CWE-611: Improper Restriction of XML External Entity Reference ('XXE')
Weakness ID: 611
The software processes an XML document that can contain XML entities with URIs that resolve to documents outside of the intended sphere of control, causing the product to embed incorrect documents into its output.
XML documents optionally contain a Document Type Definition (DTD), which, among other features, enables the definition of XML entities. It is possible to define an entity by providing a substitution string in the form of a URI. The XML parser can access the contents of this URI and embed these contents back into the XML document for further processing.
By submitting an XML file that defines an external entity with a file:// URI, an attacker can cause the processing application to read the contents of a local file. For example, a URI such as "file:///c:/winnt/win.ini" designates (in Windows) the file C:\Winnt\win.ini, or file:///etc/passwd designates the password file in Unix-based systems. Using URIs with other schemes such as http://, the attacker can force the application to make outgoing requests to servers that the attacker cannot reach directly, which can be used to bypass firewall restrictions or hide the source of attacks such as port scanning.
Once the content of the URI is read, it is fed back into the application that is processing the XML. This application may echo back the data (e.g. in an error message), thereby exposing the file contents.
XXE is an acronym used for the term "XML eXternal Entities"
Time of Introduction
Technical Impact: Read application
data; Read files or
If the attacker is able to include a crafted DTD and a default entity
resolver is enabled, the attacker may be able to access arbitrary files
on the system.
Technical Impact: Bypass protection
The DTD may include arbitrary HTTP requests that the server may
execute. This could lead to other attacks leveraging the server's trust
relationship with other entities.
The software could consume excessive CPU cycles or memory using a URI
that points to a large file, or a device that always returns data such
as /dev/random. Alternately, the URI could reference a file that
contains many nested or recursive entity references to further slow down
CWE-918 (SSRF) and CWE-611 (XXE) are closely related, because they both involve web-related technologies and can launch outbound requests to unexpected destinations. However, XXE can be performed client-side, or in other contexts in which the software is not acting directly as a server, so the "Server" portion of the SSRF acronym does not necessarily apply.