Lab: Exploiting blind XXE to exfiltrate data using a malicious external DTD
Summary: PortSwigger's lab presents a stock-check feature that parses XML input without displaying the result, and the exercise is to exfiltrate the contents of the /etc/hostname file. Because the Academy firewall blocks interactions between labs and arbitrary external systems, the lab requires the provided exploit server or Burp Collaborator's default public server, with the Collaborator payload placed in a malicious DTD file and an external entity definition inserted between the XML declaration and the stockCheck element.
Key Takeaway: Blind XXE matters because the parser gives no output to the attacker, so data leaves through an out-of-band channel - here a malicious external DTD pointing at a server the attacker controls - which means a response that looks clean tells you nothing about whether data left the network.
Read the original at PortSwigger Research ↗
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