XML External Entities (XXE)
An XML External Entity attack is a type of attack against an application that parses XML input. This attack occurs when XML input containing a reference to an external entity is processed by a weakly configured XML parser. This attack may lead to the disclosure of confidential data, denial of service, server side request forgery, port scanning from the perspective of the machine where the parser is located, and other system impacts.
The XML 1.0 standard defines the structure of an XML document. The standard defines a concept called an entity, which is a storage unit of some type. There are a few different types of entities, external general/parameter parsed entity often shortened to external entity, that can access local or remote content via a declared system identifier. The system identifier is assumed to be a URI that can be dereferenced (accessed) by the XML processor when processing the entity. The XML processor then replaces occurrences of the named external entity with the contents dereferenced by the system identifier. If the system identifier contains tainted data and the XML processor dereferences this tainted data, the XML processor may disclose confidential information normally not accessible by the application. Similar attack vectors apply the usage of external DTDs, external stylesheets, external schemas, etc. which, when included, allow similar external resource inclusion style attacks.
Attacks can include disclosing local files, which may contain sensitive data such as passwords or private user data, using file: schemes or relative paths in the system identifier. Since the attack occurs relative to the application processing the XML document, an attacker may use this trusted application to pivot to other internal systems, possibly disclosing other internal content via http(s) requests or launching a CSRF attack to any unprotected internal services. In some situations, an XML processor library that is vulnerable to client-side memory corruption issues may be exploited by dereferencing a malicious URI, possibly allowing arbitrary code execution under the application account. Other attacks can access local resources that may not stop returning data, possibly impacting application availability if too many threads or processes are not released.
Note that the application does not need to explicitly return the response to the attacker for it to be vulnerable to information disclosures. An attacker can leverage DNS information to exfiltrate data through subdomain names to a DNS server that he/she controls.1
Challenges covered in this chapter
| Name | Description | Difficulty |
|---|---|---|
| XXE Data Access | Retrieve the content of C:\Windows\system.ini or /etc/passwd from the server. |
⭐⭐⭐ |
| XXE DoS | Give the server something to chew on for quite a while. | ⭐⭐⭐⭐⭐ |
ℹ️ Please note that both XXE challenges described below are not
available when running the Juice Shop in either a Docker container or
on a Heroku dyno! Certain aggressive attacks against the underlying XML
parser caused the process to die from "Segmentation Fault" (segfault)
errors. This happens despite the fact that the parsing actually happens
in a sandbox with a timeout. While it is unfortunate to not have XXE
challenges on containerized environments, this somewhat nicely shows how
incredibly dangerous ill-configured XML parsers actually are.
Retrieve the content of C:\Windows\system.ini or /etc/passwd from the server
In this challenge you are tasked to disclose a local file from the server the Juice Shop backend is hosted on.
- You already found the leverage point for this challenge if you solved Use a deprecated B2B interface that was not properly shut down.
- This challenge sounds a lot harder than it actually is, which amplifies how bad the underlying vulnerability is.
- Doing some research on typical XEE attack patterns basically gives away the solution for free.
Give the server something to chew on for quite a while
Similar to Let the server sleep for some time this challenge is about performing a stripped-down denial-of-service attack. But this one is going against an entirely different leverage point.
- The leverage point for this is obviously the same as for the XXE Tier 1 challenge above.
- You can only solve this challenge by keeping the server busy for >2sec with your attack.
- The effectiveness of attack payloads for this challenge might depend on the operating system the Juice Shop is running on.
1. https://owasp.org/www-community/vulnerabilities/XML_External_Entity_(XXE)_Processing ↩