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As a result of the influence of MS-DOS, 8.3 file names of the form xxxxxxxx.xxx, where x denotes an alphanumeric character, are generally supported by modern systems. On some platforms, file names are case sensitive, and on other platforms, they are case insensitive. VU#439395 is an example of a vulnerability resulting from a failure to deal appropriately with case sensitivity issues [VU#439395].  Developers should generate file and path names using a safe subset of ASCII characters and, for security critical applications, only accept names that use these characters.

Noncompliant Code Example

In the following noncompliant code example, unsafe characters are used as part of a file name.

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A platform is free to define its own mapping of the unsafe characters. For example, when tested on an Ubuntu Linux distribution, this noncompliant code example resulted in the following file name:

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Compliant Solution

Use a descriptive file name, containing only the subset of ASCII previously described.

File f = new File("name.ext");
OutputStream out = new FileOutputStream(f);

Noncompliant Code Example

This noncompliant code example creates a file with input from the user without sanitizing the input.

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No checks are performed on the file name to prevent troublesome characters. If an attacker knew this code was in a program used to create or rename files that would later be used in a script or automated process of some sort, the attacker could choose particular characters in the output file name to confuse the later process for malicious purposes.

Compliant Solution

This compliant solution  uses a whitelist to reject file names containing unsafe characters.  Further input validation may be necessary, for example, to ensure that a file or directory name does not end with a period.

public static void main(String[] args) throws Exception {
  if (args.length < 1) {
    // Handle error
  }
  String filename = args[0];

  Pattern pattern = Pattern.compile("[^A-Za-z0-9._]");
  Matcher matcher = pattern.matcher(filename);
  if (matcher.find()) {
    // File name contains bad chars; handle error
  }
  File f = new File(filename);
  OutputStream out = new FileOutputStream(f);
  // ...
}

Exceptions

FIO99-EX0: A program may accept a file or path name that uses "unsafe" characters provided that the developer has determined that the file is not used in a restricted sink such as a command interpreter, shell, parser,logger, or other complex subsystem that attaches a particular meaning to these characters.

Risk Assessment

Failing to use only a safe subset of ASCII can result in misinterpreted data.

Related Guidelines

Bibliography

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      Image RemovedRule 00: Input Validation and Data Sanitization (IDS)