How to get session value in .ashx handler?

I am calling cross domain from an iFrame, passing username and password from iFrame to cross domain. in cross domain aspx page i got username and password and can set/get session value. Now when I am calling a .ASHX handler from the aspx page, Session values are not being retrieved from handler. it giving null reference error. I have used IRequiresSessionState.

--The scenario is happening in iOS safari browser only, rest are working fine.

How to resolve?