CWE-611: Improper Restriction of XML External Entity Reference (Base)

Description: The product 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.

Extended Description: 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:Winntwin.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.


Alternate Terms: ::TERM:XXE:DESCRIPTION:An acronym used for the term XML eXternal Entities::

Applicable Platforms: ::LANGUAGE NAME:XML:LANGUAGE PREVALENCE:Undetermined::TECHNOLOGY CLASS:Web Based:TECHNOLOGY PREVALENCE:Undetermined::

Modes of Introduction: ::PHASE:Implementation:NOTE:REALIZATION: This weakness is caused during implementation of an architectural security tactic.::



Common Consequences: ::SCOPE:Confidentiality:IMPACT:Read Application Data:IMPACT:Read Files or Directories:NOTE: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.::SCOPE:Integrity:IMPACT:Bypass Protection Mechanism:NOTE: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.::SCOPE:Availability:IMPACT:DoS: Resource Consumption (CPU):IMPACT:DoS: Resource Consumption (Memory):NOTE:The product 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 parsing.::

Detection Methods: ::METHOD:Automated Static Analysis:DESCRIPTION:Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect sources (origins of input) with sinks (destinations where the data interacts with external components, a lower layer such as the OS, etc.):EFFECTIVENESS:High::

Potential Mitigations: ::PHASE:Implementation System Configuration:DESCRIPTION:Many XML parsers and validators can be configured to disable external entity expansion.::

Observed Examples: ::REFERENCE:CVE-2022-42745:DESCRIPTION:Recruiter software allows reading arbitrary files using XXE:LINK:https://www.cve.org/CVERecord?id=CVE-2022-42745::REFERENCE:CVE-2005-1306:DESCRIPTION:A browser control can allow remote attackers to determine the existence of files via Javascript containing XML script.:LINK:https://www.cve.org/CVERecord?id=CVE-2005-1306::REFERENCE:CVE-2012-5656:DESCRIPTION:XXE during SVG image conversion:LINK:https://www.cve.org/CVERecord?id=CVE-2012-5656::REFERENCE:CVE-2012-2239:DESCRIPTION:XXE in PHP application allows reading the application's configuration file.:LINK:https://www.cve.org/CVERecord?id=CVE-2012-2239::REFERENCE:CVE-2012-3489:DESCRIPTION:XXE in database server:LINK:https://www.cve.org/CVERecord?id=CVE-2012-3489::REFERENCE:CVE-2012-4399:DESCRIPTION:XXE in rapid web application development framework allows reading arbitrary files.:LINK:https://www.cve.org/CVERecord?id=CVE-2012-4399::REFERENCE:CVE-2012-3363:DESCRIPTION:XXE via XML-RPC request.:LINK:https://www.cve.org/CVERecord?id=CVE-2012-3363::REFERENCE:CVE-2012-0037:DESCRIPTION:XXE in office document product using RDF.:LINK:https://www.cve.org/CVERecord?id=CVE-2012-0037::REFERENCE:CVE-2011-4107:DESCRIPTION:XXE in web-based administration tool for database.:LINK:https://www.cve.org/CVERecord?id=CVE-2011-4107::REFERENCE:CVE-2010-3322:DESCRIPTION:XXE in product that performs large-scale data analysis.:LINK:https://www.cve.org/CVERecord?id=CVE-2010-3322::REFERENCE:CVE-2009-1699:DESCRIPTION:XXE in XSL stylesheet functionality in a common library used by some web browsers.:LINK:https://www.cve.org/CVERecord?id=CVE-2009-1699::



Notes: ::TYPE:Relationship:NOTE: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.::