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Wiki Markup According to the Java API \[ [API 2006|AA. Bibliography#API 06]\], class {{] for class java.io.File}}:

A path namepathname, whether abstract or in string form, may be either absolute or relative. An absolute path name pathname is complete in that no other information is required to locate the file that it denotes. A relative path namepathname, in contrast, must be interpreted in terms of information taken from some other path namepathname.

An absolute Absolute or relative path names may contain aliasesfile links such as symbolic (soft) links, hard links, shortcuts, shadows, symbolic links and shortcuts (aliases, hereafter) rather than canonical paths, which refer to the actual files or directories that these point to. These aliases aliases, and junctions. These file links must be fully resolved before any file validation operations are performed. For instance, resolving example, the final target of a symbolic link called trace may yield its actual path on the file system, such as, might be the path name /home/system/trace. Path names may also contain special file names that make validation difficult:

1. "." refers to the directory itself.
2. Inside a directory, the special file name ".." refers to the directory's parent directory.

In addition to these specific issues, a wide variety of operating system–specific and file system–specific naming conventions make validation difficult.

Canonicalizing The process of canonicalizing file names makes it easier to verify an aliasvalidate a path name. More than one alias path name can refer to a single directory or file. Further, the textual representation of an alias a path name may yield little or no information regarding the directory or file to which it refers. Consequently, all aliases path names must be fully resolved or canonicalized before validation. This is necessary because untrusted user input may allow

Validation may be necessary, for example, when attempting to restrict user access to files within a particular directory or to otherwise make security decisions based on the name of a file name or path name. Frequently, these restrictions can be circumvented by an attacker by exploiting a directory traversal or path equivalence vulnerability. A directory traversal vulnerability allows an I/O operation to escape the a specified operating directory. Violation of this rule can result in information disclosure and malicious modification of files existing in directories other than the specified one. A path equivalence vulnerability occurs when an attacker provides a different but equivalent name for a resource to bypass security checks.

Canonicalization contains an inherent race window between the time the program obtains the canonical path name and the time it opens the file. While the canonical path name is being validated, the file system may have been modified and the canonical path name may no longer reference the original valid file. Fortunately, this race condition can be easily mitigated. The canonical path name can be used to determine whether the referenced file name is in a secure directory (see FIO00-J. Do not operate on files in shared directories for more information). If the referenced file is in a secure directory, then, by definition, an attacker cannot tamper with it and cannot exploit the race condition.

This recommendation is a specific This rule is an instance of IDS01-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 linksallows the user to specify the path of an image file to open. By prepending /img/ to the directory, this code enforces a policy that only files in this directory should be opened. The program also uses the isInSecureDir() method defined in FIO00-J. Do not operate on files in shared directories. 

However, the user can still specify a file outside the intended directory by entering an argument that contains ../ sequences.  An attacker can also create a link in the /img directory that refers to a directory or file outside of that directory. The path name of the link might appear to reside in the /img directory and consequently pass validation, but the operation will actually be performed on the final target of the link, which can reside outside the intended directory.

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

  if (!validateisInSecureDir(absPathfile)) {  // Validation
    throw new IllegalArgumentException();
  }		  
}

The application intends to restrict the user from operating on files outside the /tmp directory and uses a validate() method to enforce this condition. An attacker who can create symbolic links in /tmp can cause the program to pass validation checks by supplying the unresolved path. All file operations performed are reflected in the file pointed to by the symbolic link.

Let {{argv\[0\]}} be the string {{filename}}, where {{/tmp/filename}} is a symbolic link that points to the file {{/dirname/filename}} present on the local file system. The validation passes because the root directory of the compiled path name is still {{/tmp}}, but the operations are carried out on the file {{/dirname/filename}}.

Note that File.getAbsolutePath() actually does resolve all symbolic links, aliases and short cuts on Windows and Macintosh platforms. Nevertheless, the JLS lacks any guarantee that this behavior is present on all platforms or that it will continue in future implementations.

FileOutputStream fis = new FileOutputStream(file);
// ...

Noncompliant Code Example

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(getCanonicalPath())

This compliant solution uses the noncompliant code example attempts to mitigate the issue by using the File.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 which fully resolves the argument and constructs a canonicalized path. Special file names such as dot dot (..) are also removed so that the input is reduced to a canonicalized form before validation is carried out. An attacker cannot use ../ sequences to break out of the specified directory when the validate() method is present.

public static void main(String[] args) throws IOException {
  File f = new File("/tmp/" + args[0]);
  String canonicalPath = f.getCanonicalPath();
 
  if (!validate(canonicalPath)) {  // Validation
    throw new IllegalArgumentException();
  }
}

The  For example, the path /img/../etc/passwd resolves to /etc/passwd. The getCanonicalPath() method throws a security exception when used within in applets as because 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 actions read and write.

grant codeBase "file:/home/programpath/" {
  permission java.io.FilePermission "/tmp", "read, write";
};

See rule 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.Unfortunately, the canonicalization is performed after the validation, which renders the validation ineffective.

FileOutputStreamFile fisfile = 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 can specify files outside the intended directory.

File f = new File("/img/" + args[0]);
if (!isInSecureDir(file)) {
  throw new IllegalArgumentException();
}
String canonicalPath = ffile.getCanonicalPath();		  
FileOutputStream fis = new FileOutputStream(fcanonicalPath);
// ...

Compliant

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Solution (getCanonicalPath())

This compliant solution obtains the file name from the untrusted user input, canonicalizes it, and then validates it against the intended file namea list of benign path names. It operates on the specified file only when validation succeeds, that is, only if the file is one of the two valid files file1.txt or file2.txt in /img/java.

File ffile = new File("/img/" + args[0]);
if (!isInSecureDir(file)) {
  throw new IllegalArgumentException();
}
String canonicalPath = ffile.getCanonicalPath();

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

if (!canonicalPath.equals("/img/java/file2.txt")) {
   // Validation
Invalid file;  // Do somethinghandle error
}

FileOutputStreamFileInputStream fis = new FileOutputStreamFileInputStream(f);		

Compliant

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Solution (Security Manager)

A comprehensive solution way to handle this issue is to grant the application the permissions to read operate only on files present within the specifically intended files or directories. Grant these permissions by to specifying intended directory—the /img directory in this example. This compliant solution specifies the absolute path of the program in the its security policy file and granting grants java.io.FilePermission with the canonicalized absolute path of the file or directory as the target name and with the action set to readtarget /img/java and the read action.
This solution requires that the /img directory is a secure directory, as described in FIO00-J. Do not operate on files in shared directories.

// All files in /img/java can be read
grant codeBase "file:/home/programpath/" {
  permission java.io.FilePermission "/img/java", "read";
};

See rule 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 attacksand path equivalence vulnerabilities.

Other Languages

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Automated Detection

Related Vulnerabilities

CVE-2005-0789 describes a directory traversal vulnerability in LimeWire 3.9.6 through 4.6.0 that allows remote attackers to read arbitrary files via a .. (dot dot) in a magnet request.

CVE-2008-5518 describes multiple directory traversal vulnerabilities in the web administration console in Apache Geronimo Application Server 2.1 through 2.1.3 on Windows that allow remote attackers to upload files to arbitrary directories.

Related Guidelines

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Related Vulnerabilities

Bibliography

Android Implementation Details

This rule is applicable in principle to Android. Please refer to the Android-specific instance of this rule: DRD08-J. Always canonicalize a URL received by a content provider.

Bibliography

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Image Added Image Added Image AddedIDS09-J. Exclude user input from format strings      Image Removed      IDS05-J. Limit the size of files passed to ZipInputStream