Is there an example how to configure UdfAccess setting in firebird.conf?
Well, there's one right there in the firebird.conf, but perhaps it isn't obvious enough. Here are the basic settings ('None' to disallow UDFs completely and 'Full' to allow them anywhere) which you probably understood yourself:
UdfAccess = None
UdfAccess = Full
And here is that tricky Restrict setting:
UdfAccess = Restrict C:\some\directory
For multiple directories, use something like this:
UdfAccess = Restrict C:\some\directory;C:\some\other\directory
For Linux users:
UdfAccess = Restrict /some/directory
In the default setting 'Restrict UDF', 'UDF' is a directory relative to root directory of Firebird installation.