While Cruz's site do not list Social Security as an issue. His quotes on the issue are conflicting. On his site, he says : the current payroll tax system will be abolished, while maintaining full funding for Social Security and Medicare. Else where he says that he will allow younger workers to keep a portion of their payroll taxes.
Ending payroll taxes will complicate Social Security's benefit formula. Benefits are based on earnings which trigger the taxes. If we end the payroll tax, we will remove the disincentive to participate. Like the Fair Tax, we would expect to see significant increases in the number of beneficiaries and the size of their claims.
If you think that Social Security is broken now, just wait until it is free.