Is Open Source Software risky for Software Startups?

This is a question which is independent of whether the origins are open or not. Infringement of software patents has been given a knockback by recent Supreme Court rulings over Alice v CLS. But you might be surprised at how much "patented" stuff actually comes from originally open source, from recollection one content distribution service derived from a proxy cache server released in the 90s. Fortunately if you look at most open-source software, they tend to end up in the tool-chain (for which it benefits other developers), languages (which benefit everyone) and now cloud components. Certain tech giants (eg IBM) have acknowledged the power of the developer community by promising not to assert their patents against bona fide projects (the backlash would be serious ... see Linksys outing) though others have kept silent reserving the right to selectively trip up competitors. As for infringement, that's a matter of doing your homework and either paying for a clearance search or joining one of the defensive patent pools.