Delta CRLs are Not Accessible via HTTP When Hosted on IIS

If you are running a Microsoft PKI in your environment then chances are you will have (or at least you should have) configured at least one HTTP based distribution point (CDP) for your Certificate Revocation Lists. If you are only publishing full CRLs then you will have no problems however if you are publishing Delta CRLs, the smaller, faster to process kind which list only certificates revoked since the last full publish then you may encounter an issue if you are using an IIS website to publish these.

The problem lies in the filename used for the CRLs. In my lab for example, my Certificate Authority issues a CRL file name rjglab-CA.crl and the delta files are named the same as the full CRL but they are appended with the plus character making the file name rjglab-CA+.crl. In it’s native configuration, IIS does not permit the use of the plus character because that character falls into the realms of IIS Request Mapping and the request handler.

HTTP Error Downloading Delta CRL

We can see in the screenshot above what the error code and message given by IIS is when we try to download the Delta CRL in the default configuration.

For an IIS webserver hosting your CRL and Delta CRL, we need to change the behaviour of IIS to allow this plus character to be permitted which luckily is easily done. First off, open IIS Manager on the server which is hosting and making available to clients your Delta CRL file. From the server home in IIS, open the Request Filtering page and from this page, select the Edit Feature Settings button in the Actions bar.

Request Filtering Settings in IIS

On the Edit Request Filtering Settings page under the General section, by default, Allow Double Escaping is disabled. Enable this option and then press OK.

Once you have made the change, try to download the Delta CRL file and you should find that the file is available and you can successfully download it.

Delta CRL Downloaded OK

richardjgreen

Richard works as a Cloud Consultant for Fordway Solution where his primary focus is to help customers understand, adopt and develop with Microsoft Azure, Office 365 and System Center. Richard Green is an IT Pro with over 15 years' of experience in all things Microsoft including System Center and Office 365. He has previously worked as a System Center consultant and as an internal solutions architect across many verticals. Outside of work, he loves motorbikes and is part of the orange army, marshaling for NGRRC, British Superbikes and MotoGP. He is also an Assistant Cub Scout Leader.