not stop every attack on its own. That is because attackers do not always try to break the login process. Sometimes, they try to work around it.

Modern attacks are rarely based on one single trick. They are usually part of a wider chain, using multiple methods to get where the attacker wants to go.

So while MFA can stop a lot of password theft and basic account takeover attempts, it does not automatically protect what happens after a user has successfully signed in. That is where session cookie hijacking becomes important.

Instead of trying to beat MFA directly, attackers may steal the password and the session cookie linked to an authenticated session, then reuse that session for themselves. In other words, they are not breaking MFA. They are reusing what happens after MFA has already done its job.

What a Session Cookie Is and Why Attackers Want It

When you sign into a web app, the service needs a way to remember that you have already proved who you are. That is what a session does: it keeps you in a temporary logged-in state so you do not have to enter your password and MFA code on every click.

Session hijacking is often called cookie hijacking because cookies are commonly used to store the session identifier that keeps you authenticated. And that session identifier is valuable. Attackers want it because it gives them a shortcut.

Session tokens work like digital keys. If someone steals a valid token, they may be able to impersonate a legitimate user and potentially bypass authentication measures, including MFA. That is why session cookie hijacking is such a powerful tactic.

If an attacker can steal the cookie or token representing your active session, they are not trying to defeat the login process itself. They are trying to reuse what you have already completed and gain access to the same apps and data as if they were sitting at your keyboard.

How Session Cookie Hijacking Actually Happens

A lot of people picture account takeover as someone guessing a password or tricking a user into approving an MFA prompt. Session cookie hijacking works differently. The attacker’s goal is to steal proof that you are already logged in, then reuse it, often without triggering another sign-in challenge.

1. AiTM phishing

Adversary-in-the-middle phishing, often shortened to AiTM, is essentially a proxy login trap.

You think you are signing into a normal service, but you are actually entering your details into a convincing lookalike page sitting between you and the real site. The attacker relays the login process in real time, so everything appears to work normally, including MFA.

The key point is that the attacker is not exploiting a flaw in MFA itself. They are capturing the session after MFA has been completed, then reusing it. This approach has been used at scale, which shows just how practical and repeatable it has become for attackers.

2. Browser-in-the-Middle session stealing

Browser-in-the-middle, or BitM, is similar in principle, but even more hands-on. Rather than simply stealing a password and leaving, the attacker effectively places themselves in control of the browsing session.

Once a session token is stolen, the attacker may no longer need to perform the MFA challenge at all. So again, the attacker is not trying to authenticate instead of you. They are trying to ride along after you have already authenticated.

3. Cookie theft from the endpoint

Not every session hijack starts with a sophisticated phishing page. Sometimes the attacker simply steals session data from the device itself. If a device is compromised, valid session tokens can be extracted and reused. Because those tokens behave like digital keys, stealing them can allow an attacker to impersonate a legitimate user.

In some cases, the attacker may specifically target HTTP cookies to gain access or extract sensitive information stored in them.

MFA Is a Baseline, Not a Finish Line

MFA is still essential. It blocks a huge amount of credential theft and makes simple account takeover much harder.

But session cookie hijacking is a useful reminder that attackers do not always try to defeat the login stage. Sometimes, they simply reuse what happens afterwards. That is why the most practical response is layered and realistic.

Make phishing harder to pull off. Treat device health as part of identity. Tighten session behaviour for high-risk apps. Watch for unusual access patterns that suggest a session may be being replayed.

When those controls work together, MFA stops being a comforting checkbox and becomes what it should be: a strong baseline, backed up by protection around the session itself.

If you would like help improving login security and reducing the risk of session hijacking, get in touch with Novo IT.

Article used with permission from The Technology Press.