High prices getting even higher in places like Toronto, Vancouver, Victoria and Hamilton are making Canada's national housing market "highly vulnerable," the CMHC said Thursday.

Four times a year, the national housing agency looks at housing in the 15 largest markets in the country, and judges them on four criteria:

Overheating: Sales significantly outpace new listings.

Sales significantly outpace new listings. Price acceleration: Fast-rising prices are often a sign of speculative activity.

Fast-rising prices are often a sign of speculative activity. Overvaluation: Prices are higher than incomes, mortgage rates and other fundamentals can justify.

Prices are higher than incomes, mortgage rates and other fundamentals can justify. Overbuilding: The rental market vacancy rate or the level of unsold new buildings is higher than normal.

The CMHC then rates each city and each category on a colour-coded system, whereby green means there's little evidence of a problem, yellow means there's some signs of a problem, and red means there's definitely reason for concern.

This time around, the CMHC cited high prices in the four cities above as enough to nudge the entire country's overall rating into the red.

Overvaluation in Toronto, Hamilton, Vancouver and Victoria is enough to make the national housing picture look vulnerable, the CMHC warned. (Pete Evans/CBC)

There's "a high degree of vulnerability at the overall national level due to moderate levels of price acceleration and overvaluation," chief economist Bob Dugan said.

Outside of those cities, the CMHC waved a red flag about overbuilding in Calgary, Edmonton, Saskatoon, and Regina, but saw no reason for worry in the other categories there.

Canada's overall rating has now been in the red for seven quarters in a row, dating back to the end of 2016.