Microsoft Defender SmartScreen Security Feature Bypass Vulnerability
What it is
Microsoft Defender SmartScreen contains a security feature bypass vulnerability that could allow an attacker to evade Mark of the Web (MOTW) defenses via a specially crafted malicious file.
Known to be used in ransomware campaigns.Active threat actors have chained this vulnerability into ransomware operations — treat patching as a same-week priority, not a "next maintenance window" task. The coping action is the same one below; the urgency is higher.
Who's affected
Affects anyone running Microsoft Defender. Microsoft products in a small practice typically sit close to credentials, email, or document workflows — treat the patch as in-scope.
What to do
Apply updates per vendor instructions.
CISA action deadline: January 3, 2023. Federal agencies must complete the required action by this date. For private SMBs the deadline is advisory — but treat it as a strong recommendation, especially if you handle regulated data (HIPAA, GLBA, ABA model rules).
If you don't have someone in-house to verify the patch deployed across every endpoint — or you're not sure whether you're affected — that's exactly the kind of triage we do. Book a free 20-minute triage call.
Severity
CVSS base score: 5.4 — MEDIUM
Source
Pulled daily from the public cisagov/kev-data mirror (CC0). View the original entry on cisa.gov. CISA KEV is US-Government public-domain data; we add the SMB-vertical framing and the coping action above.