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Unfortunately, a wide variety of file system vulnerabilities can be exploited by an attacker to gain access to files for which they lack sufficient privileges, particularly when operating on files that reside in shared directories in which multiple users may create, move, or delete files. Privilege escalation is also possible when these programs run with elevated privileges. A number of file system properties and capabilities can be exploited by an attacker, including file links, device files, and shared file access. To prevent vulnerabilities, a program must operate only on files in secure directories.

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Similar to shared files, file links can be swapped out and may not always point to the intended location. As a result, file links in shared directories are untrusted and should not be operated on . See (see FIO15-J. Do not operate on untrusted file links).

Device Files

File names on many operating systems may be used to access device files. Device files are used to access hardware and peripherals. Reserved MS-DOS device names include AUX, CON, PRN, COM1, and LPT1. Character special files and block special files are POSIX device files that direct operations on the files to the appropriate device drivers.

Performing operations on device files intended only for ordinary character or binary files can result in crashes and denial-of-service (DoS) attacks. For example, when Windows attempts to interpret the device name as a file resource, it performs an invalid resource access that usually results in a crash [Howard 2002].

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On many systems, files can be simultaneously accessed by concurrent processes. Exclusive access grants unrestricted file access to the locking process while denying access to all other processes, eliminating the potential for a race condition on the locked region. The java.nio.channels.FileLock class may be used for file locking. According to the Java API, Class FileLock [API 20062014], documentation,

A file lock is either exclusive or shared. A shared lock prevents other concurrently running programs from acquiring an overlapping exclusive lock but does allow them to acquire overlapping shared locks. An exclusive lock prevents other programs from acquiring an overlapping lock of either type. Once it is released, a lock has no further effect on the locks that may be acquired by other programs.

Shared locks support concurrent read access from multiple processes; exclusive locks support exclusive write access. File locks provide protection across processes, but they do not provide protection from multiple threads within a single process. Both shared locks and exclusive locks eliminate the potential for a cross-process race condition on the locked region. Exclusive locks provide mutual exclusion; shared locks prevent alteration of the state of the locked file region (one of the required properties for a data race).

The Java API [API 20062014] documentation states that "whether or not a lock actually prevents another program from accessing the content of the locked region is system-dependent and consequently unspecified."

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Linux implements both mandatory locks and advisory locks. Advisory locks are not enforced by the operating system, which diminishes their value from a security perspective. Unfortunately, the mandatory file lock in Linux is generally impractical becausefor the following reasons:

• Mandatory mandatory locking is supported only by certain network file systems.
• file File systems must be mounted with support for mandatory locking, which is disabled by default.
• locking Locking relies on the group ID bit, which can be turned off by another process (thereby defeating the lock).
• the The lock is implicitly dropped if the holding process closes any descriptor of the file.

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In this noncompliant code example, an attacker could specify the name of a locked device or a first in, first out (FIFO) file, causing the program to hang when opening the file.:

String file = /* providedProvided by user */;
InputStream in = null;
try {
  in = new FileInputStream(file);
  // ...
} finally {
  try {
    if (in !=null) { in.close();}
  } catch (IOException x) {
    // handleHandle error
  }
}

Noncompliant Code Example

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This noncompliant code example uses the try-with-resources statement from (introduced in Java SE 7) to open the file. While this The try-with-resources statement guarantees the file's successful closure if an exception is thrown, it but this code is subject to the same vulnerabilities as the previous example.

Code Block
bgColor #ffcccc
String filename = /* providedProvided by user */; Path path = new File(filename).toPath(); try (InputStream in = Files.newInputStream(path)) { // readRead file } catch (IOException x) { // handleHandle error }

Noncompliant Code Example (

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isRegularFile())

This noncompliant code example first checks that the file is a regular file (using the new Java SE 7 NIO2 APIs NIO.2 API) before opening it.:

String filename = /* providedProvided by user */
Path path = new File(filename).toPath();
try {
  BasicFileAttributes attr =
      Files.readAttributes(path, BasicFileAttributes.class);

  // Check
  if (!attr.isRegularFile()) {
    System.out.println("Not a regular file");
    return;
  }
  // otherOther necessary checks
  
  // Use
  try (InputStream in = Files.newInputStream(path)) {
    // readRead file
  }
} catch (IOException x) {
  // handleHandle error
}

This test can still be circumvented by a symbolic link. By default, the readAttributes() method follows symbolic links and reads the file attributes of the final target of the link. The result is that the program may reference a file other than the one intended.

