Just because a piece of software is free (and "open source"), it doesn't
necessarily mean it's a good piece of software - or a "secure" one.
And, just because everyone can change its source code, it doesn't necessarily
mean that every (well-intended) person (with a knowledge of computing) has
the knowledge of the complexity (and also the patience, and the time)
necessary to spot every weaknesses in it (and solve them).
Take the "Tor" network example, that I've repeatedly mentioned in this
forum...
It was created by the US government itself, and it is said to be "private"
(or "secure"). But, already some universities' departments (with, surely, a
limited amount of personnel and money) have already discovered that it's not
as "private" (or "secure") as people thought it was.
(https://trisquel.info/en/forum/how-use-tor-trisquel#comment-26792)
