Matt Mullenweg has an interesting post on security in WordPress. He points out that, as a product becomes more popular, its security issues garner more attention:

When a product is popular, not only does it have more eyes from security professionals on it, but any problems garner a level of attention which is not quite warranted by the frequency of the general event, like Angelina Jolie having a baby. There are certainly things intrinsic to coding that can make software more or less secure, but all things being equal the software with the most eyes on it, which usually means Open Source, will be the most robust in the long term.