A longtime Pentagon official warned Wednesday that Rep.-elect George Santos (R-Nassau) poses an unprecedented challenge to national security given his recent admitted lying about his background, mysterious finances and potential access to government secrets after he takes office in January. “Every congressman has eligibility for access to classified information and that terrifies security professionals, because they haven’t been really reviewed for it,” Dan Meyer, now the managing director of the law firm Tully Rinckey’s Washington DC office, told The Post. “We don’t know whether they’re a potential espionage threat.” Reporting by The Post and other media outlets has highlighted how Santos — who has copped to lying about much of his background — somehow made millions with a company he claims leveraged his contacts to sell used luxury items like yachts for six-figure fees.