A 39-year-old mom from California holiday shopping at Macy’s Herald Square was repeatedly knifed in the back by a homeless stranger as she changed her infant daughter in the department store bathroom during a wild, unprovoked attack, officials said Friday.

The victim was changing her 9-month-old daughter’s diaper in a seventh floor bathroom at Macy’s flagship store on W. 34th St. near Broadway at about 3:15 p.m. Thursday when Kerri Aherne, 43, rushed in and repeatedly jammed a blade into the woman’s back, cops said. She also slashed her victim’s arm.

The victim’s husband heard the commotion inside the bathroom, ran in, and grabbed the assailant, holding her until police could arrive.

The knife used was recovered at the scene.

EMS rushed the mom to Bellevue Hospital , where she’s

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