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XPath injection

Need​

Implementation of input validation and sanitization to prevent XPath injection attacks.

Context​

  • Usage of Go 1.16 for developing high-performance and concurrent applications
  • Usage of Gin framework for building web applications in Go
  • Usage of gokogiri for parsing and manipulating HTML/XML documents in Go
  • Usage of gokogiri/xml for XML parsing and manipulation in Go programming language
  • Usage of net/http for creating a web server in a Node.js application
  • Usage of fmt for formatting and printing text in a specific format

Description​

Non compliant code​

package main

import (
	"fmt"
	"github.com/gin-gonic/gin"
	"github.com/moovweb/gokogiri"
	"github.com/moovweb/gokogiri/xml"
	"net/http"
)

func main() {
	r := gin.Default()

	r.GET("/search", func(c *gin.Context) {
		query := c.Query("query")

		doc, _ := gokogiri.ParseHtml([]byte("<books><book><title>Go Programming</title></book><book><title>Learning Go</title></book></books>"))
		defer doc.Free()

		xpath := doc.Root().XmlNode.Search(fmt.Sprintf("//book[title='%s']", query))
		for _, match := range xpath {
			fmt.Println(match.Content())
		}
	})

	r.Run()
}

The above code is a simple Go backend application using the Gin framework. It has a single endpoint, /search, which accepts a query parameter from the user. The query parameter is directly used in an XPath expression to search for book titles in a hardcoded XML document.

The vulnerability lies in the fact that the query parameter is directly concatenated into the XPath expression without any form of sanitization or validation. This means that an attacker could manipulate the query parameter to alter the XPath expression and potentially access data they're not supposed to.

For example, an attacker could send a query parameter like '] or '1'='1, which would result in the XPath expression //book[title='] or '1'='1']. This would return all the books, not just the ones that match the intended title. Other, more complex injections could allow the attacker to traverse the XML document in ways not intended by the application.

This is a classic example of an XPath Injection vulnerability. To fix this issue, the application should validate and sanitize all input from the user before using it in an XPath expression.

Steps​

  • Validate and sanitize user input: Implement input validation and sanitization techniques to ensure that the query parameter is safe to use in the XPath statement.
  • Use parameterized queries: Instead of directly concatenating the user input into the XPath statement, use parameterized queries or prepared statements to separate the query from the data.
  • Use XPath escaping: If you need to include user input in the XPath statement, make sure to properly escape special characters to prevent injection attacks.
  • Limit privileges: Ensure that the user executing the XPath query has the minimum required privileges to prevent unauthorized access to sensitive data.
  • Implement least privilege principle: Restrict the XPath query to only access the necessary elements and attributes, avoiding unnecessary access to sensitive data.
  • Regularly update dependencies: Keep the dependencies, such as the gin framework and gokogiri library, up to date to benefit from security patches and bug fixes.
  • Perform security testing: Conduct regular security testing, including penetration testing and code reviews, to identify and address any potential vulnerabilities.

Compliant code​

package main

import (
	"fmt"
	"github.com/gin-gonic/gin"
	"github.com/moovweb/gokogiri"
	"github.com/moovweb/gokogiri/xml"
	"net/http"
	"regexp"
)

func main() {
	r := gin.Default()

	r.GET("/search", func(c *gin.Context) {
		query := c.Query("query")

		// Validate and sanitize user input
		isValid := validateInput(query)
		if !isValid {
			c.JSON(http.StatusBadRequest, gin.H{"error": "Invalid query parameter"})
			return
		}

		doc, _ := gokogiri.ParseHtml([]byte("<books><book><title>Go Programming</title></book><book><title>Learning Go</title></book></books>"))
		defer doc.Free()

		// Use parameterized queries
		xpath := doc.Root().XmlNode.Search("//book[title=$title]", xml.SearchVariable{"title": query})
		for _, match := range xpath {
			fmt.Println(match.Content())
		}
	})

	r.Run()
}

// Validate and sanitize user input
func validateInput(input string) bool {
	// This is a simple regex for demonstration purposes, adjust according to your needs
	match, _ := regexp.MatchString("^[a-zA-Z0-9 ]*$", input)
	return match
}

The updated code includes a function validateInput that validates and sanitizes the user input. This function uses a regular expression to ensure that the input only contains alphanumeric characters and spaces. If the input is not valid, the function returns a 400 Bad Request response.

The XPath query is now parameterized using xml.SearchVariable. This separates the query from the data, preventing the possibility of XPath injection. The user input is now safely included in the XPath statement.

Remember to adjust the regular expression in the validateInput function according to your needs. This is a simple example and may not cover all possible edge cases.

Also, keep in mind the other steps mentioned earlier: limit privileges, implement the least privilege principle, regularly update dependencies, and perform regular security testing. These steps are crucial for maintaining the security of your application.

References​

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