Supplemental information about Web App Firewall policies
Following is supplemental information about particular aspects of Web App Firewall policies that system administrators who manage the Web App Firewall might need to know.
Correct but unexpected behavior
Web application security and modern web sites are complex. In a number of scenarios, a Citrix ADC policy might cause the Web App Firewall to behave differently in certain situations than a user who is familiar with policies would normally expect. Following are a number of cases where the Web App Firewall may behave in an unexpected fashion.
- Request with a missing HTTP Host header and an absolute URL. When a user sends a request, in the majority of cases the request URL is relative. That is, it takes as its starting point the Referer URL, the URL where the user’s browser is located when it sends the request. If a request is sent without a Host header, and with a relative URL, the request is normally blocked both because it violates the HTTP specification and because a request that fails to specify the host could under some circumstances constitute an attack. If a request is sent with an absolute URL, however, even if the Host header is missing, the request bypasses the Web App Firewall and is forwarded to the web server. Although such a request violates the HTTP specification, it poses no possible threat because an absolute URL contains the host.