Protecting against recently disclosed Microsoft Exchange Server vulnerabilities: CVE-2021-26855, CVE-2021-26857, CVE-2021-26858, and CVE-2021-27065

This post details the immediate deployment of Cloudflare WAF managed rules (specifically rule IDs 100179 and 100181 within the Cloudflare Specials ruleset) to protect customers against four critical, actively exploited vulnerabilities in Microsoft Exchange Server (CVE-2021-26855, CVE-2021-26857, CVE-2021-26858, and CVE-2021-27065). It explains how these rules block the SSRF, insecure deserialization, and arbitrary file upload exploits, and recommends immediate patching of on-premise systems. It also provides guidance on disabling rules if false positives occur and references external analysis of the vulnerabilities.