Improper Restriction of XML External Entity Reference

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.

See Also

CISQ Quality Measures - Security

Weaknesses in this category are related to the CISQ Quality Measures for Security. Presence of these weaknesses could reduce the security of the software.

OWASP Top Ten 2017 Category A4 - XML External Entities (XXE)

Weaknesses in this category are related to the A4 category in the OWASP Top Ten 2017.

Limit Access

Weaknesses in this category are related to the design and architecture of system resources. Frequently these deal with restricting the amount of resources that are acc...

Comprehensive CWE Dictionary

This view (slice) covers all the elements in CWE.

Weaknesses in the 2020 CWE Top 25 Most Dangerous Software Weaknesses

CWE entries in this view are listed in the 2020 CWE Top 25 Most Dangerous Software Weaknesses.

CISQ Data Protection Measures

This view outlines the SMM representation of the Automated Source Code Data Protection Measurement specifications, as identified by the Consortium for Information & So...

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