Researchers say a previously unknown Microsoft SharePoint vulnerability is being used in targeted attacks against U.S. defense contractors. The activity, first flagged by Mandiant, points to a focused campaign rather than broad opportunistic hacking.

The report adds to growing concerns about the security of widely used enterprise software, especially systems that store sensitive internal documents and communications. When a flaw is exploited before a patch is available, organizations can have little time to detect intrusions or contain damage.

The Reuters report did not name the attackers or specify how many organizations may have been affected. It also was not immediately clear whether any classified information was accessed, though defense-related targets heighten the potential risks.

The case underscores how zero-day vulnerabilities continue to pose serious threats to corporate and government networks. Security teams are typically urged to monitor for unusual activity, limit exposure of critical systems, and apply vendor guidance as soon as updates become available.