The study highlights a unique feature of student debt: Unlike other credit card balances and most other debt, it is nearly impossible to cancel student debt by filing for bankruptcy.

That leaves many borrowers trapped, behind on loans that lenders are unwilling to modify, the study said. There are more than 850,000 private loans in default, worth more than $8.1 billion, it said.

“Too many student loan borrowers are struggling to pay off private student loans that they did not understand and cannot afford,” said Richard Cordray, director of the CFPB.

The CFPB was created in the wake of the financial crisis to protect people against unfair loans, unexpected fees and other financial threats.

Lending standards for private student loans were loose during the credit bubble of the mid-2000s, the report said. Because private lenders marketed directly to students, bypassing school financial aid officers, schools did not review borrowers’ financial needs or enrollment status.

As a result, many borrowed far more than they needed to pay for tuition. The loans went to people with increasingly weak credit scores, making repayment less likely, the study said.

The report is based on data from nine lenders on over more than 5 million loans made between 2005 and 2011, as well as data from five nonprofit lenders. It was required under a sweeping overhaul of financial rules passed by Congress in 2010.

It said that lenders have been more careful since the financial crisis reduced the amount of credit available. For example, in 2011, more than 90 percent of private student loans required a co-signer, compared with 67 percent in 2008.

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