42 CFR §411.26

1. (a)Subrogation. With respect to services for which Medicare paid, CMS is subrogated to any individual, provider, supplier, physician, private insurer, State agency, attorney, or any other entity entitled to payment by a primary payer.
2. (b)Right to intervene. CMS may join or intervene in any action related to the events that gave rise to the need for services for which Medicare paid.