CVE-2021-26855

This vulnerability lets a remote attacker break into Microsoft Exchange Server and run code on it, even without a login. Think of Exchange as the front-desk computer for your email—if someone can run code there, they can potentially redirect emails, steal data, or set up ransomware. Because it can be triggered over the network without authentication, it’s the kind of issue that can be actively used by attackers.

This matters if your business runs Microsoft Exchange Server 2013 Cumulative Update 22/23, Microsoft Exchange Server 2016 Cumulative Update 13/14/19, or Microsoft Exchange Server 2019 (including Cumulative Update 2/3/4/8). It’s relevant when your Exchange server is reachable over the network, because the flaw does not require authentication or user interaction. Treat it as a risk when an attacker can reach the Exchange server endpoint from outside your organization (for example, via an internet-facing setup).

This is RED because confirmed real-world exploitation has been tied to ransomware activity, and it appears in CISA’s Known Exploited Vulnerabilities list. Attackers can use it to run code remotely without any login, so the window between exposure and compromise can be short—patching should be treated as immediate priority.