Can you Trust your Friend, the Browser?

A post for the sufficiently paranoid, that is, aimed at system administrators.

Usually, sysadmins aim at securing their machines by making sure nobody can gain root access to it,  since that would give outside users the privileges to do anything: Install software, spambots, illegal data, spy on the machine.

But what about something as simple as a browser? A browser has user level privileges, but can both connect to the outside world and run local commands. Hacking a browser gives an outside party all information on the surfing behavior of a person, the work he is doing, and contacts he has. Moreover, since a browser accesses the outside world 'erratically,' its hard to see its compromised. One outside going http connection to some machine infrequently is hard to notice, and it has a big cache of temporary data where a lot can be hidden. Moreover, a browser is a large body of software, it consists of hundreds to thousands of packages, where any compromised package may mean a compromised browser. To an outside party, it may be more interesting to hack the browsers of a company, or al Qa'ida, than to gain root access to the machines.

And now for the paranoid plot. The hackers on Mozilla can normally be trusted, we can assume they are either teenagers, or your normal smelly hippy do-good nerd. But when it comes to trust: Do you trust the malignant hacker aimed at making money through spam? Or, for that matter, the Chinese, the Israeli, IBM?

One backdoor in the open-source project Mozilla, which may be as big as ten lines, would give, say the Chinese, who have a substantial cybercrime division, spying access to each and every user who installed Firefox. And subsequently all data, or, through exploits, root access. That is not to say that IE can be trusted that much more, but still...

I guess if you would derive the probability from all probabilities of all possibly interested parties who subsequently also did it and got away with it, you'll end up with a chance bigger than one.