CWE - CWE-611: Improper Restriction of XML External Entity Reference (4.17)

Weakness ID: 611

Vulnerability Mapping: ALLOWED This CWE ID may be used to map to real-world vulnerabilities
Abstraction: Base Base - a weakness that is still mostly independent of a resource or technology, but with sufficient details to provide specific methods for detection and prevention. Base level weaknesses typically describe issues in terms of 2 or 3 of the following dimensions: behavior, property, technology, language, and resource.

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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:\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.

Alternate Terms

XXE	An acronym used for the term "XML eXternal Entities"

Common Consequences

This table specifies different individual consequences associated with the weakness. The Scope identifies the application security area that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in exploiting this weakness. The Likelihood provides information about how likely the specific consequence is expected to be seen relative to the other consequences in the list. For example, there may be high likelihood that a weakness will be exploited to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.

Impact	Details
Read Application Data; Read Files or Directories	Scope: Confidentiality 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.
Bypass Protection Mechanism	Scope: Integrity 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.
DoS: Resource Consumption (CPU); DoS: Resource Consumption (Memory)	Scope: Availability 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.

Potential Mitigations

Phase(s)	Mitigation
Implementation; System Configuration	Many XML parsers and validators can be configured to disable external entity expansion.

Relationships

This table shows the weaknesses and high level categories that are related to this weakness. These relationships are defined as ChildOf, ParentOf, MemberOf and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as PeerOf and CanAlsoBe are defined to show similar weaknesses that the user may want to explore.

Relevant to the view "Research Concepts" (View-1000)

Nature	Type	ID	Name
ChildOf	Class - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource.	610	Externally Controlled Reference to a Resource in Another Sphere
PeerOf	Class - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource.	441	Unintended Proxy or Intermediary ('Confused Deputy')

Relevant to the view "Software Development" (View-699)

Nature	Type	ID	Name
MemberOf	Category - a CWE entry that contains a set of other entries that share a common characteristic.	19	Data Processing Errors

Relevant to the view "Weaknesses for Simplified Mapping of Published Vulnerabilities" (View-1003)

Nature	Type	ID	Name
ChildOf	Class - a weakness that is described in a very abstract fashion, typically independent of any specific language or technology. More specific than a Pillar Weakness, but more general than a Base Weakness. Class level weaknesses typically describe issues in terms of 1 or 2 of the following dimensions: behavior, property, and resource.	610	Externally Controlled Reference to a Resource in Another Sphere

Relevant to the view "Architectural Concepts" (View-1008)

Nature	Type	ID	Name
MemberOf	Category - a CWE entry that contains a set of other entries that share a common characteristic.	1015	Limit Access

Modes Of Introduction

The different Modes of Introduction provide information about how and when this weakness may be introduced. The Phase identifies a point in the life cycle at which introduction may occur, while the Note provides a typical scenario related to introduction during the given phase.

Phase	Note
Implementation	REALIZATION: This weakness is caused during implementation of an architectural security tactic.

Applicable Platforms

This listing shows possible areas for which the given weakness could appear. These may be for specific named Languages, Operating Systems, Architectures, Paradigms, Technologies, or a class of such platforms. The platform is listed along with how frequently the given weakness appears for that instance.

Languages	XML (Undetermined Prevalence)
Technologies	Class: Web Based (Undetermined Prevalence)

Selected Observed Examples

Note: this is a curated list of examples for users to understand the variety of ways in which this weakness can be introduced. It is not a complete list of all CVEs that are related to this CWE entry.

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

Detection Methods

Method

Details

Automated Static Analysis

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

Memberships

This MemberOf Relationships table shows additional CWE Categories and Views that reference this weakness as a member. This information is often useful in understanding where a weakness fits within the context of external information sources.

Nature	Type	ID	Name
MemberOf	Category - a CWE entry that contains a set of other entries that share a common characteristic.	990	SFP Secondary Cluster: Tainted Input to Command
MemberOf	Category - a CWE entry that contains a set of other entries that share a common characteristic.	1030	OWASP Top Ten 2017 Category A4 - XML External Entities (XXE)
MemberOf	View - a subset of CWE entries that provides a way of examining CWE content. The two main view structures are Slices (flat lists) and Graphs (containing relationships between entries).	1200	Weaknesses in the 2019 CWE Top 25 Most Dangerous Software Errors
MemberOf	Category - a CWE entry that contains a set of other entries that share a common characteristic.	1308	CISQ Quality Measures - Security
MemberOf	View - a subset of CWE entries that provides a way of examining CWE content. The two main view structures are Slices (flat lists) and Graphs (containing relationships between entries).	1337	Weaknesses in the 2021 CWE Top 25 Most Dangerous Software Weaknesses
MemberOf	View - a subset of CWE entries that provides a way of examining CWE content. The two main view structures are Slices (flat lists) and Graphs (containing relationships between entries).	1340	CISQ Data Protection Measures
MemberOf	Category - a CWE entry that contains a set of other entries that share a common characteristic.	1349	OWASP Top Ten 2021 Category A05:2021 - Security Misconfiguration
MemberOf	View - a subset of CWE entries that provides a way of examining CWE content. The two main view structures are Slices (flat lists) and Graphs (containing relationships between entries).	1350	Weaknesses in the 2020 CWE Top 25 Most Dangerous Software Weaknesses
MemberOf	View - a subset of CWE entries that provides a way of examining CWE content. The two main view structures are Slices (flat lists) and Graphs (containing relationships between entries).	1387	Weaknesses in the 2022 CWE Top 25 Most Dangerous Software Weaknesses
MemberOf	Category - a CWE entry that contains a set of other entries that share a common characteristic.	1396	Comprehensive Categorization: Access Control

Vulnerability Mapping Notes

Usage	ALLOWED (this CWE ID may be used to map to real-world vulnerabilities)
Reason	Acceptable-Use
Rationale	This CWE entry is at the Base level of abstraction, which is a preferred level of abstraction for mapping to the root causes of vulnerabilities.
Comments	Carefully read both the name and description to ensure that this mapping is an appropriate fit. Do not try to 'force' a mapping to a lower-level Base/Variant simply to comply with this preferred level of abstraction.

