CISA Warns BlueHammer Flaw Is Now Exploited in Ransomware Attacks
Vulnerabilities mentionedAll →
| CVE | Vulnerability | CVSS | EPSS | Flags | Affected | Exposure | Published |
|---|---|---|---|---|---|---|---|
| CVE-2026-33825 | Local Privilege Escalation in Microsoft Defender Antimalware Platform CVE-2026-33825 is an insufficient granularity of access control flaw (CWE-1220) in Microsoft Defender Antimalware Platform that allows an authorized attacker to elevate privileges locally. It is triggered by an attacker who already holds a low-privileged foothold on a machine running Defender, with no user interaction required. Successful exploitation has high impact on confidentiality, integrity, and availability (CVSS 3.1: 7.8), granting elevated local rights that facilitate defense evasion, persistence, or ransomware activity. Any organization running Microsoft Defender on Windows endpoints and servers is potentially affected. The flaw is actively exploited in the wild, was added to CISA's Known Exploited Vulnerabilities catalog on 2026-04-22 with ransomware use confirmed, and is one of three Microsoft Defender zero-days reported as exploited, two of which were still unpatched at the time of reporting. Do: Apply Microsoft's April 2026 Patch Tuesday updates for the Defender Antimalware Platform (platform/security intelligence updates) per vendor instructions, consistent with CISA BOD 22-01 timelines for KEV entries, and note reports that two of the three exploited Defender zero-days were still unpatched, so monitor for follow-up fixes. Prioritize patching internet-reachable and high-value Windows hosts, and hunt for signs of local privilege escalation and ransomware precursor activity on systems that cannot be updated immediately. | 7.8 | 7% | KEV ransomware |
| masshundreds of millions of Windows endpoints and servers (Defender is the default antimalware on Windows) |
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CISA confirms BlueHammer (CVE-2026-33825) is now used in ransomware attacks to gain SYSTEM privileges through Microsoft Defender.
BlueHammer, tracked as CVE-2026-33825, has moved from proof-of-concept noise to real ransomware attacks in the wild, the US CISA confirms.
BlueHammer allows attackers to escalate privileges locally in Microsoft Defender. The vulnerability, along with two other zero-days dubbed RedSun, and UnDefend, was disclosed by a researcher known as Chaotic Eclipse after criticizing Microsoft’s handling of the disclosure.
In Mid April, Huntress researchers reported attackers were exploiting the three Windows flaws to target systems, though the victims and attackers remain unknown.
Huntress said it saw real-world exploitation of all three flaws. Attackers used BlueHammer starting April 10, 2026, then followed with RedSun and UnDefend proof-of-concept exploits on April 16.
Researchers believe attackers are using public exploit code released online by Chaotic Eclipse.
The controversial researcher Chaotic Eclipse publicly disclosed multiple Microsoft-related issues before patches were ready.
CISA added the BlueHammer flaw to its Known Exploited Vulnerabilities catalog on April 22 and later updated the entry to note ransomware use.
BlueHammer is dangerous because it can give attackers full SYSTEM-level access. With this control, ransomware groups can turn off security tools, install malware, and spread further inside the infected device.
The US agency did not provide technical details about the attacks exploiting this issue or the ransomware group behind them.
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