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
Determines the user-controllable input that is
used without proper validation as part of XPath
queries
Determines the structure of queries that accept
such input
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
Description
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
Description
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
Scope
Technical Impact
Note
Confidentiality
Access_Control
Authorization
Gain privileges / assume
identity
Confidentiality
Read application
data
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
Description
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.
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
Description
