Issue Summary

A specific feature flag on Confluence's backend that increases the security of the product doesn't allow for Relative paths in the CSS properties.
The FF in question is: pdf.export.settings.ssrf.check

Steps to Reproduce

1. If your Confluence Cloud site has the security feature flag enabled, navigate to any space in Space Settings > Look and Feel > PDF Export
2. Under the PDF stylesheet section, add this piece of CSS:

       @top-right {
           background-image: url(/download/attachments/12344/image.png?api=v2);
           background-repeat: no-repeat;
           height: auto;
           width: 10em;
       }
   

3. Hit the Save button.

Expected Results

The settings are saved as usual

Actual Results

A 500 error is thrown with the following description:

message: java.lang.IllegalArgumentException: Host name may not be null
java.lang.RuntimeException: java.lang.IllegalArgumentException: Host name may not be null
com.atlassian.confluence.extra.flyingpdf.config.ConfigurePdfExportAction.ssrfCheck(ConfigurePdfExportAction.java:162)
com.atlassian.confluence.extra.flyingpdf.config.ConfigurePdfExportAction.doEdit(ConfigurePdfExportAction.java:252)
ava.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

Workaround

Update the URL parameters to use the Absolute path of the file:

    @top-right {
        background-image: url(https://SITENAME.atlassian.net/wiki/download/attachments/12344/image.png?api=v2);
        background-repeat: no-repeat;
        height: auto;
        width: 10em;
    }