Hydrogen cyanide, or HCN, is not a strong acid. In solution, hydrogen cyanide is a weak acid that is mostly HCN with minimal amounts of hydronium and cyanide ionized. A weak acid partially ionizes in solution. The solution moves backwards and forwards to create the products and reactants.
As a rule, most acids that begin with H in the chemical formula are weak acids, but there are exceptions.
Hydrogen cyanide has a pKa of 9.2. The carbon has a triple bond with the nitrogen and a single bond with hydrogen. It is a weak acid because the carbon is reluctant to donate the hydrogen atom because the bond keeps the molecule stable. However, the substance is volatile, making it an incredibly toxic gas.