Stored cross-site scripting (XSS)

Need

Prevention of persistent cross-site scripting attacks

Context

• Usage of Go 1.16 for building 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("/", func(c *gin.Context) {
		c.String(200, c.Query("input"))
	})

	r.Run()
}

The above Go code is a simple web server using the Gin framework. It has a single route, /, which simply returns a string of whatever is passed to it via the input query parameter.

This code is vulnerable to Stored Cross-Site Scripting (XSS) attacks. The server takes whatever string is passed to it in the input query parameter and directly sends it back to the client without any sanitization or encoding. This means that if an attacker were to pass a string of malicious JavaScript code as the input parameter, the server would simply return that code back to the client, where it would be executed in the user's browser.

For example, an attacker could send a request like http://localhost:8080/?input=<script>/*Malicious JavaScript code*/</script>. The server would then return that script back to the client, causing the malicious code to be executed in the user's browser.

This could allow an attacker to perform various malicious actions, such as stealing the user's session cookies or injecting malicious software into their browser.

Steps

• Sanitize user input: Implement input validation and filtering to remove any potentially malicious code.
• Use context-aware output encoding: Encode user-generated content before displaying it to prevent the execution of any embedded scripts.
• Implement Content Security Policy (CSP): Set a strict CSP header to restrict the types of content that can be loaded on the page.
• Enable XSS protection mechanisms: Enable built-in XSS protection mechanisms provided by the framework or use third-party libraries to mitigate XSS attacks.
• Educate developers: Provide training and guidelines to developers on secure coding practices to prevent XSS vulnerabilities in the future.

Compliant code

package main

import (
	"github.com/gin-gonic/gin"
	"github.com/microcosm-cc/bluemonday"
)

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

	r.GET("/", func(c *gin.Context) {
		input := c.Query("input")
		p := bluemonday.UGCPolicy()
		sanitizedInput := p.Sanitize(input)
		c.String(200, sanitizedInput)
	})

	r.Run()
}

The original code was vulnerable to Stored Cross-Site Scripting (XSS) attacks because it directly used the user input without any sanitization or validation. This could allow an attacker to inject malicious JavaScript code into the application fields, which could then be executed in the browser of any user who views the page.

The fixed code mitigates this vulnerability by sanitizing the user input using the bluemonday library, which is a HTML sanitizer written in Go. The bluemonday.UGCPolicy() function returns a policy that allows a broad selection of HTML elements and attributes that are safe for user generated content. This policy is then used to sanitize the user input, effectively removing any potentially malicious code.

This fix addresses the first two steps in the vulnerability mitigation strategy:

1. Sanitize user input: The user input is sanitized using the bluemonday library to remove any potentially malicious code.
2. Use context-aware output encoding: The sanitized user input is then safely outputted to the page.

The remaining steps (implementing a Content Security Policy, enabling XSS protection mechanisms, and educating developers) are not directly related to the code, but are important measures that should be taken to further secure the application.

References

• 010.Stored cross-site scripting (XSS)