I'm just getting into learning REST and came across this great application that instantly lets me test out requests (Advanced Rest Client). The question arose that since it is an extension to the browser and I had to log my credentials in to run the request, is it safe to use? I see nothing but great reviews for this extension and it has 350,000+ downloads so I would assume it's safe. Assuming doesn't get me anywhere though.
I worked this summer around 3 browsers: Firefox, Google Chrome and Internet Explorer: by principle any extension is a source of vulnerability if not an exploit in itself.
Trust me it is not the number of people who use REST that will make you comfortable to use it because there had been several and more popular extensions that were discovered finally later to be too dangerous.
You can find examples of such malicious extensions (even if most of them were used as spywares and adwares) on here and there, for example.
By using an extension you are basically throwing to the garbage all the hard work that the browser's developers have done to protect you.
Think of extension as program that you install directly on your computer. If your extension is a virus, then you just installed a virus.
The team developing the browser is trying really, really hard to protect you. But, by installing an extension (think virus) you destroy that. If the OS or Browser is compromised then you cannot safely browse.
Your system is only as secure as the weakest link and here the weakest link is the extension. I can trust my OS, I can trust my browser but I'm usually not sure when I install any other program or extension.
In general, don't install extension or keep them to a minimum.