Kourtney is the latest Kardashian to deal with paternity claims and this time someone has come out to question Mason's father Scott Disick.
Kourtney, 33, has been asked to allow her three-year-old son to undergo a paternity test.
She is reportedly furious with the allegations, made by male model Michael Girgenti.
He has claimed that he and Kourtney slept together in March 2009 while she and Scott were on a break.
Michael, 26, spoke to In Touch magazine telling them they slept together nine months before the baby was born.
A legal rep for the Keeping Up With The Kardashians star has described the allegations as "preposterous and an outrageous lie."
They went on to say that not only did Kourtney not sleep with the model, she never even gave him her contact details.
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They met briefly at a photo shoot and never met again.
Michael claimed to In Touch: "After [Mason] was born, and I saw photos of him, I began to wonder.
"I would definitely want to acknowledge Mason if he's my kid - he deserves to know his real father."
This is not the first time the Kardashian clan have had to deal with such allegations.
Younger sibling Khloe Kardashian has dealt with regular reports that she does not share a biological father with her sisters Kim and Kourtney.
She has hit back at these claims as fiercely as Kourtney is now.
Similarly half-sister Kylie, 15, has denied rumours that Bruce Jenner is not her real father.
She took to Twitter to slam the reports: "Yes, my father is my father and no my parents aren't getting a divorce.
"You guys can't believe any of these magazines. It's all bulls**t."