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Overview of Cookie (Session) Hijacking

Overview of Cookie (Session) Hijacking
Cookie (or session) hijacking occurs when an attacker obtains a user’s valid session cookie—often called a “session token”—and uses it to impersonate the legitimate user. Once logged in, web applications typically keep users authenticated by referencing the session token within subsequent requests. If a malicious actor steals (or forges) that token, they can trick the server into thinking they are the logged-in user.

How Cookie Hijacking Affects 2FA
Two-Factor Authentication (2FA) is supposed to add an extra layer of security beyond passwords. However, once a legitimate user has successfully passed 2FA, the server sets a session cookie that identifies them as authenticated. If that cookie is hijacked:

  1. Bypassing the Login Process

    • Normally, to access an account, you need a username, password, and the one-time code (2FA).
    • But if the attacker has the session cookie, they can skip all these steps. The server simply “sees” the session token and assumes the user has already passed 2FA.
  2. Limited 2FA Checks

    • Many web applications request 2FA only at the initial login. Afterward, they rely on session cookies to confirm the user’s identity.
    • This means attackers who hijack a valid session cookie effectively bypass any 2FA challenges as long as the session remains active.
  3. Extended Attack Window

    • If session tokens last a long time (e.g., “Remember Me” features or poorly managed session timeouts), attackers can use these stolen cookies for extended periods, even after the user logs out—if the token is not invalidated on logout or properly refreshed on re-authentication.

Common Ways Attackers Hijack Cookies

  1. Network Sniffing: If a user visits a site over an unencrypted connection (HTTP instead of HTTPS) on public Wi-Fi, an attacker can eavesdrop on the traffic and steal session cookies.
  2. Cross-Site Scripting (XSS): A vulnerability in a website’s code that lets the attacker run malicious scripts in the victim’s browser. These scripts can extract session cookies.
  3. Session Fixation: The attacker forces a known session ID onto the user. After the user logs in, the attacker uses that same session token to access the account.
  4. Malware or Keyloggers: Malicious software on a user’s device can read local storage or cookies directly from the user’s browser.
  5. Phishing & Social Engineering: Tricking users into sending their session cookie or session token to an attacker-controlled site.

Why This Makes You Vulnerable

  • 2FA is rendered ineffective if the attacker obtains the session token after you’ve authenticated. The entire point of 2FA is to prove your identity at login—but once you’re logged in, the cookie does that for you.
  • Users often assume 2FA alone keeps them safe, but security depends on the entire chain (secure connections, proper session management, XSS prevention, and so forth). Any weakness in these areas can undermine 2FA.
  • Attackers can move quickly and often invisibly: Once they have the cookie, the legitimate user might not even know their session is being shared.

Protecting Against Cookie Hijacking

  1. Use HTTPS Everywhere: Ensures session cookies aren’t exposed in plaintext.
  2. Set Secure & HttpOnly Flags:
    • Secure Flag: Ensures the cookie is only sent over HTTPS.
    • HttpOnly Flag: Prevents JavaScript from directly reading the cookie, mitigating some XSS attacks.
  3. Short Session Lifetimes & Idle Timeout: Minimizes how long a hijacked session can remain valid.
  4. Re-verify Sensitive Actions: Prompt the user for 2FA (or at least re-check credentials) when performing high-risk actions (e.g., changing passwords, transferring funds).
  5. Monitor Active Sessions: Allow users to see active sessions and revoke suspicious ones.
  6. Implement Proper Logout: Invalidate the server-side session on logout, so old tokens won’t work.
  7. Beware of XSS: Patch vulnerabilities that allow attackers to run malicious code in your site’s context.

Key Takeaway:
Cookie session hijacking compromises an authenticated session by stealing its identifying token. Once an attacker has that valid token, 2FA no longer protects the account during that session. Good security practices—such as enforcing HTTPS, setting cookie flags, implementing short session lifetimes, re-checking 2FA for sensitive actions, and protecting against XSS—are critical to prevent attackers from bypassing 2FA through session hijacking.