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Common Weakness Enumeration

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ID

CWE-776: Improper Restriction of Recursive Entity References in DTDs ('XML Entity Expansion')

Weakness ID: 776
Vulnerability Mapping: ALLOWEDThis CWE ID may be used to map to real-world vulnerabilities
Abstraction: BaseBase - 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 uses XML documents and allows their structure to be defined with a Document Type Definition (DTD), but it does not properly control the number of recursive definitions of entities.
+ Extended Description
If the DTD contains a large number of nested or recursive entities, this can lead to explosive growth of data when parsed, causing a denial of service.
+ Alternate Terms
XEE:
XEE is the acronym commonly used for XML Entity Expansion.
Billion Laughs Attack
XML Bomb:
While the "XML Bomb" term was used in the early years of knowledge of this issue, the XEE term seems to be more commonly used.
+ Common Consequences
Section HelpThis 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.
ScopeImpactLikelihood
Availability

Technical Impact: DoS: Resource Consumption (Other)

If parsed, recursive entity references allow the attacker to expand data exponentially, quickly consuming all system resources.
+ Potential Mitigations

Phase: Operation

If possible, prohibit the use of DTDs or use an XML parser that limits the expansion of recursive DTD entities.

Phase: Implementation

Before parsing XML files with associated DTDs, scan for recursive entity declarations and do not continue parsing potentially explosive content.
+ Relationships
Section HelpThis 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" (CWE-1000)
NatureTypeIDName
ChildOfClassClass - 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.405Asymmetric Resource Consumption (Amplification)
ChildOfClassClass - 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.674Uncontrolled Recursion
CanFollowVariantVariant - a weakness that is linked to a certain type of product, typically involving a specific language or technology. More specific than a Base weakness. Variant level weaknesses typically describe issues in terms of 3 to 5 of the following dimensions: behavior, property, technology, language, and resource.827Improper Control of Document Type Definition
Section HelpThis 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 "Software Development" (CWE-699)
NatureTypeIDName
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.19Data Processing Errors
Section HelpThis 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 "Weaknesses for Simplified Mapping of Published Vulnerabilities" (CWE-1003)
NatureTypeIDName
ChildOfClassClass - 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.674Uncontrolled Recursion
+ Modes Of Introduction
Section HelpThe 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.
PhaseNote
Implementation
Operation
+ Applicable Platforms
Section HelpThis 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)

+ Likelihood Of Exploit
Medium
+ Demonstrative Examples

Example 1

The DTD and the very brief XML below illustrate what is meant by an XML bomb. The ZERO entity contains one character, the letter A. The choice of entity name ZERO is being used to indicate length equivalent to that exponent on two, that is, the length of ZERO is 2^0. Similarly, ONE refers to ZERO twice, therefore the XML parser will expand ONE to a length of 2, or 2^1. Ultimately, we reach entity THIRTYTWO, which will expand to 2^32 characters in length, or 4 GB, probably consuming far more data than expected.

(attack code)
Example Language: XML 
<?xml version="1.0"?>
<!DOCTYPE MaliciousDTD [
<!ENTITY ZERO "A">
<!ENTITY ONE "&ZERO;&ZERO;">
<!ENTITY TWO "&ONE;&ONE;">
...
<!ENTITY THIRTYTWO "&THIRTYONE;&THIRTYONE;">
]>
<data>&THIRTYTWO;</data>

+ Observed Examples
ReferenceDescription
XEE in XML-parsing library.
XML bomb / XEE in enterprise communication product.
"Billion laughs" attack in XMPP server daemon.
XML bomb in web server module
Parsing library allows XML bomb
+ Detection Methods

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
Section HelpThis 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.
NatureTypeIDName
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.1030OWASP Top Ten 2017 Category A4 - XML External Entities (XXE)
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.1349OWASP Top Ten 2021 Category A05:2021 - Security Misconfiguration
MemberOfCategoryCategory - a CWE entry that contains a set of other entries that share a common characteristic.1416Comprehensive Categorization: Resource Lifecycle Management
+ Vulnerability Mapping Notes

Usage: ALLOWED

(this CWE ID could 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.
+ Taxonomy Mappings
Mapped Taxonomy NameNode IDFitMapped Node Name
WASC44XML Entity Expansion
+ References
[REF-676] Amit Klein. "Multiple vendors XML parser (and SOAP/WebServices server) Denial of Service attack using DTD". 2002-12-16. <https://seclists.org/fulldisclosure/2002/Dec/229>. URL validated: 2023-04-07.
[REF-678] Didier Stevens. "Dismantling an XML-Bomb". 2008-09-23. <https://blog.didierstevens.com/2008/09/23/dismantling-an-xml-bomb/>. URL validated: 2023-04-07.
[REF-679] Robert Auger. "XML Entity Expansion". <http://projects.webappsec.org/w/page/13247002/XML%20Entity%20Expansion>. URL validated: 2023-04-07.
[REF-680] Elliotte Rusty Harold. "Tip: Configure SAX parsers for secure processing". 2005-05-27. <https://web.archive.org/web/20101005080451/http://www.ibm.com/developerworks/xml/library/x-tipcfsx.html>. URL validated: 2023-04-07.
[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-682] Blaise Doughan. "Preventing Entity Expansion Attacks in JAXB". 2011-03-11. <http://blog.bdoughan.com/2011/03/preventing-entity-expansion-attacks-in.html>. URL validated: 2023-04-07.
+ Content History
+ Submissions
Submission DateSubmitterOrganization
2009-06-30
(CWE 1.5, 2009-07-27)
CWE Content TeamMITRE
+ Modifications
Modification DateModifierOrganization
2010-02-16CWE Content TeamMITRE
updated Taxonomy_Mappings
2010-12-13CWE Content TeamMITRE
updated Relationships
2011-06-01CWE Content TeamMITRE
updated Common_Consequences
2012-05-11CWE Content TeamMITRE
updated Demonstrative_Examples
2013-02-21CWE Content TeamMITRE
updated Alternate_Terms, Applicable_Platforms, Description, Name, Observed_Examples, References, Relationships
2017-11-08CWE Content TeamMITRE
updated Likelihood_of_Exploit, References
2018-03-27CWE Content TeamMITRE
updated Relationships
2019-06-20CWE Content TeamMITRE
updated Relationships, Type
2020-02-24CWE Content TeamMITRE
updated Applicable_Platforms, Relationships
2021-10-28CWE Content TeamMITRE
updated Relationships
2022-04-28CWE Content TeamMITRE
updated Related_Attack_Patterns
2023-01-31CWE Content TeamMITRE
updated Description
2023-04-27CWE Content TeamMITRE
updated Detection_Factors, References, Relationships
2023-06-29CWE Content TeamMITRE
updated Mapping_Notes
+ Previous Entry Names
Change DatePrevious Entry Name
2013-02-21Unrestricted Recursive Entity References in DTDs ('XML Bomb')
Page Last Updated: July 16, 2024