Single Tab Security: Is it Worth It?

TL;DR

Forcing a website to open in only one tab at a time can improve security, but it’s not a silver bullet. It mainly defends against certain session hijacking and credential stuffing attacks by making it harder for attackers to use multiple sessions simultaneously. It’s complex to implement reliably and can annoy users. Weigh the benefits carefully against usability concerns.

How Single Tab Mode Helps cyber security

The idea behind single tab mode is simple: if a user can only have one instance of your website open, it limits an attacker’s ability to exploit multiple sessions at once. Here’s how:

• Session Hijacking Resistance: If someone steals a session cookie (e.g., through XSS), they can’t easily use it if the user is already logged in elsewhere. The existing tab prevents a new one from opening with that stolen cookie.
• Credential Stuffing Mitigation: Attackers often try many username/password combinations simultaneously across multiple tabs. Single tab mode slows them down significantly, potentially triggering rate limiting or other security measures.
• Reduced Attack Surface: Fewer open browser instances mean fewer places for malicious scripts to run.

Implementing Single Tab Mode – Step-by-Step

There are several ways to achieve this, with varying degrees of reliability and complexity. We’ll focus on a JavaScript approach as it’s most common.

1. Detect Existing Tabs

The core is checking if another tab is already open. We use localStorage for this.

window.onload = function() {
  if (window.localStorage.getItem('single_tab') === 'true') {
    // Another tab is open, close this one
    window.close();
  } else {
    // This is the first tab, set a flag
    window.localStorage.setItem('single_tab', 'true');

    window.onbeforeunload = function() {
      window.localStorage.removeItem('single_tab'); // Clear on close
    };
  }
};

Explanation:

• window.onload ensures the code runs after the page is fully loaded.
• We check if single_tab exists in localStorage and is set to ‘true’. If it does, another tab is open, so we close this one.
• If no other tab is found, we set single_tab to ‘true’ to indicate this tab is active.
• window.onbeforeunload removes the flag when the tab is closed or refreshed, allowing a new tab to open.

2. Handling Multiple Windows (Not Just Tabs)

The above only covers tabs within the same browser window. To handle multiple windows, you need a more robust approach.

function checkSingleTab() {
  if (window.name !== 'main') {
    window.close();
  } else {
    // Set a name to identify the main window.
    window.name = 'main';
  }
}

checkSingleTab();

Explanation:

• We check if the window.name is ‘main’. If it’s not, we close the window.
• The first window sets its name to ‘main’, preventing subsequent windows from opening.

3. Combining Approaches for Better Coverage

Use both methods together for maximum effectiveness:

window.onload = function() {
  checkSingleTab(); // Check window name first

  if (window.localStorage.getItem('single_tab') === 'true') {
    // Another tab is open, close this one
    window.close();
  } else {
    // This is the first tab, set a flag
    window.localStorage.setItem('single_tab', 'true');

    window.onbeforeunload = function() {
      window.localStorage.removeItem('single_tab'); // Clear on close
    };
  }
};

4. Server-Side Reinforcement (Recommended)

JavaScript can be bypassed. Implement session management on the server that limits active sessions per user. This is crucial.

• Session Tracking: Store a list of active session IDs for each user.
• Limit Sessions: Allow only one (or a limited number) of active sessions per user. If a new session is attempted while the limit is reached, reject it or invalidate older sessions.

Important Considerations

• User Experience: This can be frustrating for users who legitimately want multiple tabs. Provide clear messaging explaining why this restriction exists.
• Browser Compatibility: localStorage is widely supported, but older browsers might have issues. Test thoroughly.
• Privacy Concerns: Be transparent about using localStorage and its purpose.
• False Positives: Incognito mode or multiple browser profiles can cause false positives (closing legitimate tabs).