"A friend of mine once had a great job working for a major studio. She had a nice office and a large desk and windows that got the morning sun, and she had her name on the door in gold lettering with a title something like "assistant acquisitions manager." Her job was gathering high concepts, great lines of dialogue, and fresh, dramatic situations from scripts and treatments that were submitted to the studio. A world-class speed-reader, she would scan the scripts and treatments quickly, then pass the gold she'd mine from these scripts to any producer working for the studio who could use them. She was a hired thief, which did not square well with her self-image. Her conscience bothered her so badly that she wrote a scathing magazine piece about the practice, quit, and moved to Vermont to become a creative writing coach.
One way to protect yourself from getting ripped off is to turn your high concept into a damn good thriller as a novel before you pitch it to Hollywood for a film. That way, you are somewhat protected. Of course, they can still steal your high concept, because high concepts are just ideas and you cannot copyright an idea. My guess is, if you have a high concept that tickles producers' fancies, they will be busy two days later with their own screenwriters who they know can develop your high concept into a damn good thriller. What need do they have of you? Hollywood is a small town where it's who you know that counts."
Chapter from James N. Frey's book "How To Write A Damn Good Thriller" (2010), pp. 4 - 5, available here:
https://www.amazon.com/Write-Damn-Thriller-Step-Step/dp/0312575076/