The incident happened between 1:45am and 2:45am on Sunday at the 85-year-old’s home on Hawthornhill Road in Dumbarton.
She was taken by ambulance to the Royal Alexandra Hospital in Paisley where she is being treated for head and facial injuries.
Hospital staff have described her condition as serious, but stable.
It is not yet known if anything was stolen in the incident, but it is believed the attacker fled with a parasol.
Detective Inspector Ogilvie Ross said: “We believe he may have left the address with a large garden parasol and I would urge anyone who saw someone in the area acting suspiciously, possibly carrying this item, or anyone who found or saw a parasol discarded, to please contact us.
“No matter how insignificant you think the information you hold is, get in touch.
“Additional police patrols remain in the area to provide reassurance and anyone with concerns should speak to officers.”
The suspect is described as being around 6ft and of slim build.
He was wearing dark clothing and was last seen heading in the direction of Castlehill Road.