Lack of data validation - Content Spoofing

Need

Implementation of robust data validation and authentication mechanisms

Context

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

Description

Non compliant code

package main

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

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

	r.POST("/upload", func(c *gin.Context) {
		file, _, err := c.Request.FormFile("file")
		if err != nil {
			c.JSON(500, gin.H{
				"message": err.Error(),
			})
			return
		}

		out, err := os.Create("uploaded_file")
		if err != nil {
			c.JSON(500, gin.H{
				"message": err.Error(),
			})
			return
		}
		defer out.Close()

		_, err = io.Copy(out, file)
		if err != nil {
			c.JSON(500, gin.H{
				"message": err.Error(),
			})
			return
		}

		c.JSON(200, gin.H{
			"message": "file uploaded successfully",
		})
	})

	r.Run()
}

The application has an endpoint /upload where it accepts file uploads from clients. The uploaded file is then written to the server with the filename uploaded_file.

The vulnerability lies in the fact that the application does not validate if a file with the same name has already been uploaded. This means that if an attacker uploads a file with the same name, it will overwrite the existing file on the server without any checks or validation.

Additionally, the application does not verify the authenticity of the client making the upload request. This means that an unauthorized attacker can generate a token without credentials and replace files already uploaded by a client.

This lack of data validation can lead to content spoofing where an attacker can upload malicious files to the server, potentially causing harm to the system or other users who download or access these files.

Steps

• Implement authentication and authorization mechanisms to ensure that only authorized users can upload files.
• Validate the uploaded file to ensure it meets the expected format and size.
• Check if the file already exists before saving it to prevent overwriting existing files.
• Use a secure file naming convention to prevent malicious users from spoofing file names.
• Implement input validation to prevent any potential injection attacks.
• Consider implementing file integrity checks to detect any tampering with the uploaded files.

Compliant code

package main

import (
	"github.com/gin-gonic/gin"
	"io"
	"os"
	"errors"
	"net/http"
)

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

	r.Use(AuthMiddleware()) // Add authentication middleware

	r.POST("/upload", func(c *gin.Context) {
		file, header, err := c.Request.FormFile("file")
		if err != nil {
			c.JSON(http.StatusInternalServerError, gin.H{
				"message": err.Error(),
			})
			return
		}

		// Validate file size and format here

		filename := header.Filename
		out, err := os.OpenFile(filename, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0666)
		if err != nil {
			if os.IsExist(err) {
				c.JSON(http.StatusBadRequest, gin.H{
					"message": "file already exists",
				})
			} else {
				c.JSON(http.StatusInternalServerError, gin.H{
					"message": err.Error(),
				})
			}
			return
		}
		defer out.Close()

		_, err = io.Copy(out, file)
		if err != nil {
			c.JSON(http.StatusInternalServerError, gin.H{
				"message": err.Error(),
			})
			return
		}

		c.JSON(http.StatusOK, gin.H{
			"message": "file uploaded successfully",
		})
	})

	r.Run()
}

// AuthMiddleware is a sample middleware for authentication and authorization
func AuthMiddleware() gin.HandlerFunc {
	return func(c *gin.Context) {
		// Implement your authentication and authorization logic here
		// If not authorized, return error
		// c.AbortWithStatusJSON(http.StatusUnauthorized, gin.H{"error": "unauthorized"})
		// return

		c.Next()
	}
}

The updated code includes several security improvements to address the identified vulnerability:

1. Authentication and Authorization: The AuthMiddleware function is a placeholder for authentication and authorization logic. This middleware should be updated to verify the user's credentials and permissions before allowing them to upload a file.

2. File Validation: Before the file is saved, the application should validate the file's size and format to ensure it meets the expected criteria. This validation logic should be added where the comment // Validate file size and format here is located.

3. File Existence Check: The os.OpenFile function is used with the os.O_CREATE and os.O_EXCL flags. This combination of flags will cause the function to fail if the file already exists, preventing an existing file from being overwritten.

4. Secure File Naming: The filename is taken directly from the uploaded file's header. This could be updated to use a secure naming convention, such as generating a unique filename for each upload.

5. Error Handling: The application now returns more specific error messages, which can help with troubleshooting while not revealing sensitive information.

6. Input Validation and File Integrity Checks: These are not directly included in the updated code, but should be considered as additional security measures. Input validation can prevent injection attacks, while file integrity checks can detect tampering with the uploaded files.

References

• 189.Lack of data validation - Content Spoofing