It's 6 of one, half-dozen of the other.
Anyone can view the source of an open source project, which means anyone can find vulnerabilities in it. Specifically, hackers wishing to exploit the software, as well as users withing to audit and fix the software. But, someone who knows what they're doing has to actually look at the source for that to matter; and this rarely happens.
Hackers must black-box closed source software to find exploits, which make it more difficult than finding them in open source software; the flip-side is that they can only by fixed by the few people who have the source. If the hacker doesn't disclose the exploit and the people with access to the code don't look for it, it goes unpatched forever.
Open source software does provide an advantage to both sides, hackers can find exploits more easily and users can fix them more easily; with closed source, you're at the mercy of the vendor to fix their code but, at the same time, it's more difficult for a hacker to find a vulnerability without access to the source.
Then, we consider how good fuzzing techniques have gotten and... well, as it becomes easier to find vulnerabilities in closed source software, open source starts to look better.