Referencing NIST: Improper Input Validation vulnerability in Apache Tomcat. Tomcat did not limit HTTP/0.9 requests to the GET method. If a security constraint was configured to allow HEAD requests to a URI but deny GET requests, the user could bypass that constraint on GET requests by sending a (specification invalid) HEAD request using HTTP/0.9.
SMG does not currently implement any security constraint on endpoints to allow HEAD requests but deny GET requests, hence is not vulnerable to this issue.