The flaw was apparently found in the software used by a DHS vendor to process personnel security investigations and has been immediately addressed.
There is no evidence that the information contained in the system - names, social security numbers, date of birth - were actually stolen or accesses at all, but potentially affected employees, contractors, inactive applicants, and former employees and contractors will be notified of the increased risk of identity theft they might face and the steps they can take to minimize it.
"DHS is evaluating all legal options and is engaged with the vendorís leadership to pursue all costs incurred mitigating the damages," the notice says, but doesn't say which vendor it was.
The vulnerability was first discovered by a law enforcement partner who then notified the DHS.
"DHS believes that employees who submitted background investigation information, and individuals who received a DHS clearance, between July 2009 and May 2013, primarily for positions at DHS HQ, Customs and Border Protection (CBP), and Immigration and Customs Enforcement (ICE), may be affected," the notice says, then adds that other information provided in the standard security questionnaire was not accessible.