Learn German: Dein Passwort braucht zehn Zeichen, sonst hat der Hacker Zugriff. - Your password needs ten characters, otherwise the hacker will have access.

sonst is an adverbial connector, meaning it takes up the first position (Position 1) in the second clause. In German main clauses, the conjugated verb must always be in the second position. So, the verb hat comes immediately after sonst, pushing the subject der Hacker to the third position.

Could I put sonst later in the second clause?

Yes! Because sonst is an adverb, it can move around. You could say: ..., der Hacker hat sonst Zugriff. In this case, der Hacker takes the first position, the verb hat stays in the second position, and sonst moves to the middle of the sentence. Putting sonst at the beginning simply puts more emphasis on the 'otherwise' warning.

Is Zeichen singular or plural here?

It is plural here, meaning 'characters'. The noun das Zeichen (the character or sign) belongs to a group of German nouns that do not change their form in the plural. You can easily tell it is plural in this sentence because of the number zehn (ten) right before it.

Why does it say hat ... Zugriff instead of using a specific verb for 'to access'?