Protecting Your Firm from “Session Hijacking”: Why MFA Alone Won’t Stop Cookie Theft

Article summary: Multi-factor authentication confirms who you are at login. Session hijacking attacks after that point, stealing the cookie that proves login already happened. By the time a session cookie is stolen, MFA is irrelevant. Session hijacking protection requires a different set of controls: shorter session lifetimes, conditional access policies, and monitoring for anomalous session behavior.

Multi-factor authentication (MFA) remains one of the most effective ways to protect business accounts. It stops the vast majority of attacks that rely on stolen or guessed passwords, making it an essential part of any cybersecurity strategy.

Unfortunately, attackers have adapted. Rather than trying to defeat MFA, many now target what happens after a user has successfully signed in.

This technique, known as session hijacking, focuses on stealing a session cookie, a small piece of data that tells an application you’ve already verified your identity. If an attacker obtains that cookie, they may be able to access your account without knowing your password or completing the second authentication step.

Protecting against session hijacking requires more than enabling MFA. It also requires security controls that can detect and stop suspicious activity after a user has signed in.

Why MFA Alone Cannot Stop Session Hijacking

Multi-factor authentication protects the sign-in process. Session hijacking targets what happens after a user has successfully signed in. The two security controls operate at different stages of the authentication process.

After a successful login, applications issue a session cookie or authentication token that tells the service the user has already verified their identity. If an attacker steals that session, they can often continue using it without entering a password or responding to another MFA prompt. From the application’s perspective, it appears to be the same authenticated user continuing their work.

This is why session hijacking has become such an effective attack technique. In its 2026 Identity Exposure Report, SpyCloud reported recovering 8.6 billion stolen cookies and session artifacts from malware infections, highlighting the growing scale of session theft.

How Session Cookies Get Stolen

Adversary-in-the-Middle (AiTM) phishing

An AiTM attack uses a proxy between the victim and the real login page. 

The victim authenticates normally, including MFA. The proxy captures the session cookie as it is issued and relays it to the attacker in real time.

Commercial phishing kits like Tycoon 2FA and EvilProxy make this technique available to attackers with no technical background. 

HP Wolf Security reported that, among the documented Microsoft 365 incidents it analyzed, token theft was the most common MFA bypass technique, accounting for 31% of cases.

Infostealer malware

Infostealer malware is designed to steal sensitive information from an infected device, including the browser session cookies that keep users signed in to websites and cloud applications. Malware families such as LummaC2, RedLine, and Vidar quietly collect this data and send it to attackers.

Stolen sessions are not always used immediately. Attackers may wait days or even weeks before exploiting them, making unauthorized access harder to detect.

Malicious or compromised browser extensions

Browser extensions with broad permissions can access sensitive information stored by the browser, including authentication data. 

If an employee installs a malicious or compromised extension, it may be able to steal active session information without the user’s knowledge.

Security Controls That Help Prevent Session Hijacking

Conditional access policies

Conditional access goes beyond verifying a username and password. It evaluates factors such as device health, location, user risk, and sign-in behavior before allowing access. 

If an attacker attempts to use a stolen session from an unfamiliar device or suspicious location, conditional access policies can block or limit access even when the session appears valid.

Microsoft Entra and Google BeyondCorp both support conditional access, making it one of the most direct controls available.

Short session lifetimes and re-authentication 

The longer a session remains active, the more time an attacker has to abuse a stolen session cookie. Limiting session duration and requiring users to re-authenticate before accessing sensitive applications or changing security settings reduces the opportunity for a hijacked session to cause damage.

Continuous session monitoring

Modern identity platforms can detect unusual session activity, such as access from an unexpected location, impossible travel, multiple active sessions from different regions, or sudden changes in device characteristics. 

Our post on moving beyond basic compliance audits covers why continuous monitoring catches exactly these patterns, which point-in-time reviews miss.

Phishing-resistant MFA for high-value accounts

Phishing-resistant authentication methods, such as FIDO2 security keys and passkeys, make it much harder for attackers to steal credentials through adversary-in-the-middle (AiTM) phishing attacks.

For administrators, executives, and other users with access to sensitive systems, they provide an additional layer of protection against modern account compromise.

MFA Is Still Essential

Multi-factor authentication remains one of the most effective ways to protect business accounts and should be enabled wherever possible.

The challenge is that session hijacking targets what happens after a user has successfully signed in. Reducing that risk requires additional safeguards, such as conditional access policies, shorter session lifetimes, continuous monitoring, and phishing-resistant authentication for high-value accounts.

Is Your Session Security Configured to Match the Threat?

Enabling multi-factor authentication is an important first step, but it should not be the last. Protecting business accounts today also means securing what happens after users sign in.

Unbound Digital can assess your Microsoft 365 or Google Workspace environment, identify gaps in your authentication and session security, and implement the layered controls needed to better protect your business from modern account takeover attacks. Call us at 423-467-7777 or contact us online to get started.

Article FAQs

What is a session cookie?

A session cookie is a small piece of data that a website or cloud application creates after you successfully sign in. It lets the application remember that you’ve already authenticated, so you don’t have to enter your password or complete MFA every time you click to a new page.

How does an adversary-in-the-middle (AiTM) attack work?

In an AiTM attack, a victim is tricked into signing in through an attacker-controlled website that relays the login to the legitimate service. The victim enters their credentials and completes MFA as usual, but the attacker captures the authenticated session and uses it to access the account.

What should I do if I think a session has been hijacked?

Change the account password immediately, revoke all active sessions, and review recent sign-in activity for suspicious access. Revoking active sessions forces users to authenticate again, invalidating any stolen session cookies that an attacker may be using.