From the CCC:
The Eucharist and the unity of Christians:
1399: The Eastern churches that are not in full communion with the Catholic Church celebrate the Eucharist with great love. "These Churches, although separated from us, yet possess true sacraments, above all- by apostolic succession- the priesthood and the Eucharist, whereby they are still joined to us in closest intimacy". A certain communion "in sacris", and so in the Eucharist, "given suitable circumstances and the approval of Church authority, is not merely possible but is encouraged".