The 33-year-old has been asked to allow Mason be given a paternity test to prove that the 26-year-old model is not his real father.
Michael Girgenti claims he slept with Kourtney nine months before Mason was born, when she and Scott were apparently on a break.
And although the Kardashian has kept quiet about the claims, her sister Khloe Kardashian took to Twitter to post a reflective statement.
Khloe, 28, has previously had to deny rumours that Robert Kardashian was not her biological father, while their half-sister Kylie, 15, recently denied that Bruce Jenner is not her real father.
It's like Star Wars all over again.
[ Related article: Kim Kardashian boasts that Kourtney Kardashian gained WAY more weight than her when she was pregnant ]
[ Related article: Kourtney Kardashian and Kim Kardashian strip down to show off their bikini bodies for spray tans ]