CAPEC-197 Detalhe

An adversary submits data to a target application which contains nested exponential data expansion to produce excessively large output. Many data format languages allow the definition of macro-like structures that can be used to simplify the creation of complex structures. However, this capability can be abused to create excessive demands on a processor's CPU and memory. A small number of nested expansions can result in an exponential growth in demands on memory.

Fluxo de Execução

[Survey the target] An adversary determines the input data stream that is being processed by a data parser that supports using subsitituion on the victim's side.

• Use an automated tool to record all instances of URLs to process requests.
• Use a browser to manually explore the website and analyze how the application processes requests.

[Craft malicious payload] The adversary crafts a malicious message containing nested exponential expansion that completely uses up available server resources. See the "Example Instances" section for details on how to craft this malicious payload.

[Send the message] Send the malicious crafted message to the target URL.

Pré-requisitos

This type of attack requires that the target must receive input but either fail to provide an upper limit for entity expansion or provide a limit that is so large that it does not preclude significant resource consumption.

Habilidades Necessárias

Ability to craft nested data expansion messages.

Recursos Necessários

None: No specialized resources are required to execute this type of attack.

Mitigações

Design: Use libraries and templates that minimize unfiltered input. Use methods that limit entity expansion and throw exceptions on attempted entity expansion.
Implementation: For XML based data - disable altogether the use of inline DTD schemas when parsing XML objects. If a DTD must be used, normalize, filter and use an allowlist and parse with methods and routines that will detect entity expansion from untrusted sources.

Fraquezas Relacionadas

Referências

REF-64

Multiple vendors XML parser (and SOAP/WebServices server) Denial of Service attack using DTD
Amit Klein.
http://www.securityfocus.com/archive/1/303509

REF-65

Attacking & Defending Web Services
Pete Lindstrom.
http://www.webtorials.com/main/comnet/cn2003/web-service/24.pdf

REF-66

Tip: Configure SAX parsers for secure processing
Elliotte Rusty Harold.
http://www.ibm.com/developerworks/xml/library/x-tipcfsx.html

REF-67

XML Denial of Service Attacks and Defenses
Bryan Sullivan.
http://msdn.microsoft.com/en-us/magazine/ee335713.aspx

REF-67

XML Denial of Service Attacks and Defenses
Bryan Sullivan.
http://msdn.microsoft.com/en-us/magazine/ee335713.aspx

Submissão

Modificações