CAPEC Details
Name XML Quadratic Expansion
Likelyhood of attack Typical severity
Medium High
Summary An adversary exploits a few properties of XML(substitution entities and inline DTDs) to cause a denial of service situation due to excessive memory being allocated to fully expand the XML. The result of this denial of service could cause the application to freeze or crash.
Prerequisites This type of attack requires a server that accepts XML data and parses the data.
Solutions Design: Use libraries and templates that minimize unfiltered input. Use methods that limit entity expansion and throw exceptions on attempted entity expansion. Implementation: Disable altogether the use of inline DTD schemas in your XML parsing objects. If must use DTD, normalize, filter and use an allowlist and parse with methods and routines that will detect entity expansion from untrusted sources.
Related Weaknesses
CWE ID Description
CWE-770 Allocation of Resources Without Limits or Throttling
Related CAPECS
CAPEC ID Description
CAPEC-230 Applications often need to transform data in and out of the XML format by using an XML parser. It may be possible for an adversary to inject data that may have an adverse effect on the XML parser when it is being processed. By nesting XML data and causing this data to be continuously self-referential, an adversary can cause the XML parser to consume more resources while processing, causing excessive memory consumption and CPU utilization. An adversary's goal is to leverage parser failure to their advantage. In most cases this type of an attack will result in a XML Denial of Service (XDoS) due to an application becoming unstable, freezing, or crashing. However it may be possible to cause a crash resulting in arbitrary code execution, leading to a jump from the data plane to the control plane [REF-89]. XDoS is most closely associated with web services, SOAP, and Rest, because remote service requesters can post malicious XML payloads to the service provider designed to exhaust the service provider's memory, CPU, and/or disk space. The main weakness in XDoS is that the service provider generally must inspect, parse, and validate the XML messages to determine routing, workflow, security considerations, and so on. It is exactly these inspection, parsing, and validation routines that XDoS targets. This attack exploits the loosely coupled nature of web services, where the service provider has little to no control over the service requester and any messages the service requester sends.