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Excessive privileges - Temporary Files

Need​

Restriction of privileges and access modes for temporary files

Context​

  • Usage of Go 1.16 for building efficient and scalable applications
  • Usage of gin-gonic/gin for building web applications in Go
  • Usage of io/ioutil for reading and writing files in Node.js
  • Usage of log for logging and debugging purposes

Description​

Non compliant code​

package main

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

func main() {
    r := gin.Default()
    r.GET("/createTempFile", func(c *gin.Context) {
        tempFile, err := ioutil.TempFile("", "sample")
        if err != nil {
            log.Fatal(err)
        }

        defer tempFile.Close()

        tempFile.WriteString("This is a sample text for the temporary file.")
    })

    r.Run()
}

In the above code, a temporary file is being created using the ioutil.TempFile() function. This function creates a new temporary file in the directory "" (which is the system's default directory for temporary files), opens the file for reading and writing, and returns the resulting *os.File. The name of the file is generated by taking the prefix "sample", and appending a random string to it.

However, the code does not set any restrictions on the file permissions of the temporary file. This means that the file can be accessed, read, and modified by all users in the system, not just the user who created the file.

This is a security vulnerability, as it can allow an attacker to read sensitive information from the file, or modify its contents, potentially leading to a variety of attacks such as information disclosure, unauthorized data modification, or even code execution if the temporary file is used in a context that allows for code execution.

Furthermore, the use of the TempFile() function without a specified directory can potentially lead to the creation of the temporary file in a directory that is shared between users, further increasing the risk of unauthorized access or modification.

To fix this vulnerability, the code should set restrictive file permissions on the temporary file, and consider creating the temporary file in a secure directory that is not shared between users.

Steps​

  • Avoid creating temporary files without proper restrictions on privileges and access modes.
  • Set appropriate permissions for the temporary file to restrict access to only the necessary users or processes.
  • Use a secure and unique naming convention for the temporary file to prevent potential file name collisions and unauthorized access.
  • Consider using a dedicated directory for storing temporary files instead of the default system temporary directory.
  • Ensure that the temporary file is properly cleaned up after its use to prevent accumulation of unnecessary files.
  • Regularly review and update the code to incorporate any security patches or best practices related to handling temporary files.

Compliant code​

package main

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

func main() {
    r := gin.Default()
    r.GET("/createTempFile", func(c *gin.Context) {
        tempFile, err := ioutil.TempFile("/secure/temp", "sample")
        if err != nil {
            log.Fatal(err)
        }

        defer tempFile.Close()

        // Set appropriate permissions
        err = os.Chmod(tempFile.Name(), 0600)
        if err != nil {
            log.Fatal(err)
        }

        tempFile.WriteString("This is a sample text for the temporary file.")
        
        // Delete the file after use
        defer os.Remove(tempFile.Name())
    })

    r.Run()
}

The updated code now creates temporary files in a dedicated directory (/secure/temp) instead of the default system temporary directory. This helps to isolate the temporary files from other system files and reduces the risk of unauthorized access.

The os.Chmod function is used to set the permissions of the temporary file to 0600 (read and write access for the owner only). This restricts access to the temporary file to only the necessary users or processes.

The os.Remove function is used to delete the temporary file after it has been used. This ensures that the temporary file is properly cleaned up to prevent accumulation of unnecessary files.

The naming convention for the temporary file is still secure and unique as it is handled by the ioutil.TempFile function.

Regular reviews and updates of the code should be carried out to incorporate any security patches or best practices related to handling temporary files.

References​

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