The Cambridge dictionary spells it as one word “cybersecurity”. The Cambridge dictionary is from the United Kingdom.

The Merriam Webster dictionary spells it as one word “cybersecurity”. The Merriam Webster dictionary is a United States dictionary.

The National Institute of Standards and Technology, a U.S. government organization spells cybersecurity as one word.

The United Kingdom National Cyber Security Centre spells cybersecurity as two separate words.

The Australian Cyber Security Centre (ACSC) spells cybersecurity as two separate words.

The Canadian Centre for Cyber Security spells cybersecurity as two separate words.

It looks like the U.S. government is the outlier among governments.

Using Google’s trend tool it is clear that spelling cybersecurity as two separate words is more popular. This applies worldwide, in the United States, and in the U.K. Based on observation, spelling cybersecurity as two separate words seems more popular.

It depends. The Oxford and Merriam Webster dictionaries spell cybersecurity as one word. NIST spells it as one word, however other government organizations spell it as two separate words. Spell checkers generally don’t flag either method of spelling as incorrect. The only conclusion is that both spellings, cybersecurity and cyber security are correct.