Bsrgvty

I just added a new, underprivileged "desktop user," and I was surprised to discover that it can see the files in my home folder.

What is the rational for setting up such lax permissions?

A Public folder exists in your Home directory (/home/user) for sharing files with other users. If an other user wants to get access to this Public folder, the execute bit for the world should be set on the Home directory.

If you do not need to allow others to access your home folder (other humans or users like www-data for a webserver), you'll be fine with chmod o-rwx "$HOME" (remove read/write/execute from "other", equivalent to chmod 750 "$HOME" since the default permission is 750). Otherwise, you should change the umask setting too to prevent newly created files from getting read permissions for the world by default.

For a system-wide configuration, edit /etc/profile; per-user settings can be configured in ~/.profile. I prefer the same policy for all users, so I'd edit the /etc/profile file and append the line:

umask 027

You need to re-login to apply these changes, unless you're in a shell. In that case, you can run umask 027 in the shell.

Now to fix the existing permissions, you need to remove the read/write/execute permissions from other:

chmod -R o-rwx ~

Now if you decide to share the ~/Public folder to everyone, run the next commands:

• 
  chmod o+x ~ - allow everyone to descend in the directory (x), but not get a directory listing (r should not be added)


• 
  find ~/Public -type f -exec chmod o+r ; - allow everyone to read the files in ~/Public
  


• 
  find ~/Public -type d -exec chmod o+rx ; - allow everyone to descend into directories and list their contents

If you are use GNU coreutils (e.g. on Ubuntu, not on a embedded system having only busybox), then the previous two commands using find and chmod can be replaced by this single command that recursively makes folders and files readable (and additionally adds the execute (descend) bit for directories only):

chmod -R o+rX ~/Public

According to an Ubuntuforms.org staff member, it is to make it easier to share files between new users.

You can change the permission to either 700 or 750 if you don't want the files readable and executable by others.

Command is:

chmod 750 $HOME

Note: Ubuntu default is 755

According to Mark Shuttleworth,

"The majority of users of Ubuntu systems either have exclusive use of the
machine (personal laptop) or are sharing with friends and relatives. We
assume that the people who share the machine are either trusted, or in a
position to hack the machine (boot from USB!) trivially. As a result,
there is little to no benefit"

... from removing those permissions.

You can read the User Management section of the Ubuntu Server Guide which covers the necessary details. The User Profile Security paragraph will probably answer your questions - officially.

I think Lekensteyn's answer can be improved by replacing the last two find commands with chmod using -X option (note the capital X). The two find commands can be replaced with

chmod -R o+rX ~/Public

This differentiates appropriately between files and directories, but does have the additional effect of allowing others to run executable files.

Since it is privacy that interests you (judging from the tags that were applied) it is very possible that setting permissions is insufficient (see ignis's answer). The answer may be something along the lines of an encrypted home directory. This solution is specifically designed against the attack by another user of a computer. It will, of course, be unable to stop another user from damaging your files (by simply removing ~/.Private directory, thus erasing all of your files), but they will be unable to mount the directory and see the files without your password.

The easiest way to achieve that is during the installation process, there is a check box, stating "Encrypt your home directory" and you need to select that.

Since it is unlikely that you will want to reinstall just for that (and because it still carries all the risks that are entailed with doing it without reinstall), you can do the following:

sudo apt-get install encryptfs-utils
encryptfs-migrate-home

If you really need a high level of security: please re-install and make sure to choose the option to encrypt your entire disk. This will require a passphrase to even start the machine. You may of course also encrypt your home folder once more on top of this, with some performance degradation; though not noticeable for normal use.

Please note, that encrypting your home folder will disable applications like Dropbox. Dropbox is not secure storage that respects privacy anyway, so that may be a trite point. However, if you do need secure and private storage in the cloud, I would personally recommend MEGAsync since only you would have the keys to access the data.