Skip to main content

Inappropriate coding practices - invalid file

Need​

Enforcement of proper file validation and verification

Context​

  • Usage of Swift for iOS and macOS app development
  • Usage of Foundation for building responsive and accessible web interfaces

Description​

Non compliant code​

import Foundation

func readFileFromPath(path: String) -> String {
    do {
        let fileUrl = URL(fileURLWithPath: path)
        let fileExtension = fileUrl.pathExtension

        // Reading file content without verifying its extension
        let fileContent = try String(contentsOf: fileUrl, encoding: .utf8)

        return fileContent
    } catch {
        print("Error: \\(error)")
        return ""
    }
}

The above Swift code represents a function readFileFromPath(path: String) which reads the file content from a given file path. The problem with this code is that it does not verify the file extension before reading its content. This is a bad practice as the content of the file might not be compatible with its extension.

For instance, if a png image file is saved with a .txt extension, the function would still attempt to read it as a text file, which could lead to an error or unexpected behavior. This inappropriate coding practice poses a vulnerability as it could cause the application to crash or behave unpredictably, and it could potentially be exploited to cause harm to the application or its data.

To fix this vulnerability, the code should verify that the file's content is compatible with its extension before attempting to read it. This could be done by comparing the file's MIME type with its extension, or by using a library or API that can accurately determine the file's true type.

Steps​

  • Validate the file extension before reading the file content.
  • Implement a whitelist or blacklist of allowed file extensions.
  • Ensure that the file extension matches the expected content type.
  • Consider using a file signature or magic number to verify the file content.
  • Implement proper error handling and logging for file reading operations.

Compliant code​

import Foundation

func readFileFromPath(path: String) -> String {
    do {
        let fileUrl = URL(fileURLWithPath: path)
        let fileExtension = fileUrl.pathExtension

        // Whitelist of allowed file extensions
        let allowedExtensions = ["txt", "json", "xml"]

        // Check if the file extension is in the whitelist
        guard allowedExtensions.contains(fileExtension) else {
            print("Error: Invalid file extension")
            return ""
        }

        // Reading file content
        let fileContent = try String(contentsOf: fileUrl, encoding: .utf8)

        return fileContent
    } catch {
        print("Error: \\(error)")
        return ""
    }
}

The updated code now includes a whitelist of allowed file extensions. Before reading the file content, the code checks if the file's extension is in the whitelist. If the extension is not in the whitelist, the function prints an error message and returns an empty string.

This approach helps to ensure that only files with the expected extensions are processed. It can help to prevent issues such as processing a binary file as a text file, which could lead to unexpected behavior or security vulnerabilities.

In addition, the code continues to include proper error handling for the file reading operation. If an error occurs while reading the file, the function prints the error and returns an empty string. This helps to ensure that the application can handle file reading errors gracefully.

References​

Last updated on