Name XML Schema Poisoning
Summary 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. Possible attacks are denial of service attacks 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. In addition, manipulation of the data types described in the schema may affect the results of calculations taken by the document reader. For example, a float field could be changed to an int field. Finally, the adversary may 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 us 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.
Solutions 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.
Related Weaknesses
CWE ID Description
CWE-15 External Control of System or Configuration Setting
CWE-472 External Control of Assumed-Immutable Web Parameter
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