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Although creating a file is generally usually accomplished with a single method call, it actually has several issues to considerthis single action raises multiple security-related questions. What should be done if the file cannot be created? What should be done if the file already exists? What should be the file's initial attributes, such as permissions?

Java provides several generations of file-handling facilities. The original input/output facilities, which included basic file handling, are in the package java.io. More comprehensive facilities were included in JDK 1.4 with the New I/O package java.nio (see New I/O APIs [Oracle 2010b]). This package Still more comprehensive facilities were included in JDK 1.7 with the New I/O 2 package java.nio.file. Both packages introduced a number of methods to handle finesupport finer-grained control over file creation.

FIO01-J. Create files with appropriate access permissions explains how to specify the permissions of a newly created file creation.

Noncompliant Code Example

This noncompliant code example tries to open a file for writing:

public void openFile_ncecreateFile(String filename) 
    throws FileNotFoundException{
  OutputStream out = new FileOutputStream(filename);
  // Work with FILEfile
}

If the file existed before being opened, its former contents will be overwritten with the contents provided by the program.

Noncompliant Code Example

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This noncompliant code example tries to avoid overwriting an existing file:

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If the file existed before being opened, its new contents will be appended to the former contents. This code is compliant only if this was the intention of the programmer.

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TOCTOU)

This noncompliant code example tries to avoid altering an existing file by creating an empty file using java.io.File.createNewfile(). If a file with the given name already exists, then createNewFile() will return false without destroying the named file's contents.

public void noAlter_ncecreateFile(String filename)
    throws FileNotFoundExceptionIOException{
  OutputStream out = new FileOutputStream(filename, true);
  if (!new File(filename).createNewFile()) {
      // File cannot be created...handle error
  } else {
      OutputStream out = new FileOutputStream(filename);
      // Work with FILEfile
  }
} 

Unfortunately, this solution is subject to a TOCTOU (time-of-check-, time-of-use) race condition. It is possible for an attacker to modify the file system after the empty file is created but before the file is opened, such that the file that is created opened is not distince from the file that is openedwas created.

Compliant Solution (Files)

This compliant solution uses the java.nio.file.Files.newOutputStream() method to atomically create the file and throw throws an exception if the file already exists:

public void createFile_cs(String filename) 
    throws FileNotFoundException{
  try (OutputStream out = new BufferedOutputStream(
         Files.newOutputStream( Paths.get(filename),
                               StandardOpenOption.CREATE_NEW))) {
    // Work with out
  } catch (IOException x) {
      // File not writable...handle error
  }
} 

Applicability

The ability to determine if whether an existing file has been opened or a new file has been created provides greater assurance that a file other than only the intended file is not opened or overwritten and that other files remain undisturbed.

Bibliography

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2011

[API	2013]	Class java.io.File Class java.nio.file.Files
[Oracle 2010b]	New I/O APIs

 

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