Violating Implicit Assumptions Regarding XML Content (aka XML Denial of Service (XDoS))
Attack Pattern ID: 82 (Standard Attack Pattern Completeness: Complete)Typical Severity: Very HighStatus: Draft
+ Description


XML Denial of Service (XDoS) can be applied to any technology that utilizes XML data. This is, of course, most distributed systems technology including Java, .Net, databases, and so on. 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.

There are three primary attack vectors that XDoS can navigate

Target CPU through recursion: attacker creates a recursive payload and sends to service provider

Target memory through jumbo payloads: service provider uses DOM to parse XML. DOM creates in memory representation of XML document, but when document is very large (for example, north of 1 Gb) service provider host may exhaust memory trying to build memory objects.

XML Ping of death: attack service provider with numerous small files that clog the system.

All of the above attacks exploit 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.

+ Attack Prerequisites

Attacker must be able to send a malicious XML payload to host, such as SOAP or REST web service.

+ Typical Likelihood of Exploit

Likelihood: High

+ Methods of Attack
  • Injection
+ Examples-Instances


Several commercial XML parsers were found to be vulnerable to XDoS through XML recursion attacks. The code fragment below is self-referencing and can result in the parser exhausting all CPU and/or memory available to it.

<!DOCTYPE evildoc [
<!ENTITY x0 hello XDoS">
<!ENTITY xevilparam &x99;&x99;"> ]>

By the time the service provider validatesthe DTD elements it is too late, because the validation routines references itself. SOAP messages are no longer allowed to accept DTDs, however there is nothing to stop developers of other applications or custom SOAP implementations from bypassing this concern.

+ Attacker Skills or Knowledge Required

Skill or Knowledge Level: Low

Crafting malicious XML content and injecting it through standard interfaces

+ Solutions and Mitigations

Design: Utilize a Security Pipeline Interface (SPI) to mediate communications between service requester and service provider The SPI should be designed to throttle up and down and handle a variety of payloads.

Design: Utilize clustered and fail over techniques, leverage network transports to provide availability such as HTTP load balancers

Implementation: Check size of XML message before parsing

+ Attack Motivation-Consequences
  • Denial of Service
+ Injection Vector

XML-capable system interfaces

+ Payload

Maliciously crafted XML

+ Activation Zone

XML inspection, parsing and validation routines

+ Payload Activation Impact

Denial of Service

+ Related Weaknesses
CWE-IDWeakness NameWeakness Relationship Type
674Uncontrolled RecursionTargeted
400Uncontrolled Resource Consumption ('Resource Exhaustion')Targeted
770Allocation of Resources Without Limits or ThrottlingTargeted
+ Related Attack Patterns
NatureTypeIDNameDescriptionView(s) this relationship pertains toView\(s\)
ChildOfCategoryCategory119Resource Depletion 
Mechanism of Attack (primary)1000
ParentOfAttack PatternAttack Pattern99XML Parser Attack 
Mechanism of Attack (primary)1000
ParentOfAttack PatternAttack Pattern147XML Ping of Death 
Mechanism of Attack (primary)1000
ParentOfAttack PatternAttack Pattern197XEE (XML Entity Expansion) 
Mechanism of Attack (primary)1000
ParentOfAttack PatternAttack Pattern229XML Attribute Blowup 
Mechanism of Attack (primary)1000
+ Purposes
  • Exploitation
+ CIA Impact
Confidentiality Impact: LowIntegrity Impact: MediumAvailability Impact: High
+ Technical Context
Architectural Paradigms
+ Content History
Gunnar Peterson2007-02-28
Sean BarnumCigital, Inc2007-03-07Review and revise
Richard StruseVOXEM, Inc2007-03-26Review and feedback leading to changes in Name
Sean BarnumCigital, Inc2007-04-16Modified pattern content according to review and feedback