BlueHammer Zero-Day Vulnerability Exploited in Ransomware Attacks Targeting Microsoft Defender

The vulnerability CVE-2026-33825, dubbed BlueHammer, was exploited in the wild as a zero-day before patches were released. The flaw affects Microsoft Defender and has been leveraged in ransomware attacks. No specific threat actors, attack timelines, or affected systems beyond Microsoft Defender were detailed. The vulnerability is listed in CISA’s Known Exploited Vulnerabilities (KEV) catalog, confirming active exploitation. The impact includes unauthorized access and ransomware deployment, though further technical specifics were not provided.