FIO07-J. Do not create temporary files in shared directories

Temporary files are typically used to

• Share data between processes
• Store auxiliary program data (for example, to save memory)
• Construct and/or load classes, JAR files and native libraries dynamically

Programmers frequently create temporary files in directories that are writable by everyone; examples include /tmp and /var/tmp on UNIX and C:\TEMP on Windows. Files in such directories may be purged regularly, for example, every night or during reboot. However, an attacker who has access to the local file system can misuse files held in world-writable directories when those files are created insecurely or remain accessible after use. For instance, an attacker who can both predict the name of a temporary file and can change or replace that file, can exploit a time-of-check time-of-use (TOCTOU) condition to cause either a failure in creating the temporary file from within program code or operating on a file determined by the attacker.

Mitigation strategies include use of:

• other IPC mechanisms such as sockets and remote procedure calls
• the low-level Java Native Interface (JNI).
• memory mapped files
• threads to share heap data within the same JVM (applies to data sharing between Java processes only)
• a secure directory that can be accessed only by application instances. When using this strategy, ensure that multiple instances of the application running on the same platform avoid competing for the same files.

Shared access to a directory is riskier than shared access to a limited number of files. Consequently, temporary files in shared directories must be

• Created with unique and unpredictable file names,
• Opened with exclusive access,
• Removed before the program exits, and
• Opened with appropriate permissions.

Secure creation of temporary files is error prone and relies on platform dependent behavior; the Operating System, and the file system are the determining factors. Code that works for a locally mounted file system, for example, may be vulnerable when used with a remotely mounted file system. Moreover, most relevant APIs are problematic. The only secure comprehensive solution is to refrain from creating temporary files in shared directories.

Unique and Unpredictable File Names

A recently identified bug in JRE and JDK version 6.0 and earlier permits an attacker who can predict the names of temporary files to write malicious JAR files via unknown vectors [[CVE 2008]]. Failure to reclaim temporary resources can cause rapid disk space exhaustion because of unreclaimed files [Secunia Advisory 20132].

Exclusive Access

Exclusive access grants unrestricted file access to the locking process while denying access to all other processes, thus eliminating the potential for a race condition on the locked region. The java.nio.channels.FileLock class facilitates file locking. According to the Java API [[API 2006]] 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; they are ineffective for 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).

"Whether or not a lock actually prevents another program from accessing the content of the locked region is system-dependent and consequently unspecified" [[API 2006]].

Microsoft Windows uses a file-locking mechanism called mandatory locking because every process attempting access to a locked file region is subject to the restriction.

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 because:

• Mandatory locking is supported only on local file systems; it lacks support for network file systems (such as NFS or AFS).
• File systems must be explicitly mounted with support for mandatory locking; this support is disabled by default.
• Locking relies on the set-group-ID bit, however that bit can be disabled by another process (thereby defeating the lock).

Removal Before Termination

Removing temporary files when they are no longer required allows file names and other resources (such as secondary storage) to be recycled. Each program is responsible for ensuring that temporary files are removed during normal operation. There is no surefire method that can guarantee the removal of orphaned files in the case of abnormal termination, even in the presence of a finally block, because the finally block may fail to execute. For this reason, many systems employ temporary file cleaner utilities to sweep temporary directories and remove old files. Such utilities can be invoked manually by a system administrator or can be periodically invoked by a system daemon. However, these utilities are themselves vulnerable to file-based exploits and often require the use of shared directories.

