U.S. retailers are hunting for evidence of new breaches leading into the holiday shopping season after a cyber intelligence firm privately warned them about payment-card-stealing malware that it said evades almost all security software. “This is by far the most sophisticated point-of-sale malware seen to date,” said Maria Noboa, lead technical analyst for privately held iSight Partners, which uncovered the malware and was due to release a technical report about it on Tuesday.