The C99 {{fopen()}} function is used to open an existing file or create a new one \[[ISO/IEC 9899:1999|AA. Bibliography#ISO/IEC 9899-1999]\]. However, {{fopen()}} does not indicate if an existing file has been opened for writing or a new file has been created. This may lead to a program overwriting or accessing an unintended file.

Noncompliant Code Example (FileOutputStream())

In this noncompliant code example, the file referenced by file is opened for writing. This example is noncompliant if the programmer's intent was to create a new file, but the referenced file already exists.

String file;
OutputStream out = new FileOutputStream(file);

Compliant Solution (Java 1.7, StandardOpenOption.CREATE_NEW)

This compliant solution uses the CREATE_NEW option from Java 1.7, which causes an exception to be thrown if the file being created already exists.

Path file = new File("file").toPath();
try (OutputStream out = Files.newOutputStream( file, StandardOpenOption.CREATE_NEW);) {
  // write to out
};

Noncompliant Code Example (FileWriter())

In this noncompliant code example, the file referenced by file is opened for writing. Again, the example is noncompliant if the programmer's intent was to create a new file, but the referenced file already exists.

String file;
Writer out = new FileWriter(file);

Compliant Solution (Java 1.7, StandardOpenOption.CREATE_NEW)

This compliant solution uses the CREATE_NEW option from Java 1.7, which causes an exception to be thrown if the file being created already exists.

Path file = new File("file").toPath();
try (BufferedWriter out = Files.newBufferedWriter( file, Charset.forName("UTF8"),
                                                   StandardOpenOption.CREATE_NEW);) {
  // write to out
};

Risk Assessment

The ability to determine if an existing file has been opened or a new file has been created provides greater assurance that a file other than the intended file is not acted upon.

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this guideline on the CERT website.

Related Guidelines

CERT C++ Secure Coding Standard: FIO03-CPP. Do not make assumptions about fopen() and file creation
CERT C Secure Coding Standard: FIO03-C. Do not make assumptions about fopen() and file creation

Bibliography

[[API 2006|AA. Bibliography#API 06]\] Class {{InputStream}}, {{DataInputStream}}
\[[J2SE 2011|AA. Bibliography#J2SE 11]\] The try-with-resources Statement
\[[Seacord 2005a|AA. Bibliography#Seacord 05]\] Chapter 7, "File I/O"

FIO01-J. Do not expose buffers created using the wrap() or duplicate() methods to untrusted code      12. Input Output (FIO)      FIO05-J. Do not create multiple buffered wrappers on a single InputStream