Can you catch herpes from kissing someone if they have a coldsore at the time?

Yes: A person with a cold sore due to herpes simplex virus is shedding a significant amount of virus from the lesion and can infect another person's mucous membranes that come into contact with the lesion if that other person has not been previously infected.

Yes: Cold sores are caused by the herpes virus (hsv-1) . It's closely related to the virus that causes genital herpes(hsv-2). It's spread by close personal contact such as kissing. Either virus can affect either location but generally hsv-1 above, hsv-2 below. Hsv infection is lifelong. The virus is most contagious at the time of an outbreak but can be spread when the infected person is asymptomatic.