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This guideline is an instance of IDS02-J. Normalize strings before validating them.

Noncompliant Code Example

This noncompliant code example accepts a file path as a command line argument and uses the File.getAbsolutePath() method to obtain the absolute file path. This method does not automatically resolve symbolic links.

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public static void main(String[] args) {
  File f = new File("/tmp/" + args[0]);
  String absPath = f.getAbsolutePath();

  if (!validate(absPath)) {  // Validation
    throw new IllegalArgumentException();
  }		  
}

Compliant Solution (getCanonicalPath())

This compliant solution uses the getCanonicalPath() method, introduced in Java 2, because it resolves all aliases, shortcuts or symbolic links consistently, across all platforms. The value of the alias (if any) is not included in the returned value. Moreover, relative references like the double period (..) are also removed so that the input is reduced to a canonicalized form before validation is carried out. An adversary cannot use ../ sequences to break out of the specified directory when the validate() method is present.

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The getCanonicalPath() method throws a security exception when used within applets as it reveals too much information about the host machine. The getCanonicalFile() method behaves like getCanonicalPath() but returns a new File object instead of a String.

Compliant Solution (Security Manager)

A comprehensive way of handling this issue is to grant the application the permissions to operate only on files present within the intended directory — /tmp in this example. This compliant solution specifies the absolute path of the program in its security policy file, and grants java.io.FilePermission with target /tmp and read and write.

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See guideline ENV02-J. Create a secure sandbox using a Security Manager for additional information on using security managers.

Noncompliant Code Example

This noncompliant code example allows the user to specify the absolute path of a file name on which to operate. The user can specify files outside the intended directory (/img in this example) by entering an argument that contains ../ sequences, and consequently violate the intended security policies of the program.

FileOutputStream fis = new FileOutputStream(new File("/img/" + args[0]));
// ...

Noncompliant Code Example

This noncompliant code example attempts to mitigate the issue by using the File.getCanonicalPath() method, which fully resolves the argument and constructs a canonicalized path. For example, the path /img/../etc/passwd resolves to /etc/passwd. Validation without canonicalization remains insecure because the user remains able to specify files outside the intended directory.

File f = new File("/img/" + args[0]);
String canonicalPath = f.getCanonicalPath();		  
FileOutputStream fis = new FileOutputStream(f);
// ...

Compliant Solution

This compliant solution obtains the file name from the untrusted user input, canonicalizes it and then validates it against the intended file name. It operates on the specified file only when validation succeeds.

File f = new File("/img/" + args[0]);
String canonicalPath = f.getCanonicalPath();

if (canonicalPath.equals("/img/java/file1.txt")) {  // Validation
   // Do something
}

if (!canonicalPath.equals("/img/java/file2.txt")) {  // Validation
   // Do something
}

FileOutputStream fis = new FileOutputStream(f);		

Compliant solution (Security Manager)

A comprehensive solution is to grant the application the permissions to read only the specifically intended files or directories. Grant these permissions by to specifying the absolute path of the program in the security policy file and granting java.io.FilePermission with the canonicalized absolute path of the file or directory as the target name and with the action set to read.

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See guideline ENV02-J. Create a secure sandbox using a Security Manager for additional information on using security managers.

Risk Assessment

Using path names from untrusted sources without first canonicalizing them and then validating them can result in directory traversal attacks.

Related Vulnerabilities

CVE-2005-0789, CVE-2008-5518

Other Languages

This guideline appears in the C Secure Coding Standard as FIO02-C. Canonicalize path names originating from untrusted sources.

This guideline appears in the C++ Secure Coding Standard as FIO02-CPP. Canonicalize path names originating from untrusted sources.

Bibliography

\[[API 2006|AA. Bibliography#API 06]\] [method getCanonicalPath()|http://java.sun.com/javase/6/docs/api/java/io/File.html#getCanonicalPath()]
\[[API 2006|AA. Bibliography#API 06]\] [method getCanonicalFile()|http://java.sun.com/javase/6/docs/api/java/io/File.html#getCanonicalFile()]
\[[Harold 1999|AA. Bibliography#Harold 99]\]
\[[MITRE 2009|AA. Bibliography#MITRE 09]\] [CWE ID 171|http://cwe.mitre.org/data/definitions/171.html] "Cleansing, Canonicalization, and Comparison Errors", [CWE ID 647|http://cwe.mitre.org/data/definitions/647.html] "Use of Non-Canonical URL Paths for Authorization Decisions"

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