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XXE
https://portswigger.net/web-security/xxe

External Entity Injection

XML external entity injection (also known as XXE) is a web security vulnerability that allows an attacker to interfere with an application's processing of XML data. It often allows an attacker to view files on the application server filesystem, and to interact with any backend or external systems that the application itself can access.
Certain applications will occasionally have you post an XML document to do an action. Improper handling of these XML documents can lead to what's known as XML External Entity Injection(XXE). XXE is when an attacker is able to use the ENTITY feature of XML to load resources from outside the website directory, for example, XXE would allow an attacker to load the contents of /etc/passwd.
Payloads: https://github.com/swisskyrepo/PayloadsAllTheThings/tree/master/XXE Injection#classic-xxe (Put into Burp repeater and try different payloads).

XXE Lab Walkthrough

​Link​
Visit a product page, click "Check stock", and intercept the resulting POST request in Burp Suite.
Insert the following external entity definition in between the XML declaration and the stockCheck element:
<!DOCTYPE test [ <!ENTITY xxe SYSTEM "file:///etc/passwd"> ]>
Then replace the productId number with a reference to the external entity: &xxe;
The response should contain "Invalid product ID:" followed by the contents of the /etc/passwd file.

Exploiting XXE to perform SSRF attacks

Aside from retrieval of sensitive data, the other main impact of XXE attacks is that they can be used to perform server-side request forgery (SSRF). This is a potentially serious vulnerability in which the server-side application can be induced to make HTTP requests to any URL that the server can access.
To exploit an XXE vulnerability to perform an SSRF attack, you need to define an external XML entity using the URL that you want to target and use the defined entity within a data value. If you can use the defined entity within a data value that is returned in the application's response, then you will be able to view the response from the URL within the application's response, and so gain two-way interaction with the backend system. If not, then you will only be able to perform blind SSRF attacks (which can still have critical consequences).
In the following XXE example, the external entity will cause the server to make a back-end HTTP request to an internal system within the organization's infrastructure:
<!DOCTYPE foo [ <!ENTITY xxe SYSTEM "http://internal.vulnerable-website.com/"> ]>

XXE attacks via file upload

Some applications allow users to upload files which are then processed server-side. Some common file formats use XML or contain XML subcomponents. Examples of XML-based formats are office document formats like DOCX and image formats like SVG. Lab​
Create a local SVG image with the following content:
<?xml version="1.0" standalone="yes"?><!DOCTYPE test [ <!ENTITY xxe SYSTEM "file:///etc/hostname" > ]><svg width="128px" height="128px" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" version="1.1"><text font-size="16" x="0" y="16">&xxe;</text></svg>
Post a comment on a blog post, and upload this image as an avatar.
When you view your comment, you should see the contents of the /etc/hostname file in your image. Then use the "Submit Solution" button to submit the value of the server hostname.
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Outline
External Entity Injection
XXE Lab Walkthrough
Exploiting XXE to perform SSRF attacks
XXE attacks via file upload