Your student loan servicer may not have your best interest at heart. And that means you need to be vigilant.

The Pennsylvania Attorney General's office on Thursday filed a lawsuit against Navient, now the largest student loan servicer in the U.S. and formerly part of Sallie Mae. The suit alleges the company "harmed countless student loan borrowers" by "peddling risky and expensive subprime loans that they knew or should have known were likely to default."

"The allegations are completely unfounded and the case was filed without any review of Pennsylvania residents' customer accounts," Navient said in a statement.

National student loan debt has climbed to $1.4 trillion as of 2017, according to the Student Loan Report, which tracks student loans. The average debt per student borrower is now $27,857, and the student loan default rate is 10.7 percent.

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For existing borrowers, there likely isn't any immediate action needed, said Mark Kantrowitz, publisher and vice president of strategy at Cappex.com, a provider of data on colleges, admissions and scholarships. "If you haven't experienced any problems with them, then there isn't any reason to switch," he said.

Kantrowitz recommends that any borrower who encounters problems with their servicer to reach out to the Consumer Financial Protection Bureau.

"The lenders tend to be really responsive to complaints received on the CFPB," Kantrowitz said. "It's not always going to be in your favor, but they will respond."