As if companies didn’t have enough to worry about, a wave of hostile bids may be around the corner.

With equity markets in free fall over the past few weeks, financial and strategic buyers are on the lookout for struggling companies they can snatch up on the cheap. The blood was already in the water as companies’ defenses against hostile takeovers have eroded over the years in favor of more shareholder-friendly practices. Few have so-called “poison pills” in place and only 10% of companies in the S&P 500 have staggered boards,...