MFA is a strong front-door lock. But it’s not the only thing that decides whether someone can get in.
After you sign in, your browser keeps you logged in using a session token (often stored as a cookie). It’s the digital version of a wristband at an event: once you’ve been checked, the wristband proves you belong there.
If an attacker steals that wristband, they may not need to beat your MFA prompt at all.
That’s the core of session cookie hijacking. The attacker isn’t “cracking” MFA. They’re skipping it by replaying your already authenticated session.
This isn’t a reason to stop using MFA. It’s a reason to stop treating MFA as the finish line.
Why MFA isn’t a “game over” control
MFA is still one of the best upgrades most businesses can make, but it doesn’t end an attack on its own.
The reason is that attackers don’t always try to beat the login step. They try to go around it.
Cloudflare notes that “attackers are finding new ways to circumvent MFA” and that modern incidents are rarely one isolated technique. They’re “part of a chain of attacks.”
In other words, MFA can block a lot of credential theft, but it doesn’t automatically protect what happens after a user successfully signs in.
That’s where session cookie hijacking comes in.
What a session cookie is and why attackers want it
When you sign into a web app, the site needs a way to remember that you’ve already proved who you are.
That’s what a session is: a temporary “logged-in” state that saves you from entering your password and MFA code on every click.
Kaspersky explains that session hijacking is “sometimes called cookie hijacking” because cookies are commonly used to store the session identifier that keeps you authenticated.
Proofpoint describes session tokens as digital “keys” that let a user stay authenticated. It warns that stealing valid tokens lets attackers impersonate legitimate users and potentially bypass authentication measures “like MFA.” That’s why session cookie hijacking is so highly leveraged.
If an attacker can steal the cookie or token that represents your active session, they’re not trying to defeat the login process. They’re attempting to reuse what you already completed and access the same apps and data as if they were sitting at your keyboard.
How session cookie hijacking actually happens
AiTM phishing – Adversary-in-the-middle (AiTM) phishing is the “proxy login” trap. You think you’re signing into a normal service, but you’re actually signing into a lookalike page that sits between you and the real site.
The attacker relays the login in real time, so everything appears to work, including MFA.
Browser-in-the-Middle session stealing. It’s similar in spirit, but it’s even more “hands- on” from the attacker’s side. Instead of stealing a password and running away, the attacker effectively places themselves in control of the browsing session.
Cookie theft from the endpoint. Not every session hijack starts with a fancy proxy. Sometimes, the attacker simply steals session data from the device itself, allowing attackers to impersonate legitimate users.
MFA is a baseline, not a finish line
MFA is still essential. It blocks a huge amount of credential theft and makes basic account takeover harder.
But session cookie hijacking is a reminder that attackers don’t always try to defeat the login step. Sometimes, they reuse what happens after it.
The practical response is layered and realistic. When those controls work together, MFA stops being a checkbox and becomes a strong baseline backed by protections around the session itself.