A year after China's financial regulators squared up to the systemic perils of "shadow banking," the threat is shifting to a booming corporate bond market, and risky borrowers' debt is finding its way into products aimed at retail investors.

An opaque network of trust companies and nonbank lenders had grown their annual market to a hefty 2.9 trillion yuan ($450 billion) in loans before regulators stepped in, spooked by rising defaults on wealth-management products (WMPs) backed by such high-interest shadow lending.

Now the high-risk borrowers who took those loans, such as unlisted real-estate firms struggling with a stagnant property market and financing companies backing shoddy local government investment, are finding a new avenue of funding after regulators began allowing unlisted companies to issue bonds on public exchanges.