I want to stop changing my mind, I've used all three of the forms cyber security, cyber-security or cybersecurity at different times. There have been previous discussions on this (e.g. here and here) and I'm looking for a reasoned recommendation. I know there is never a simple answer in language due to the constant evolution of speech, but a reasoned consensus should be possible. Don't waste time on what cyber means, concentrate of the usage form. Why not the cybersecurity form, common in the America's, yet tagged as a misspelled in my UK version of Microsoft Word. The cyber security form is common in Europe yet I come across many documents in which both versions, and all three versions are used interchangeably. My preference would be to use the hyphen form to strengthen the specific computer/software related version of security. The single word form seems incorrect since I can think of no other security word without a hyphenated prefix (self-security is a rare hyphenated example). Other types of security are separated words, e.g. information security, financial security, physical security. Usage without the hyphen makes sense in other forms, e.g. cyberphobia. Let's see the consensus :)
Where different forms of a word exist and especially where they differ in different countries, and where your audience is international, it is probably worth choosing one and sticking to it, and mentioning the other two terms in a small note or addendum, if it's in a written document or report.
At least if you are consistent, it looks less messy and you can more easily do a 'global replace' later if it finally changes to one or the other.
So I don't think there is a 'right answer' at this point - it's just 'how you deal with it - without driving yourself crackers!
First of all, let me state that the reasoning in this answer, or rather in the answer I cite in the form of an article on LinkedIn, is rather technical. I think the answer is appropriate, because the term cyber(_)security is itself a technical term. Therefore, a technical approach to your question is warranted.
This article on LinkedIn by L. Rawson argues that cybersecurity should be used exclusively as adjective and cyber security should be used exclusively as noun. I am going to quote the most important part of her argument, but I suggest anyone interested in her reasoning read the entire article. Citation:
"Now, in applying the Friedman and Callimahos technical rule, the single word was only used when it was an adjective, and the two words were ONLY used when the word was the object of a noun." source 1
I should add that she bases her reasoning on Military Cryptanalytics, a book by William F. Friedman. The NSA very conveniently put a version of this book on their website (warning, opens pdf file).
I should also say that in the LinkedIn article it is acknowledged that the terms are used interchangeably and that it is rather a matter of preference. I posted this specific view as an answer because I think it's sound reasoning (as opposed to just saying you like one spelling over the other).
Source 1: Rawson, L. (2017, September 18). Which word should I use? Cybersecurity or Cyber Security. Retrieved March 20, 2018, from https://www.linkedin.com/pulse/which-word-should-i-use-cybersecurity-cyber-security-linda-rawson/