You can check your signatures to find out by following the guidance here: https://support.f5.com/csp/article/K45558510
if there is a signature and you’ve applied it it should block. If there is no signature you will need to add a parameter to do so.
We have added a dedicated signature on the last signature update that mitigates this vulnerability:
200007038 - DotNetNuke - GetCSS Arbitrary File Read
Also on a side note, the POC exploit for this vulnerability is matched by the following signature:
200000042 - ASP.NET configuration file access (web.config) (Parameter)