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

This lab has a stock check feature that fetches data from an internal system. To solve the lab, change the stock check URL to access the admin interface at http://localhost/admin and delete the user carlos. The developer has deployed an anti-SSRF defense you will need to bypass | Karthikeyan Nagaraj

Description

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.

Hint

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

Solution

1. Visit a product page, click “Check stock”, and intercept the resulting POST request in Burp Suite.
2. Insert the following parameter entity definition in between the XML declaration and the stockCheck element:

This will import the Yelp DTD, and then redefine the ISOamso entity, triggering an error message containing the contents of the /etc/passwd file.

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Author: Karthikeyan Nagaraj ~ Cyberw1ng