Noncompliant Code Example (Predictable File Names)

This noncompliant code example hardcodes the name of a temporary file; consequently, the file's name is predictable. Even though there is a built-in check to detect whether a file still exists after its creation, this check creates a TOCTOU race condition that an attacker can exploit, by altering or deleting the file between the check and the read.

class TempFile{
  public static void main(String[] args) throws IOException{
    File f = new File("tempnam.tmp");
    FileOutputStream fop = new FileOutputStream(f);
    String str = "Data";
    
    if (f.exists()) {
      fop.write(str.getBytes());
      fop.close();
    } else { 
      System.out.println("This file does not exist"); 
    }
  }      
}

Noncompliant Code Example (createTempFile(), deleteOnExit())

This noncompliant code example improves over the previous noncompliant code example by using the method File.createTempFile() to generate a unique temporary filename based on two parameters, a prefix and an extension. This is the only method currently designed and provided for producing unique file names; unfortunately, the names produced can be easy to predict. Mitigate this vulnerability by using a good random number generator to produce the prefix.

This example also attempts to use the deleteOnExit() method to ensure that the temporary file is deleted when the JVM terminates. However, according to the Java API [[API 2006]] Class File, method deleteOnExit() documentation:

Deletion will be attempted only for normal termination of the virtual machine, as defined by the Java Language Specification. Once deletion has been requested, it is not possible to cancel the request. This method should consequently be used with care.
Note: this method should not be used for file-locking, as the resulting protocol cannot be made to work reliably.

Consequently, the file is not deleted if the JVM terminates unexpectedly. A longstanding bug on Windows based systems reported as Bug ID: 4171239 [[SDN 2008]] causes JVMs to fail to delete a file when deleteOnExit() is invoked before the associated stream or RandomAccessFile is closed.

class TempFile {
  public static void main(String[] args) throws IOException{
    File f = File.createTempFile("tempnam",".tmp");
    FileOutputStream fop = new FileOutputStream(f);
    String str = "Data";
    try {
      fop.write(str.getBytes());
      fop.flush();        
    } finally {
      // Stream/file still open; file will
      // not be deleted on Windows systems
      f.deleteOnExit(); // Delete the file when the JVM terminates
    }
  }       
}

Noncompliant Code Example (POSIX, Java 1.7)

This noncompliant code example creates a temporary file using the newest features of Java 1.7's NIO facility. It uses the createTempFile() method, which creates an unpredictable name. (The actual method by which the name is created is implementation-defined and undocumented.) The file is specifically created with POSIX file permissions denying access to the file to everyone except the file's creator, a similar permission set can be devised for Windows. Furthermore, the createTempFile() will throw an exception if the file already existed. The file is opened using the try-with-resources construct, which automatically closes the file whether or not an exception occurs. And finally, the file is opened with the Java 1.7 DELETE_ON_CLOSE option, which serves to remove the file automatically when it is closed.

class TempFile {
  public static void main(String[] args) {
    // POSIX file permissions for exclusive read/write
    Set<PosixFilePermission> perms = PosixFilePermissions.fromString("rw-------");
    FileAttribute att = PosixFilePermissions.asFileAttribute( perms);
    Path tempFile = null;
    try {
      tempFile = Files.createTempFile( "file", ".myapp", att);
      try (BufferedWriter writer = Files.newBufferedWriter(tempFile, Charset.forName("UTF8"),
                                                           StandardOpenOption.DELETE_ON_CLOSE)) {
          // write to file
      }
      System.out.println("Temporary file write done, file erased");
    } catch (FileAlreadyExistsException x) {
      System.err.println("File exists: " + tempFile);
    } catch (IOException x) {
      // Some other sort of failure, such as permissions.
      System.err.println("Error creating temporary file: " + x);
    }
  }
}

Despite the new Java 1.7 features, this example still has several vulnerabilities. There is no mechanism to open the file with exclusive access, a feature provided by standard POSIX. Consequently the temporary file, once created, is still accessible and modifiable by anyone with access to its containing directory. Also, because the creation of the file and the opening of the file are distinct operations, this program is still vulnerable to a time-of-check-time-of-use (TOCTOU) race condition.

To work around the file/stream termination issue, always attempt to terminate the resource normally before invoking deleteOnExit(). Using File.io.delete() to immediately delete the file is good practice, when possible; this avoids improper JVM termination related issues. Moreover, although unreliable, System.gc() may be invoked to free up related resources. Sometimes, the resources to be deleted cannot be closed first; see, for example, Bug ID: 4635827 [[SDN 2008]]. There is no known workaround for this case.

