XPath Injection
Attack Pattern ID: 83 (Detailed Attack Pattern Completeness: Complete)Typical Severity: HighStatus: Draft
+ Description

Summary

An attacker can craft special user-controllable input consisting of XPath expressions to inject the XML database and bypass authentication or glean information that he normally would not be able to. XPath Injection enables an attacker to talk directly to the XML database, thus bypassing the application completely. XPath Injection results form the failure of an application to properly sanitize input used as part of dynamic XPath expressions used to query an XML database. In order to successfully inject XML and retrieve information from a database, an attacker:

Attack Execution Flow

  1. Determines the user-controllable input that is used without proper validation as part of XPath queries

  2. Determines the structure of queries that accept such input

  3. Crafts malicious content containing XPath expressions that is not validated by the application and is executed as part of the XPath queries.

+ Attack Prerequisites

XPath queries used to retrieve information stored in XML documents

User-controllable input not properly sanitized before being used as part of XPath queries

+ Typical Likelihood of Exploit

Likelihood: High

+ Methods of Attack
  • Injection
+ Examples-Instances

Description

Consider an application that uses an XML database to authenticate its users. The application retrieves the user name and password from a request and forms an XPath expression to query the database. An attacker can successfully bypass authentication and login without valid credentials through XPath Injection. This can be achieved by injecting the query to the XML database with XPath syntax that causes the authentication check to fail. Improper validation of user-controllable input and use of a non-parameterized XPath expression enable the attacker to inject an XPath expression that causes authentication bypass.

+ Attacker Skills or Knowledge Required

Skill or Knowledge Level: Low

XPath Injection shares the same basic premises with SQL Injection. An attacker must have knowledge of XPath synax and constructs in order to successfully leverage XPath Injection

+ Resources Required

None

+ Probing Techniques

The attacker tries to inject characters that can cause an XPath error, such as single-quote ('), or content that may cause a malformed XPath expression. If the injection of such content into the input causes an XPath error and the resulting error is displayed unfiltered, the attacker can begin to determine the nature of input validation and structure of XPath expressions used in queries.

+ Indicators-Warnings of Attack

Too many exceptions generated by the appplication as a result of malformed XPath queries

+ Solutions and Mitigations

Strong input validation - All user-controllable input must be validated and filtered for illegal characters as well as content that can be interpreted in the context of an XPath expression. Characters such as a single-quote(') or operators such as or (|), and (&) and such should be filtered if the application does not expect them in the context in which they appear. If such content cannot be filtered, it must at least be properly escaped to avoid them being interpreted as part of XPath expressions.

Use of parameterized XPath queries - Parameterization causes the input to be restricted to certain domains, such as strings or integers, and any input outside such domains is considered invalid and the query fails.

Use of custom error pages - Attackers can glean information about the nature of queries from descriptive error messages. Input validation must be coupled with customized error pages that inform about an error without disclosing information about the database or application.

+ Attack Motivation-Consequences
  • Privilege Escalation
  • Information Leakage
+ Injection Vector

User-controllable input used as part of dynamic XPath queries

+ Payload

XPath expressions intended to defeat checks run by XPath queries

+ Activation Zone

XML database

+ Payload Activation Impact

The impact of payload activation is that it is interpreted as part of the XPath expression used in the query, thus enabling an attacker to modify the expression used by the query.

+ Related Weaknesses
CWE-IDWeakness NameWeakness Relationship Type
91XML Injection (aka Blind XPath Injection)Targeted
74Failure to Sanitize Data into a Different Plane ('Injection')Secondary
20Improper Input ValidationSecondary
390Detection of Error Condition Without ActionSecondary
713OWASP Top Ten 2007 Category A2 - Injection FlawsTargeted
707Improper Enforcement of Message or Data StructureTargeted
+ Related Attack Patterns
NatureTypeIDNameDescriptionView(s) this relationship pertains toView\(s\)
ChildOfAttack PatternAttack Pattern250XML Injection 
Mechanism of Attack (primary)1000
ChildOfCategoryCategory372WASC Threat Classification 2.0 - WASC-39 - XPath Injection 
WASC Threat Classification 2.0333
+ Relevant Security Requirements

Special characters in user-controllable input must be escaped before use by the application.

Only use parameterized XPath expressions to query the XML database.

Custom error pages must be used to handle exceptions such that they do not reveal any information about the architecture of the application or the database.

+ Related Security Principles

  • Reluctance to Trust

  • Failing Securely

  • Defense in Depth

+ Related Guidelines

  • Never Use Input as Part of a Directive to any Internal Component

  • Handle All Errors Safely

+ Purposes
  • Penetration
  • Exploitation
+ CIA Impact
Confidentiality Impact: HighIntegrity Impact: HighAvailability Impact: Medium
+ Technical Context
Architectural Paradigms
Client-Server
SOA
Frameworks
All
Platforms
All
Languages
All
+ References

CWE - XML Injection

CWE - Input Validation

CWE - Improper Error Handling

+ Content History
Submissions
SubmitterDateComments
Chiradeep B Chhaya2007-01-30Second Draft
Modifications
ModifierOrganizationDateComments
Malik HamroCigital, Inc2007-02-27Reformat to new schema and review
Sean BarnumCigital, Inc2007-03-05Review and revise
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