XML External Entity

An XML External Entity (XXE) attack is a vulnerability that abuses features of XML parsers/data. It often allows an attacker to interact with any backend or external systems that the application itself can access and can allow the attacker to read the file on that system. They can also cause Denial of Service (DoS) attack or could use XXE to perform Server-Side Request Forgery (SSRF) inducing the web application to make requests to other applications. XXE may even enable port scanning and lead to remote code execution. There are two types of XXE attacks: in-band and out-of-band (OOB-XXE). 1) An in-band XXE attack is the one in which the attacker can receive an immediate response to the XXE payload.

2) out-of-band XXE attacks (also called blind XXE), there is no immediate response from the web application and attacker has to reflect the output of their XXE payload to some other file or their own server.

XXE Payload

1) The first payload we'll see is very simple.

<!DOCTYPE replace [<!ENTITY name "feast"> ]> <userInfo> <firstName>falcon</firstName> <lastName>&name;</lastName> </userInfo>

As we can see we are defining a ENTITY called name and assigning it a value feast. Later we are using that ENTITY in our code.

2) We can also use XXE to read some file from the system by defining an ENTITY and having it use the SYSTEM keyword <?xml version="1.0"?> <!DOCTYPE root [<!ENTITY read SYSTEM 'file:///etc/passwd'>]> <root>&read;</root> Here again, we are defining an ENTITY with the name read but the difference is that we are setting it value to `SYSTEM` and path of the file. If we use this payload then a website vulnerable to XXE(normally) would display the content of the file /etc/passwd.

In a similar manner, we can use this kind of payload to read other files but a lot of times you can fail to read files in this manner or the reason for failure could be the file you are trying to read.

Exploitation

1) Let's see how the website would look if we'll try to use the payload for displaying the name.

On the left side, we can see the burp request that was sent with the URL encoded payload and on the right side we can see that the payload was able to successfully display name falcon feast

2) Now let's try to read the /etc/passwd