A benefit under Canadian social security legislation paid to a resident of the U.S. is taxable in the U.S. as though it were a benefit under the U.S. Social Security Act (Canadian Treaty, Art. XVIII(5)(b))
Exception: A type of benefit that is not subject to Canadian tax when paid to residents of Canada is exempt from United States tax (U.S. - Canadian Treaty, Art. XVIII(5)(b))
Thus, a maximum of 85% of Canadian social security paid to a U.S. resident may be subject to U.S. taxes. However, if Canadian social security is amended to make benefits means-tested at the source and not subject to tax, the benefits will not be subject to U.S. tax (Canadian Protocol Treas. Tech. Explanation Pg. 4.).
States do not have treaties with foreign countries. Be sure to check the taxpayer’s U.S. resident state tax laws for how Canadian Social Security benefits are taxed by the state. For example, California Revenue & Taxation Code Sec 17087(a) exempts only U.S. Social Security and there is no provision under either the California statute or the Internal Revenue Code to specifically exempt foreign social security from taxation by California.