Notes

Relationship

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.

Taxonomy Mappings

Mapped Taxonomy Name	Node ID	Fit	Mapped Node Name
WASC	43		XML External Entities
Software Fault Patterns	SFP24		Tainted input to command

Related Attack Patterns

CAPEC-ID	Attack Pattern Name
CAPEC-221	Data Serialization External Entities Blowup

References

[REF-496]	OWASP. "XML External Entity (XXE) Processing". <https://www.owasp.org/index.php/XML_External_Entity_(XXE)_Processing>.
[REF-497]	Sascha Herzog. "XML External Entity Attacks (XXE)". 2010-10-20. <https://owasp.org/www-pdf-archive/XML_Exteral_Entity_Attack.pdf>. (URL validated: 2023-04-07)
[REF-498]	Gregory Steuck. "XXE (Xml eXternal Entity) Attack". <https://www.beyondsecurity.com/>. (URL validated: 2023-04-07)
[REF-499]	WASC. "XML External Entities (XXE) Attack". <http://projects.webappsec.org/w/page/13247003/XML%20External%20Entities>.
[REF-500]	Bryan Sullivan. "XML Denial of Service Attacks and Defenses". 2009-09. <https://learn.microsoft.com/en-us/archive/msdn-magazine/2009/november/xml-denial-of-service-attacks-and-defenses>. (URL validated: 2023-04-07)
[REF-501]	Chris Cornutt. "Preventing XXE in PHP". <https://websec.io/2012/08/27/Preventing-XXE-in-PHP.html>. (URL validated: 2023-04-07)

Content History

Submissions
Submission Date	Submitter	Organization
2007-05-07 (CWE Draft 6, 2007-05-07)	Anonymous Tool Vendor (under NDA)
2007-05-07 (CWE Draft 6, 2007-05-07)
Modifications
Modification Date	Modifier	Organization
2023-10-26	CWE Content Team	MITRE
2023-10-26	updated Observed_Examples
2023-06-29	CWE Content Team	MITRE
2023-06-29	updated Mapping_Notes
2023-04-27	CWE Content Team	MITRE
2023-04-27	updated Detection_Factors, References, Relationships
2023-01-31	CWE Content Team	MITRE
2023-01-31	updated Alternate_Terms, Common_Consequences, Description
2022-06-28	CWE Content Team	MITRE
2022-06-28	updated Relationships
2021-10-28	CWE Content Team	MITRE
2021-10-28	updated Relationships
2021-07-20	CWE Content Team	MITRE
2021-07-20	updated Relationships
2020-12-10	CWE Content Team	MITRE
2020-12-10	updated Relationships
2020-08-20	CWE Content Team	MITRE
2020-08-20	updated Relationships
2020-02-24	CWE Content Team	MITRE
2020-02-24	updated Applicable_Platforms, Relationships
2019-09-19	CWE Content Team	MITRE
2019-09-19	updated Relationships
2019-06-20	CWE Content Team	MITRE
2019-06-20	updated Name, Type
2019-01-03	CWE Content Team	MITRE
2019-01-03	updated Related_Attack_Patterns
2018-03-27	CWE Content Team	MITRE
2018-03-27	updated Relationships
2017-11-08	CWE Content Team	MITRE
2017-11-08	updated Modes_of_Introduction, References, Relationships, Relevant_Properties
2015-12-07	CWE Content Team	MITRE
2015-12-07	updated Relationships
2014-07-30	CWE Content Team	MITRE
2014-07-30	updated Relationships, Taxonomy_Mappings
2013-02-21	CWE Content Team	MITRE
2013-02-21	updated Alternate_Terms, Applicable_Platforms, Background_Details, Common_Consequences, Description, Name, Observed_Examples, Potential_Mitigations, References, Relationship_Notes, Relationships, Taxonomy_Mappings
2012-05-11	CWE Content Team	MITRE
2012-05-11	updated Relationships
2011-06-01	CWE Content Team	MITRE
2011-06-01	updated Common_Consequences
2011-03-29	CWE Content Team	MITRE
2011-03-29	updated Name
2010-09-27	CWE Content Team	MITRE
2010-09-27	updated Background_Details, Other_Notes
2010-02-16	CWE Content Team	MITRE
2010-02-16	updated Taxonomy_Mappings
2008-09-08	CWE Content Team	MITRE
2008-09-08	updated Description, Relationships, Observed_Example, Other_Notes, Taxonomy_Mappings
2008-07-01	Eric Dalci	Cigital
2008-07-01	updated Time_of_Introduction
Previous Entry Names
Change Date	Previous Entry Name
2011-03-29	Information Leak Through XML External Entity File Disclosure
2013-02-21	Information Exposure Through XML External Entity Reference
2019-06-20	Improper Restriction of XML External Entity Reference ('XXE')


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CWE-611: Improper Restriction of XML External Entity Reference

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