ABSTRACT

Constructing a dynamic XPath query with user input may allow an attacker to modify the statement's meaning.

EXPLANATION

XPath injection occurs when:

1. Data enters a program from an untrusted source.



2. The data used to dynamically construct an XPath query.



Example 1: The following code dynamically constructs and executes an XPath query that retrieves an e-mail address for a given account ID. The account ID is read from an HTTP request, and is therefore untrusted.


...
string acctID = Request["acctID"];
string query = null;
if(acctID != null) {
StringBuffer sb = new StringBuffer("/accounts/account[acctID='");
sb.append(acctID);
sb.append("']/email/text()");
query = sb.toString();
}

XPathDocument docNav = new XPathDocument(myXml);
XPathNavigator nav = docNav.CreateNavigator();
nav.Evaluate(query);
...


Under normal conditions, such as searching for an e-mail address that belongs to the account number 1, the query that this code executes will look like the following:

/accounts/account[acctID='1']/email/text()

However, because the query is constructed dynamically by concatenating a constant base query string and a user input string, the query only behaves correctly if acctID does not contain a single-quote character. If an attacker enters the string 1' or '1' = '1 for acctID, then the query becomes the following:

/accounts/account[acctID='1' or '1' = '1']/email/text()

The addition of the 1' or '1' = '1 condition causes the where clause to always evaluate to true, so the query becomes logically equivalent to the much simpler query:

//email/text()

This simplification of the query allows the attacker to bypass the requirement that the query only return items owned by the authenticated user; the query now returns all e-mail addresses stored in the document, regardless of their specified owner.

REFERENCES

[1] Standards Mapping - OWASP Top 10 2010 - (OWASP 2010) A1 Injection

[2] Standards Mapping - OWASP Top 10 2007 - (OWASP 2007) A2 Injection Flaws

[3] Standards Mapping - OWASP Top 10 2004 - (OWASP 2004) A6 Injection Flaws

[4] Standards Mapping - Security Technical Implementation Guide Version 3 - (STIG 3) APP3510 CAT I

[5] Standards Mapping - Common Weakness Enumeration - (CWE) CWE ID 643

[6] Standards Mapping - Payment Card Industry Data Security Standard Version 1.2 - (PCI 1.2) Requirement 6.3.1.1, Requirement 6.5.2

[7] Standards Mapping - Payment Card Industry Data Security Standard Version 2.0 - (PCI 2.0) Requirement 6.5.1

[8] Standards Mapping - Payment Card Industry Data Security Standard Version 1.1 - (PCI 1.1) Requirement 6.5.6

[9] Standards Mapping - FIPS200 - (FISMA) SI

[10] Standards Mapping - Web Application Security Consortium 24 + 2 - (WASC 24 + 2) XPath Injection