It's meant to be basically impossible because we will did sterilize the landers. We know separately there are already rocks going from mars to earth and earth to mars, because we found some on the earth.
Allan Hills 84001 famously caused some controversy in 1996 when scientist believed it contained evidence for life on Mars, which ended in an announcement from the President. In the end the consensus seems to be that there was no evidence for life.
[1] https://en.wikipedia.org/wiki/Northwest_Africa_7034 [2] https://en.wikipedia.org/wiki/Allan_Hills_84001