Federal Civilian Executive Branch (FCEB) agencies are being advised to update their Sitecore instances by September 25, 2025, following the discovery of a security flaw that has come under active exploitation in the wild. The vulnerability, tracked as CVE-2025-53690, carries a CVSS score of 9.0 out of a maximum of 10.0, indicating critical severity. "Sitecore Experience Manager (XM), Experience Platform (XP), Experience Commerce (XC), and Managed Cloud contain a deserialization of untrusted data vulnerability involving the use of default machine keys," the U.S. Cybersecurity and Infrastructure Security Agency (CISA) said. "This flaw allows attackers to exploit exposed ASP.NET machine keys to achieve remote code execution." Google-owned Mandiant, which discovered the active ViewState…