IDS06-J. Use a subset of ASCII for file and path names

File path names containing particular characters can be troublesome and can cause unexpected behavior resulting in vulnerabilities. If untrusted data is used in the construction of a file or path name, the following characters and patterns must be disallowed:

• Leading dashes: Leading dashes can cause problems when programs are called with the file name as a parameter because the first character or characters of the file name might be interpreted as an option switch.
• Control characters, such as newlines, carriage returns, and escape: Control characters in a file name can cause unexpected results from shell scripts and in logging.
• Spaces: Spaces can cause problems with scripts and when double quotes aren't used to surround the file name.
• Invalid character encodings: Character encodings can make it difficult to perform proper validation of file and path names. (See rule IDS11-J. Eliminate non-character code points before validation.)
• Characters other than letters, numbers, and portable punctuation: These characters may be used as name-separator character. Including them in a file or path name can cause unexpected and potentially insecure behavior.

As a result of the influence of MS-DOS, file names of the form xxxxxxxx.xxx, where x denotes an alphanumeric character, are generally supported by modern systems.

In addition to the letters of the English alphabet ("A" through "Z" and "a" through "z"), the digits ("0" through "9"), and the space, only the following characters are portable:

% & + , - . : = _

Only these characters should be considered for use in file and path names. Punctuation characters not on this list are not unconditionally safe for file names even if they are portably available. These characters or patterns can cause problems for scripts and automated parsing, but because they are not commonly used, it is best to disallow their use to reduce potential problems. Interoperability concerns also exist because different operating systems handle these punctuation characters in file and path names in an implementation-defined manner.

This is an instance of rule IDS00-J. Sanitize untrusted data passed across a trust boundary.

Noncompliant Code Example

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

File f = new File("A\uD8AB");
OutputStream out = new FileOutputStream(f);

An implementation is free to define its own mapping of the non-"safe" characters. For example, when tested on an Ubuntu Linux distribution, this noncompliant code example resulted in the following file name:

A?

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 it.

public static void main(String[] args) throws Exception {
  if (args.length < 1) {
    // handle error
  }
  File f = new File(args[0]);
  OutputStream out = new FileOutputStream(f);
  // ...
}

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, they could choose particular characters in the output file name to confuse the later process for malicious purposes.

Compliant Solution

In this compliant solution, the program uses a whitelist to reject unsafe file names.

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()) {
    // filename contains bad chars, handle error
  }
  File f = new File(filename);
  OutputStream out = new FileOutputStream(f);
  // ...
}

Similarly, all file names originating from untrusted sources must be sanitized to ensure they contain only safe characters.

Risk Assessment

Failing to use only the subset of ASCII that is guaranteed to work can result in misinterpreted data.

Related Guidelines

Bibliography

IDS05-J. Limit the size of files passed to ZipInputStream            IDS07-J. Do not pass untrusted, unsanitized data to the Runtime.exec() method