Well, that threat always exists, just like with any other software. A software developer might sneak some malware into an upgraded version of their software, but with open source, software will always have a bit more transparency than closed-source, since any user can see the code and check for potential issues. That means other developers and users can check the source code for any potential issues or malicious code and alert the community. That's the benefit of open source software. Not only does it allow any programmer with the ability to improve upon the source code, it also means that there are many people looking at the source code for vulnerabilities and flaws to report. There is strength in numbers, after all, and open source projects are no exception.