An adversary corrupts or modifies the content of XML schema information passed between a client and server for the purpose of undermining the security of the target. XML Schemas provide the structure and content definitions for XML documents. Schema poisoning is the ability to manipulate a schema either by replacing or modifying it to compromise the programs that process documents that use this schema.
Likelihood Of Attack
Low
Typical Severity
High
Relationships
This 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.
Nature
Type
ID
Name
ChildOf
Standard 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.
Determine if XML schema is local or remote: Because this attack differs slightly if the target uses remote XML schemas versus local schemas, the adversary first needs to determine which of the two are used.
Experiment
Gain access to XML schema: The adversary gains access to the XML schema so that they can modify the contents.
Techniques
For a local scenario, the adversary needs access to the machine that the schema is located on and needs to gain permissions to alter the contents of the file.
For a remote scenario, the adversary needs to be able to sniff HTTP traffic that contains an XML schema.
Exploit
Poison XML schema: Once the adversary gains access to the XML schema, they will alter it to achieve a desired effect. Locally, they can simply modify the file. For remote schemas, the adversary will alter the schema in transit by performing an adversary in the middle attack.
Techniques
Cause a denial of service by modifying the schema so that it does not contain required information for subsequent processing. For example, the unaltered schema may require a @name attribute in all submitted documents. If the adversary removes this attribute from the schema then documents created using the new grammar may lack this field, which may cause the processing application to enter an unexpected state or record incomplete data.
Manipulation of the data types described in the schema may affect the results of calculations. For example, a float field could be changed to an int field.
Change the encoding defined in the schema for certain fields allowing the contents to bypass filters that scan for dangerous strings. For example, the modified schema might use a URL encoding instead of ASCII, and a filter that catches a semicolon (;) might fail to detect its URL encoding (%3B).
Prerequisites
Some level of access to modify the target schema.
The schema used by the target application must be improperly secured against unauthorized modification and manipulation.
Resources Required
Access to the schema and the knowledge and ability modify it. Ability to replace or redirect access to the modified schema.
Consequences
This 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.
Scope
Impact
Likelihood
Availability
Unreliable Execution
Resource Consumption
Integrity
Modify Data
Confidentiality
Read Data
Mitigations
Design: Protect the schema against unauthorized modification.
Implementation: For applications that use a known schema, use a local copy or a known good repository instead of the schema reference supplied in the XML document. Additionally, ensure that the proper permissions are set on local files to avoid unauthorized modification.
Implementation: For applications that leverage remote schemas, use the HTTPS protocol to prevent modification of traffic in transit and to avoid unauthorized modification.
Example Instances
XML Schema Poisoning Attacks can often occur locally due to being embedded within the XML document itself or being located on the host within an improperaly protected file. In these cases, the adversary can simply edit the XML schema without the need for additional privileges. An example of the former can be seen below:
If the 'name' attribute is required in all submitted documents and this field is removed by the adversary, the application may enter an unexpected state or record incomplete data. Additionally, if this data is needed to perform additional functions, a Denial of Service (DOS) may occur.
XML Schema Poisoning Attacks can also be executed remotely if the HTTP protocol is being used to transport data. :
<?xml version="1.0"?> <!DOCTYPE contact SYSTEM "http://example.com/contact.dtd"[ <note> <name>John Smith</name> <phone>555-1234</phone> <email>jsmith@email.com</email> <address>1 Example Lane</address> </note></capec:Code>
The HTTP protocol does not encrypt the traffic it transports, so all communication occurs in plaintext. This traffic can be observed and modified by the adversary during transit to alter the XML schema before it reaches the end user. The adversary can perform a Adversary-in-the-Middle (CAPEC-94) Attack to alter the schema in the same way as the previous example and to acheive the same results.
Related Weaknesses
A Related Weakness relationship associates a weakness with this attack pattern. Each association implies a weakness that must exist for a given attack to be successful. If multiple weaknesses are associated with the attack pattern, then any of the weaknesses (but not necessarily all) may be present for the attack to be successful. Each related weakness is identified by a CWE identifier.
Description
This 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.
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