Although creating a file is usually accomplished with a single method call, this 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]). 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 support finer-grained control of over file creation.
The rule FIO01-J. Create files with appropriate access permissions addresses the issue of explains how to formally indicate specify the permissions of a newly created file.
Noncompliant Code Example
This noncompliant code example tries to open a file for writing:
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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, any new data written out will be appended to the former contents. This code is compliant only if this behavior matches the intent 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.
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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 open call is opened, such that the file that is opened is not distince from the file that was 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:
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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 whether an existing file has been opened or a new file has been created provides greater assurance that only the intended file is opened or overwritten and that other files remain undisturbed.
Bibliography
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