You can also read Free and Open Source Software – Common Misconception #1
When marking exams, I often read statements like “Because anybody can access the source code, malicious users could easily alter FOSS and damage your software or data” and “The source code could easily be changed by accident”. To understand why these statements are wrong, you need to understand that writing computer programs is a hard job (I know because I used to do it for a living, and I still do it for fun).
To change the source code of a program, you need to download the source, open it in a text editor or programmer’s editor, alter it (correctly!), then compile it into an executable file. This makes it rather hard to “accidentally” change the source code.
Secondly, if a user changes a program’s source code to do something malicious, they still need to install this new, malicious, program on your computer. If they have access to your computer to install software, they could just install any virus or malicious program – at this point, FOSS is irrelevant!
Ask yourself: do you use any of the following software: Firefox, OpenOffice/LibreOffice, VLC, VirtualBox, Linux, Audacity, GIMP, Azureus, 7-Zip? If so, you use FOSS. Provided you downloaded the software from its official site, your computer is no less secure. If you are foolish enough to download and install software from unknown or untrusted sites, you may be exposed to malicious software – but this applies to all types of software, not just FOSS.
Still not convinced?
FOSS is used by organisations all over the world, including the US Department of Defense, the Federal Aviation Administration, the US Postal Service; governments in France, Mexico, Cuba, and many more countries; by the New York Stock Exchange, CERN; by Internet giants like Wikipedia, Google, Amazon, and many more. I’m pretty sure these organisations value the security of their systems!