Security constraints defined by annotations of Servlets in Apache Tomcat 9.0.0.M1 to 9.0.4, 8.5.0 to 8.5.27, 8.0.0.RC1 to 8.0.49 and 7.0.0 to 7.0.84 were only applied once a Servlet had been loaded. Because security constraints defined in this way apply to the URL pattern and any URLs below that point, it was possible - depending on the order Servlets were loaded - for some security constraints not to be applied. This could have exposed resources to users who were not authorised to access them.
Apache Tomcat 7.x through 7.0.70 and 8.x through 8.5.4, when the CGI Servlet is enabled, follows RFC 3875 section 4.1.18 and therefore does not protect applications from the presence of untrusted client data in the HTTP_PROXY environment variable, which might allow remote attackers to redirect an application's outbound HTTP traffic to an arbitrary proxy server via a crafted Proxy header in an HTTP request, aka an "httpoxy" issue. NOTE: the vendor states "A mitigation is planned for future releases of Tomcat, tracked as CVE-2016-5388"; in other words, this is not a CVE ID for a vulnerability.