Unusual Web Server Child Process Creation
This detection flags a parent and child process pair on internet-facing Windows servers, where the parent is a web server process and the child is the process it launched. The web server processes covered are w3wp, httpd, nginx and Tomcat, including versioned Tomcat service executables such as tomcat9.exe and Tomcat running under java.exe, which is identified by the Catalina bootstrap class in its command line. A server is treated as internet-facing based on the most recent status recorded for it. Instead of matching known web shell names, the query builds a baseline from the previous 30 days of process activity on each device and alerts when a pair appears in the latest window that has never been seen on that device before, so it can catch web shell activity it has never seen before. Because the baseline is held per device, a pair that is normal on one server does not hide the same pair appearing on another. The rule runs every 24 hours, as this is the only Defender custom detection frequency with a 30-day lookback, and the alert window covers the last 25 hours so that a slightly delayed run does not leave a gap. The baseline and alert windows are separated by ingestion time rather than event time, so events that arrive late are still evaluated.
Microsoft Sentinel (KQL)