Noncompliant Code Example (

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NOFOLLOW_LINKS)

This noncompliant code example checks the file by calling the readAttributes() method with the NOFOLLOW_LINKS link option to prevent the method from following symbolic links. This approach allows the detection of symbolic links because the isRegularFile() check is carried out on the symbolic link file and not on the final target of the link.

String filename = /* providedProvided by user */;
Path path = new File(filename).toPath();
try {
  BasicFileAttributes attr = Files.readAttributes(
      path, BasicFileAttributes.class, LinkOption.NOFOLLOW_LINKS);

  // Check
  if (!attr.isRegularFile()) {
    System.out.println("Not a regular file");
    return;
  }
  // otherOther necessary checks

  // Use
  try (InputStream in = Files.newInputStream(path)) {
    // readRead file
  };
} catch (IOException x) {
  // handleHandle error
}

This code is still vulnerable to a time-of-check, time-of-use (TOCTOU) race condition. For example, an attacker can replace the regular file with a file link or device file after the code has completed its checks but before it opens the file.

Noncompliant Code Example (POSIX

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: Check-Use-Check)

This noncompliant code example performs the necessary checks and then opens the file. After opening the file, it performs a second check to make sure that the file has not been moved and that the file opened is the same file that was checked. This approach reduces the chance that an attacker has changed the file between checking and then opening the file. In both checks, the file's fileKey attribute is examined. This The fileKey attribute serves as a unique key for identifying files and is a more reliable than the path name as on indicator of the file's identity than its path name.

The SE 7 Documentation [J2SE 2011] describes the fileKey attribute:

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As noted in the documentation, the FileKey cannot be used if it is not available. The fileKey() method returns null on Windows. Consequently, this solution is available only available on POSIX systems (actually any systems where systems such as POSIX in which fileKey() does not return null).

String filename = /* providedProvided by user */;
Path path = new File(filename).toPath();
try {
  BasicFileAttributes attr = Files.readAttributes(
      path, BasicFileAttributes.class, LinkOption.NOFOLLOW_LINKS);
  Object fileKey = attr.fileKey();

  // Check
  if (!attr.isRegularFile()) {
    System.out.println("Not a regular file");
    return;
  }
  // otherOther necessary checks

  // Use
  try (InputStream in = Files.newInputStream(path)) {

    // Check
    BasicFileAttributes attr2 = Files.readAttributes(
        path, BasicFileAttributes.class, LinkOption.NOFOLLOW_LINKS
    );
    Object fileKey2 = attr2.fileKey();
    if (!fileKey.equals(fileKey2)) {
      System.out.println("File has been tampered with");
    }

    // readRead file
  };
} catch (IOException x) {
  // handleHandle error
}

While Although this code goes to great lengths to prevent an attacker from successfully tricking it into opening the wrong file, it still has several vulnerabilities:

• The TOCTOU race condition still exists between the first check and open. During this race window, an attacker can replace the regular file with a symbolic link or other nonregular file. The second check detects this this race condition but does not eliminate it.
• An attacker could subvert this code by letting the check operate on a regular file, substituting the nonregular file for the open, and then resubstituting the regular file to circumvent the second check. This vulnerability exists because Java lacks any a mechanism to obtain file attributes from a file by any means other than the file name, and the binding of the file name to a file object is reasserted every time the file name is used in an operation. Consequently, an attacker can still swap a file for a nefarious file, such as a symbolic link.
• A system with hard links allows an attacker to construct a malicious file that is a hard link to a protected file. Hard links cannot be reliably detected by a program and can foil canonicalization attempts, which are prescribed by rule FIO16-J. Canonicalize path names before validating them.

Compliant Solution (POSIX

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: Secure Directory)

Because of the potential for race conditions and the inherent accessibility of shared directories, files must be operated on only in secure directories. Because programs may run with reduced privileges and lack the facilities to construct a secure directory, a program may need to throw an exception if it determines that a given path name is not in a secure directory.

