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Lack of data validation - Reflected Parameters

Need​

Implementation of proper input validation and sanitization

Context​

  • Usage of Go 1.16 for developing high-performance and efficient applications
  • Usage of gin-gonic/gin for building web applications in Go

Description​

Non compliant code​

package main

import (
	"github.com/gin-gonic/gin"
)

func main() {
	r := gin.Default()
	r.GET("/error", func(c *gin.Context) {
		errorMessage := c.Query("error")
		c.String(500, "An error has occurred: %s", errorMessage)
	})
	r.Run()
}

This Go application uses the Gin web framework and has a GET route, /error, which takes a query parameter error and directly includes it in the server's error response without any validation or sanitization.

The errorMessage variable is populated with the error query parameter from the user's request. This variable is then directly used in the response string that is sent back to the user.

This is a vulnerability because an attacker could include malicious scripts or special characters in the error query parameter. Since the server does not validate or sanitize this input before including it in the response, this could lead to Cross-Site Scripting (XSS) attacks. An XSS attack allows an attacker to inject client-side scripts into web pages viewed by other users. This could lead to a variety of malicious activities, such as stealing user data or performing actions on behalf of the user.

To mitigate this vulnerability, the application should sanitize and validate all user inputs before using them. This can be done by encoding the user input, which ensures that any special characters are treated as literal characters rather than executable code. Additionally, the application should implement a Content Security Policy (CSP) to restrict the types of content that can be included in the response.

Steps​

  • Implement input validation to sanitize and validate user input.
  • Use a whitelist approach to allow only specific characters or patterns in the input.
  • Encode or escape user input before displaying it in server error responses.
  • Implement output encoding to prevent XSS attacks.
  • Consider using a web application firewall (WAF) to provide an additional layer of protection against reflected XSS attacks.

Compliant code​

package main

import (
	"github.com/gin-gonic/gin"
	"net/url"
	"regexp"
)

func main() {
	r := gin.Default()
	r.GET("/error", func(c *gin.Context) {
		errorMessage := c.Query("error")
		// Validate the input using a whitelist approach
		isValid := validateInput(errorMessage)
		if isValid {
			// Encode the user input before displaying it
			errorMessage = url.QueryEscape(errorMessage)
			c.String(500, "An error has occurred: %s", errorMessage)
		} else {
			c.String(500, "An error has occurred")
		}
	})
	r.Run()
}

// validateInput checks if the input contains only alphanumeric characters
func validateInput(input string) bool {
	match, _ := regexp.MatchString("^[a-zA-Z0-9]*$", input)
	return match
}

The original code was vulnerable to XSS attacks because it included unvalidated user input in server error responses. This could allow an attacker to inject malicious scripts into the error message, which would then be executed in the user's browser when the error message is displayed.

The fixed code mitigates this vulnerability by implementing input validation and output encoding.

Firstly, the validateInput function is used to check if the user input contains only alphanumeric characters. This is a whitelist approach to input validation, which is generally more secure than a blacklist approach because it explicitly defines what is allowed rather than what is not allowed. If the input is not valid, a generic error message is returned without including the user input.

Secondly, if the input is valid, it is URL-encoded using the url.QueryEscape function before it is included in the error message. This ensures that any special characters in the user input are escaped, preventing them from being interpreted as code.

These measures help to prevent XSS attacks by ensuring that user input is both validated and properly encoded before it is included in server error responses.

References​

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