Whenever the topic of viruses comes up on any tech-oriented public forum, we are often told by clueless commenters that Linux doesn’t have viruses because it is secure by design. When pressed, such people will talk of privilege separation in Linux. Sure, Windows has privilege separation too, and it has filesystem ACLs enabled by default, which allows fine-grained control, but this privilege separation is in inferior to that in Linux since it was only introduced in 1993, whereas Unix-like operating systems have supported the concept since the 1970′s.
Such commenters may be proud of the fact that they run Ubuntu in its default configuration, which does not allow root logins, but rather uses gksudo to pop up a box requesting your password before any action is taken as root. They never start a root shell, instead they prefix every privileged shell command with “sudo”.
There are two problems with this:
- Linux has viruses.
- Privilege separation in desktop Linux is so easily broken that you may as well not bother.
Of course, Linux viruses are not viruses in the 1990 sense of the word, they are worms exploiting things like weak SSH passwords and web apps with arbitrary shell execution vulnerabilities. But the things on Windows that we call viruses these days are very similar. The reason Linux viruses target servers instead of desktops is because there are more servers than desktops, and servers are high-value because they can send larger volumes of spam.
On the second point, if you don’t believe me, try this little experiment. In your favourite Ubuntu desktop, open a terminal window, and run the following shell command:
while ! sudo -n true </dev/null >/dev/null 2>&1 ; do sleep 1; done ; sudo id </dev/null 2>&1 | cat
That will silently poll sudo until it is able to run a command without asking for a password. So while that is running, click System > Administration > Synaptic Package Manager (or any other command that runs as root), and type in your password when it prompts. The shell command will complete as soon as you hit OK.
It only logs to the syslog when it successfully runs the command as root, at which point you’re screwed anyway, since the command running as root could edit the logs.
You may have noticed that if you run sudo in one terminal window, you still need to enter your password again when you run it from another terminal window. This is because sudo stores a separate password timestamp for each pseudo-terminal. However, this mechanism is not any sort of secure, and you can easily trick sudo into thinking it is connected to any terminal to which the current user has read access. The redirections in the shell command above cause sudo to treat the terminal as “unknown”, which gives you access to the same password timestamp that is used for commands run from the desktop.
Alternatively, try this one:
gksu --description 'Update Manager' --print-pass
Convincing, yes? The point is that a malicious application that runs from your main user account can easily gain root access, either by waiting for you to do some regular sysadmin task like updating your system, or by tricking you into thinking that it is a valid system component which is asking for your password.
All that remains, then, is to find some way, by vulnerability or social engineering, to run malicious code as the main desktop user. That’s really not that far from the situation in Windows.
The most significant security difference between Linux and Windows is popularity. Anyone using a Linux desktop for security reasons would do well to switch to FreeBSD or BeOS the moment Linux desktops becomes popular enough for virus writers to start targeting them.
[Update 2011-04-26: using PolicyKit's pkexec instead of sudo to authenticate sysadmin actions would address my primary concern here. But it would have to be used exclusively. As soon as you give sudo your password, you're toast. Let's hope the distros start heading in that direction.]