Two suspects, one described as "podgy" and the other as of "heavy build," forced their way into the flat in Burns Court, Kirkintilloch, and threatened the householder and her son with a weapon.
The 64-year-old woman was not injured but her 45-year-old son was seriously assaulted and suffered lacerations to his head.
He was treated at Glasgow Royal Infirmary before being released.
Police are hunting the two suspects and have been carrying out door-to-door enquiries in the area.
The first suspect is described as around 6ft 2, in his early 30s with a heavy build. The second suspect is younger and described as "podgy."
Both men had their faces covered.
Detective Inspector Douglas Robertson said: "There doesn't appear to be a motive for this attack, however we are following up a number of lines of enquiry.
"I am appealing to local people in the area who may have seen the two suspects around the time of the incident or who may noticed something suspicious to get in touch with us, whatever information you have, it could prove vital to our investigation."
Anyone with any information should contact officers at Kirkintilloch Police Office on the non-emergency police number, 101.
Alternatively, call Crimestoppers on 0800 555 111 where information can be given anonymously.