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Unverifiable files

Need​

Ensuring file integrity and verifiability in the repository

Context​

  • Usage of Scala for building scalable and functional applications
  • Usage of java.io.File for file input/output operations in Java
  • Usage of scala.io.Source for reading input from various sources

Description​

Non compliant code​

import java.io.File
import scala.io.Source

def readFile(fileName: String): String = {
  val file = new File(fileName)
  Source.fromFile(file).getLines.mkString
}

val fileName = "/path/to/file"
val content = readFile(fileName)
println(content)

This code reads a file from a given path and prints its content. The vulnerability lies in the fact that it does not verify if the content of the file is compatible with its extension.

This could allow an attacker to upload a malicious file with a benign extension (e.g., a .txt file containing executable script). When the file is read and processed by the application, it may execute the malicious script, leading to potential security issues.

The function readFile takes a fileName as input and creates a File object. It then uses Source.fromFile(file).getLines.mkString to read the content of the file and return it as a String. The content of the file is then printed out.

At no point in this process is the content of the file verified against its extension, which is where the vulnerability lies.

Steps​

  • Validate file extensions before processing them.
  • Implement a whitelist of allowed file extensions.
  • Use a secure file upload mechanism that verifies the file content against its extension.
  • Consider using a file signature verification library to ensure the integrity of the file content.
  • Implement proper error handling and logging to handle cases where unverifiable files are encountered.

Compliant code​

import java.io.File
import scala.io.Source

// Whitelist of allowed file extensions
val allowedExtensions = List(".txt", ".doc", ".docx", ".pdf")

def validateFileExtension(fileName: String): Boolean = {
  val extension = fileName.substring(fileName.lastIndexOf("."))
  allowedExtensions.contains(extension)
}

def readFile(fileName: String): Option[String] = {
  if (validateFileExtension(fileName)) {
    val file = new File(fileName)
    try {
      Some(Source.fromFile(file).getLines.mkString)
    } catch {
      case e: Exception =>
        println(s"Error reading file $fileName: ${e.getMessage}")
        None
    }
  } else {
    println(s"Invalid file extension for file $fileName")
    None
  }
}

val fileName = "/path/to/file"
val content = readFile(fileName)
content match {
  case Some(c) => println(c)
  case None => println(s"Could not read file $fileName")
}

The updated code includes a function validateFileExtension that checks if the file extension is in the whitelist of allowed extensions. This function is called before attempting to read the file in the readFile function. If the file extension is not valid, readFile will return None and log an error message.

The readFile function now returns an Option[String] instead of a String. This allows us to handle cases where the file cannot be read (either due to an invalid extension or an error during reading) in a safe manner. If the file is read successfully, readFile will return Some(content). If the file cannot be read, readFile will return None.

Finally, when we attempt to read the file, we match on the result of readFile. If readFile returned Some(content), we print the content. If readFile returned None, we print an error message.

References​

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