Two Microsoft Defender zero-days patched, RAMPART lands, and Purview now watches Claude
Microsoft pushed patches this week for two actively exploited Defender zero-days, open-sourced a pair of agent safety tools, and extended Purview compliance coverage to Anthropic’s Claude. Here is what happened and why it matters if you run a Microsoft-heavy environment.
Two Microsoft Defender zero-days under active attack (CVE-2026-41091, CVE-2026-45498)
On May 21, Microsoft released out-of-band updates for two Defender vulnerabilities that were already being exploited in the wild. CVE-2026-41091 is a privilege escalation flaw in Malware Protection Engine 1.1.26030.3008 and earlier, caused by an improper link-following weakness that lets attackers reach SYSTEM. CVE-2026-45498 hits the Defender Antimalware Platform 4.18.26030.3011 and earlier and can trigger a denial-of-service condition. The good news: updates are delivered automatically for most environments, so you should not need to do anything manual. The less-good news: CISA added both CVEs to its Known Exploited Vulnerabilities catalog the same day and ordered federal agencies to patch by June 3. If you manage endpoints where automatic updates are disabled or delayed, check the Protection Updates page in Windows Security right now and confirm you are running Malware Protection Engine 1.1.26040.8 or Antimalware Platform 4.18.26040.7. Source: BleepingComputer, May 21, 2026.
Microsoft open-sources RAMPART and Clarity for agentic AI safety
On May 20, Microsoft’s AI Red Team published two new open-source tools aimed at making safety a first-class engineering concern for teams building agentic AI systems. RAMPART is a continuous testing framework that brings red-teaming techniques directly into the developer loop. It focuses first on cross-prompt injection attacks, supports statistical (probabilistic) test runs because LLM behavior is non-deterministic, and is designed to make red-team findings and production incidents reproducible. Clarity is a desktop and web tool that helps teams think through the “why” of a feature before writing code, producing a structured spec that tracks dependencies and flags when implementation drifts from the original design. Both are available on GitHub today. For teams shipping agents on Azure or inside Microsoft 365 Copilot, this is the tooling you want to plug into your CI/CD pipeline before anything goes to production. Source: Microsoft Security Blog, May 20, 2026.
Microsoft Purview compliance coverage now extends to Anthropic Claude
Also announced on May 21 as part of Microsoft’s monthly security roundup, a new Claude Compliance API integration lets Microsoft Purview detect and investigate Claude usage alongside other cloud applications your organization already monitors. Security and compliance teams get visibility into Claude activity without deploying separate tooling. This is a direct parallel to the existing ChatGPT Enterprise integration Purview already supports. If your organization has given employees access to Claude through Claude.ai or the API and you are using Purview for DLP or insider-risk policies, this is worth turning on. The integration is available now. Source: Microsoft Security Blog, May 21, 2026.
What to watch
The Defender CVEs are the most urgent item this week. Check your endpoint management console to confirm the Antimalware Platform update has landed across your fleet, and pay particular attention to any systems where Windows Update is managed manually or through a WSUS/MECM ring that has not yet approved the update.
