From the Document: "The Office of the Director of National Intelligence (ODNI) reported that, as of October 2013, over 5.1 million federal employees (civilian and military) and contractors held--or were individuals eligible to hold--a security clearance due to a favorable adjudication. The number of personnel determined eligible for clearance or who currently hold a security clearance underscores the importance of security-clearance adjudicators conducting thorough vulnerability assessments of security-clearance applicants as these clearances may allow government personnel to gain access to classified information that, through unauthorized disclosure, can in some cases cause exceptionally grave damage to U.S. national security. Federal regulations state that these assessments should include a review of a host of factors, including an applicant's financial position and relevant tax-debt information, in addition to other available, reliable information about the person, past and present, favorable and unfavorable. Federal law does not expressly prohibit an individual with unpaid federal taxes from being granted a security clearance; however, delinquent tax debt does pose a potential vulnerability that is to be considered in making a broader determination of whether an applicant should be granted a security clearance. Specifically, federal regulations state that an individual who is financially overextended is at risk of having to engage in illegal acts to generate funds, and that adjudicating officials must weigh an individual's inability or unwillingness to satisfy debts, such as federal tax debts, as they relate to an individual's financial and personal conduct when making the security-clearance determination."
GAO-14-686R DOD Clearance Hoders with Unpaid Federal Taxes
Government Accountability Office: http://www.gao.gov/