The FBI is warning of a campaign by a mysterious Internet hacking group to breach numerous federal agencies’ computer systems and the resulting theft of thousands of people’s personal information.

According to a memo obtained by Reuters, hackers associated with Anonymous exploited a flaw in Adobe Systems’ ColdFusion software — used to construct websites — to accomplish the break-ins.

Among the affected agencies were the U.S. Army, Department of Energy and the Department of Health and Human Services, although the FBI memo reportedly warns the damage could be far more widespread.