CWE-643: Improper Neutralization of Data within XPath Expressions ('XPath Injection')

Description

The product uses external input to dynamically construct an XPath expression used to retrieve data from an XML database, but it does not neutralize or incorrectly neutralizes that input. This allows an attacker to control the structure of the query.

The net effect is that the attacker will have control over the information selected from the XML database and may use that ability to control application flow, modify logic, retrieve unauthorized data, or bypass important checks (e.g. authentication).

Potential Impact

Access Control

Bypass Protection Mechanism

Confidentiality

Read Application Data

Demonstrative Examples

Consider the following simple XML document that stores authentication information and a snippet of Java code that uses XPath query to retrieve authentication information:

Informative

<users><user><login>john</login><password>abracadabra</password><home_dir>/home/john</home_dir></user><user><login>cbc</login><password>1mgr8</password><home_dir>/home/cbc</home_dir></user></users>

The Java code used to retrieve the home directory based on the provided credentials is:

Bad

XPath xpath = XPathFactory.newInstance().newXPath();XPathExpression xlogin = xpath.compile("//users/user[login/text()='" + login.getUserName() + "' and password/text() = '" + login.getPassword() + "']/home_dir/text()");Document d = DocumentBuilderFactory.newInstance().newDocumentBuilder().parse(new File("db.xml"));String homedir = xlogin.evaluate(d);

Assume that user "john" wishes to leverage XPath Injection and login without a valid password. By providing a username "john" and password "' or ''='" the XPath expression now becomes

Attack

//users/user[login/text()='john' or ''='' and password/text() = '' or ''='']/home_dir/text()

This lets user "john" login without a valid password, thus bypassing authentication.

Mitigations & Prevention

Implementation

Use parameterized XPath queries (e.g. using XQuery). This will help ensure separation between data plane and control plane.

Implementation

Properly validate user input. Reject data where appropriate, filter where appropriate and escape where appropriate. Make sure input that will be used in XPath queries is safe in that context.

Detection Methods

Automated Static Analysis High — Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then sea

Taxonomy Mappings

WASC: 39 — XPath Injection
Software Fault Patterns: SFP24 — Tainted input to command

Frequently Asked Questions

What is CWE-643?

CWE-643 (Improper Neutralization of Data within XPath Expressions ('XPath Injection')) is a software weakness identified by MITRE's Common Weakness Enumeration. It is classified as a Base-level weakness. The product uses external input to dynamically construct an XPath expression used to retrieve data from an XML database, but it does not neutralize or incorrectly neutralizes that input. This allows a...

How can CWE-643 be exploited?

Attackers can exploit CWE-643 (Improper Neutralization of Data within XPath Expressions ('XPath Injection')) to bypass protection mechanism. This weakness is typically introduced during the Implementation phase of software development.

How do I prevent CWE-643?

Key mitigations include: Use parameterized XPath queries (e.g. using XQuery). This will help ensure separation between data plane and control plane.

What is the severity of CWE-643?

CWE-643 is classified as a Base-level weakness (Medium abstraction). Its actual severity depends on the specific context and how the weakness manifests in your application.