共有デバイスのセキュリティ対策 - 家族・職場で安全に使う方法

About 13 min read

Sharing a single computer with family, using shared terminals at work, or using PCs at libraries and internet cafes - using shared devices is common in daily life, but without proper measures, passwords and personal information can leak to other users. Passwords saved in browsers, active login sessions, browsing history, and downloaded files - on shared devices, all your traces could be visible to the next user. This article explains specific measures for safely using shared devices at home and in the workplace.

Separating User Accounts

Account Separation at the OS Level

The most fundamental and effective security measure for shared devices is creating separate OS user accounts for each person. On both Windows and macOS, assigning individual accounts to each user separates desktops, documents, browser data, and app settings. Accessing another user's files requires administrator privileges, so in everyday use, each person's data remains invisible to others.

At home, we recommend creating standard user accounts (without administrator privileges) for children, restricting software installation and settings changes. This also prevents unintentional malware installation. Set a password or PIN for each account and ensure the lock screen appears when stepping away.

Using Browser Profiles

Even when OS account separation is difficult, browser profile features can separate data within the browser. Chrome, Firefox, and Edge all have profile features where bookmarks, history, saved passwords, and extensions are independent per profile. By having each family member use their own profile, you prevent session tokens and passwords saved in the browser from being visible to other family members.

Password Management on Shared PCs

Never Save Passwords in the Browser

On shared devices, always decline the browser's "Save password?" prompt. Saved passwords can be easily accessed by other users sharing the same browser profile. Disabling automatic password saving in browser settings is the safest approach. Instead, use a password manager browser extension and always log out of the password manager when done.

Using Guest Mode and Incognito Mode

When temporarily borrowing someone else's device or using a public PC, use the browser's guest mode or incognito mode (private browsing). In these modes, browsing history, cookies, form data, and download history are automatically deleted when the browser is closed. However, downloaded files themselves remain on the device and must be manually deleted.

Note that incognito mode does not protect against network administrators or installed monitoring software. It is solely a feature to prevent local browser data from being retained. On shared workplace PCs, we recommend avoiding logging into personal accounts as much as possible while following company security policies.

Building Logout Habits and Away-from-Desk Measures

Always Log Out When Done

The most common security incident on shared devices is forgetting to log out. Email, social media, cloud storage, online banking - if you leave while still logged in, the next user can freely access your account. Make "always log out when done" a habit. It is especially important to note that many services do not log you out just by closing the browser. Explicitly click the "Log out" button.

Physical Privacy Protection

When using devices in shared spaces, be aware of shoulder surfing (looking over someone's shoulder). Attaching a privacy filter to your display makes the screen invisible from angles other than directly in front. Also, always lock the screen when stepping away. On Windows, use Win+L; on macOS, use Ctrl+Command+Q to lock instantly. Never let your guard down, even for brief absences.

In the workplace, practice a clear screen policy (locking the screen when away) alongside a clean desk policy (not leaving confidential information on the desk when away). <AmazonLink keyword="プライバシーフィルター" locale={locale} className="amazon-inline-link">Privacy filters (Amazon)</AmazonLink> are relatively inexpensive and can significantly reduce information leakage risks in shared offices.