- Security of Deb and Other Files
You can find a .deb file for a package somewhere on the Internet. Then you can use dpkg -i package.deb and install it. That's no better than picking up an install for Windows somewhere on the Internet. Don't do it unless you are absolutely sure of the source, and even then make sure you have all of the prerequisite packages already installed.
Deb files, safe or not, do follow a format with hashes, etc. so that they have to be rebuilt if they are changed.
Package (.deb files) in the Ubuntu repositores are generally built from source on Launchpad build computers so the contents of the .deb file matches the source, and the source can be viewed by anyone. Many packages have teams maintaining them who follow them and are on the lookout for security problems. New source package versions have to be signed properly by gpg keys using public key cryptography before they can be built.
There are now binary only packages available in the Ubuntu Software Center, so the public can't view the source of those. I don't know about these for sure, but I believe they are reviewed before they are made available.
You generally shouldn't install a package with dpkg -i package.deb, but use apt-get or the software center instead, downloading from an Ubuntu repository. You should also avoid picking up any other kind of script that you can't look at and understand completely before you run it.
The multi-user system Unix-like systems do mean that if you make a mistake you can mess up your account and its files, but not the accounts and settings of other users that have been established on the same system, nor the operating system itself.
The exception is when you run a command with sudo or have to enter a password to install a package or do other maintenance. These are the times to be very careful about the source of what you are doing. This is very similar to using UAC.
- Executable Files on Removable Media
As long as you are using due care, I don't think you need to maintain programs on removable media. Like Windows, most programs are installed as packages and therefore aren't runnable from removable media (although you could put an entire Ubuntu on a flash drive if you want).
As I mentioned above, .deb files use hashes for the files they include to see that they aren't altered by an attacker. Ubuntu repositories also have gpg keys stored on your system when you install Ubuntu, and there is a signature and chain of hashes followed down to the .deb files to keep things secure. Ubuntu is derived from Debian and that project created this approach.
There are things like autorun in Linux and other Unix-like systems. When you install packages those packages can cause programs to start at boot time, or when a user logs in to a terminal, or when a user logs into a GUI session. Most users have a (hidden by default) .bashrc file in their home directories that execute when a user logs in to a terminal.
The Ubuntu download web site not only has the .iso files for CD's and DVD's but also message digests (hashes) you can check to make sure the file you retrieved is authentic down to the bit.
Despite everything else, developers make mistakes and potential security problems can creep into software. Running supported versions of Ubuntu means that you will be offered security fixes for items in the main Ubuntu repositories, and often for items in the universe and other repositories. You should apply those fixes. Long-term-support releases like 12.04 (Precise) offer this service for a longer term than other releases of Ubuntu.
I can't personally guarantee that the precautions are perfect, but I think they are pretty good for the current state of the art.
That file seems to belong to package libruby1.9.1, which should be installed when you installed Ruby.
If that package comes from the default repositories I guess that warning should be nothing to you worry about. If it comes from a PPA then you should take a closer look.
To see from where the package comes you can use apt-cache. From my system:
$ apt-cache policy libruby1.9.1
libruby1.9.1:
Installed: (none) <-- This shows the installed version (not installed on my case)
Candidate: 1.9.3.0-1ubuntu2.5
Version table:
1.9.3.0-1ubuntu2.5 0
500 http://pt.archive.ubuntu.com/ubuntu/ precise-updates/main amd64 Packages
500 http://security.ubuntu.com/ubuntu/ precise-security/main amd64 Packages
1.9.3.0-1ubuntu1 0
500 http://pt.archive.ubuntu.com/ubuntu/ precise/main amd64 Packages
Also consider checking that file in an online scanner like VirusTotal. If only clamav marks it as a potencially threath probably it is a false positive.
Best Answer
This is probably just a false positive.
/usr/share/mime/mime.cacheis a generated file of all known mime types on your system. It's not an executable.Virus scanners detect malicious software by sets of known fingerprints (hashes). This model leads to some false positives, inevitably. Perhaps it's a coincedence a known Windows virus matches the fingerprint as found on a Linux system, perhaps it's because the fingerprint just matches a certain MIME type pattern that will match inevitably on any Linux system...
For now, I wouldn't worry about it, but just contact the ClamAV team to ask whether this is a known issue with them already.
Also make sure to stay updated with the most recent ClamAV fingerprints (
freshclam).