New data suggests that mortgage fraud—which got tougher to pull off after the collapse of the U.S. real estate market—is returning in a big way. Data prepared for The Wall Street Journal by research firm CoreLogic, examining about seven million home loans made by hundreds of lenders, show that losses from mortgage fraud—ranging from falsified credit reports to identity theft—rose 17% last year after declining 57% in the two years after its 2006 peak.