The prize for Industrial & Commercial Bank of China Ltd. isn’t stocks, bonds or currencies. It’s the grease in the wheels of all those markets: repurchase agreements.

By exploiting a loophole in rules intended to keep U.S. banks from getting“too big to fail,” the state-owned ICBC has become a go-to dealer in repos in just a few short years, alongside longtime powerhouses like Goldman Sachs Group Inc. The short-term loans allow investors to borrow money by lending securities, serving a vital role in day-to-day trading on Wall Street.

ICBC’s rise reflects not only China’s global ambitions in high finance, but also how post-crisis rules have let a whole host of new players profit from the murky world of shadow banking, largely beyond the reach of bank regulators. As big banks face tougher standards, they’re being replaced by brokers, asset managers and foreign firms like ICBC, which can use more leverage and take greater risks. That has some regulators worried non-bank lenders are once again emerging as a threat to financial stability, less than a decade after panic in the repo market wiped out Lehman Brothers. […]