Following is a POSIX-specific implementation of an isInSecureDir() method. This method ensures that the supplied file and all directories above it are owned by either the user or the system administrator, that each directory lacks write access for any other users, and that directories above the given file may not be deleted or renamed by any users other users (except than the system administrator).

public static boolean isInSecureDir(Path file) {
  return isInSecureDir(file, null);
}
public static boolean isInSecureDir(Path file, UserPrincipal user) {
   return isInSecureDir(file, user, 5);
}

/**
 * Indicates whether file lives in a secure directory relative
 * to the program's user
 * @param file Path to test
 * @param user User to test. If null, defaults to current user
 * @param symlinkDepth Number of symbolic links allowed
 * @return true if file's directory is secure.
 */
public static boolean isInSecureDir(Path file, UserPrincipal user,
                                    int symlinkDepth) {
  if (!file.isAbsolute()) {
    file = file.toAbsolutePath();
  } if (symlinkDepth <=0) {
      // Too many levels of symbolic links
      return false;
    }

  // Get UserPincipalUserPrincipal for specified user and superuser
  FileSystem fileSystem =
      Paths.get(file.getRoot().toString()).getFileSystem();
  UserPrincipalLookupService upls = 
      fileSystem.getUserPrincipalLookupService();
  UserPrincipal root = null;
  try {
    root = upls.lookupPrincipalByName("root");
    if (user == null) {
      user = upls.lookupPrincipalByName(System.getProperty("user.name"));
    }
    if (root == null || user == null) {
      return false;
    }
  } catch (IOException x) {
    return false;
  }

  // If any parent dirs (from root on down) are not secure,
  // dir is not secure
  for (int i = 1; i <= file.getNameCount(); i++) {
    Path partialPath = Paths.get(file.getRoot().toString(),
                                 file.subpath(0, i).toString());

    try {
      if (Files.isSymbolicLink(partialPath)) {
        if (!isInSecureDir(Files.readSymbolicLink(partialPath),)) {
                           user, symlinkDepth - 1)) {
          // Symbolic link, linked-to dir not secure
          return false;
        }
      } else {
        UserPrincipal owner = Files.getOwner(partialPath);
        if (!user.equals(owner) && !root.equals(owner)) {
          // dir owned by someone else, not secure
          return false;
        }
        PosixFileAttributes attr =
            Files.readAttributes(partialPath, PosixFileAttributes.class);
        Set<PosixFilePermission> perms = attr.permissions();
        if (perms.contains(PosixFilePermission.GROUP_WRITE) ||
            perms.contains(PosixFilePermission.OTHERS_WRITE)) {
          // someoneSomeone else can write files, not secure
          return false;
        }
      }
    } catch (IOException x) {
      return false;
    }
  }

  return true;
}

When checking directories, it is important to traverse from the root directory to the leaf directory to avoid a dangerous dangerous race condition whereby an attacker who has privileges to at least one of the directories can rename and re-create a directory after the privilege verification of subdirectories but before the verification of the tampered directory.

If the path contains any symbolic links, this routine will recursively invoke itself on the linked-to directory and ensure it is also secure. A symlinked directory may be secure if both its source and linked-to directory are secure. The method checks every directory in the path, ensuring that every directory is owned by the current user or the system administraor administrator and that all directories in the path prevent other users from creating, deleting, or renaming files.

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String filename = /* providedProvided by user */;
Path path = new File(filename).toPath();
try {
  if (!isInSecureDir(path)) {
    System.out.println("File not in secure directory");
    return;
  }

  BasicFileAttributes attr = Files.readAttributes(
      path, BasicFileAttributes.class, LinkOption.NOFOLLOW_LINKS);

  // Check
  if (!attr.isRegularFile()) {
    System.out.println("Not a regular file");
    return;
  }
  // otherOther necessary checks

  try (InputStream in = Files.newInputStream(path)) {
    // readRead file
  }
} catch (IOException x) {
  // handleHandle error
}

Programs with elevated privileges may need to write files to directories owned by unprivileged users. One example would be is a mail daemon that reads a mail message from one user and places it in a directory owned by another user. In such cases, the mail daemon should assume the privileges of a user when reading or writing files on behalf of that user, in which case all file access should occur in secure directories relative to that user. When a program with elevated privileges must write files on its own behalf, these files should be in secure directories relative to the privileges of the program (such as directories accessible only by the system administrator).

Exceptions

FIO00-J-EX0: Programs that operate on single-user systems or on systems that have no shared directories or no possibility of file system vulnerabilities do not need to ensure that files are maintained in secure directories before operating on them.

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Performing operations on files in shared directories can result in (in DoS ) attacks. If the program has elevated privileges, privilege escalation escalation exploits are possible.

Related Guidelines

Android Implementation Details

On Android, the SD card ( /sdcard or /mnt/sdcard ) is shared among multiple applications, thus so sensitive files should not be stored on the SD card .See: (see DRD00-J. Do not store sensitive information on external storage (SD card)).

Bibliography

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