Commonwealth Bank of Australia (ASX: CBA) shares have delivered big returns for investors over the past 12 months.

During this time, the banking giant's shares have outperformed some of Australia's best growth shares with a gain of 44%.

To put that into context, a $10,000 investment a year ago would now be worth approximately $14,400.

And that's not including the $4.75 per share it has paid out in fully franked dividends over the period.

But, as they say, all good things come to an end. And could the incredible run by CBA shares be coming to an end?

Let's see what brokers are saying about Australia's largest bank.

Could CBA shares crash 40%?

Unfortunately a large number of brokers believe that CBA's shares are vastly overvalued and at risk of crashing back down to earth.

For example, last week Citi put a sell button and $100 price target on the bank's shares. Based on its current share price of $173.84, this implies potential downside of 42% for investors over the next 12 months.

Its analysts believe that it is a case of when and not if Australia's largest bank gets hit with consensus earnings downgrades. Citi suspects that when the time comes, the bank's shares will start to pullback from record levels.

And with the banks facing funding cost pressures, deteriorating asset quality, and the likely end of share buybacks, it feels it may not be long until this happens.

Elsewhere, the team at Macquarie Group Ltd (ASX: MQG) is almost as bearish. Last week it put an underperform rating and $105.00 price target on its shares. This suggests that downside of 40% is possible from current levels. The broker said:

Despite a consistent and reasonably positive trading update, CBA remains expensive, trading at a ~26x FY25E P/E and a ~45-90% premium to peers. While we appreciate de-rating lacks near-term catalysts, we do not think the current valuation is fundamentally justified. Maintain Underperform.

But the most bearish broker out there appears to be Morgans. Its analysts responded to CBA's third quarter update by putting a reduce rating and $97.49 price target on its shares. This implies potential downside of approximately 44% for investors over the next 12 months.

Overall, while it might seem unlikely that CBA shares would drop by so much from where they currently trade, it is worth remembering that a year ago it seems equally unlikely that they would record a 40%+ gain over the past 12 months. So, don't rule out anything.