Compliant Solution (POSIX, Java 1.7, secure directory)

Because of the potential for race conditions, and the inherent accessability of shared directories, temporary files must only be created in secure directories. This compliant solution depends on an isSecureDir() method. This method ensures that file and all directories above it are owned by either the user or the superuser, that each directory does not have write access for any other users, and that directories above path may not be deleted or renamed by any other users. When checking directories, it is important to traverse from the root to the leaf to avoid a dangerous race condition whereby an attacker who has privileges to at least one of the directories can rename and recreate a directory after the privilege verification.

The file name passed to this function is first verified to be a directory, and rendered absolute if necessary. 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 function checks every directory in the path, ensuring that every directory is owned by the current user or the superuser and that all directories in the path forbid other users from deleting or renaming files.

POSIX maintains that a directory can prevent modifications to its files either by disabling group and other write access to the directory or by turning on the sticky bit. However, Java 1.7 provides no mechanism for accessing the sticky bit.

Most home directories are secure by default on standard POSIX installations, and only shared directories such as /tmp are insecure. However, since any user can create an insecure directory, a program should verify that a particular directory is actually secure, as it may not have the permissions required to create a secure directory.

Note that this method is only effective on filesystems that are fully compatible with UNIX permissions, and it may not behave normally for filesystems with other permission mechanisms, such as AFS.

public static boolean isSecureDir(Path file) {
  if (!Files.isDirectory( file)) {
    return false;
  }
  if (!file.isAbsolute()) {
    file = file.toAbsolutePath();
  }

  // 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 (!isSecureDir( Files.readSymbolicLink(partialPath))) {
          // Symbolic link, linked-to dir not secure
          return false;
        }
      } else {
        FileSystem fileSystem = partialPath.getFileSystem();
        UserPrincipalLookupService upls = fileSystem.getUserPrincipalLookupService();
        UserPrincipal root = upls.lookupPrincipalByName("root");
        UserPrincipal user = upls.lookupPrincipalByName(System.getProperty("user.name"));
        UserPrincipal owner = Files.getOwner(partialPath);
        if (!owner.equals(user) && !owner.equals(root)) {
          // 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)) {
          // someone else can write files, not secure
          return false;
        }
      }
    } catch (IOException x) {
      return false;
    }
  }

  return true;
}

This compliant solution uses the isSecureDir() method to ensure that an attacker may not tamper with the file to be opened and subsequently removed. Note that once the path name of a directory has been checked using isSecureDir(), all further file operations on that directory must be performed using the same path.

class TempFile {
  public static boolean isSecureDir(Path file) {
    // ...
  }

  public static void main(String[] args) {
    Path tempDir = new File(args[0]).toPath();
    if (!isSecureDir( tempDir)) {
      System.out.println("Temporary Directory not secure");
      return;
    }

    // POSIX file permissions for exclusive read/write
    Set<PosixFilePermission> perms = PosixFilePermissions.fromString("rw-------");
    FileAttribute att = PosixFilePermissions.asFileAttribute( perms);
    Path tempFile = null;
    try {
      tempFile = Files.createTempFile( tempDir, "file", ".myapp", att);
      try (BufferedWriter writer = Files.newBufferedWriter(tempFile, Charset.forName("UTF8"),
                                                           StandardOpenOption.DELETE_ON_CLOSE)) {
          // write to file
        }
      System.out.println("Temporary file write done, file erased");

    } catch (FileAlreadyExistsException x) {
      System.err.println("File exists: " + tempFile);
    } catch (IOException x) {
      // Some other sort of failure, such as permissions.
      System.err.println("Error creating temporary file: " + x);
    }
  }
}

Risk Assessment

Failure to follow best practices while creating, using and deleting temporary files can lead to denial of service vulnerabilities, misinterpretations and alterations in control flow.

Related Vulnerabilities

GERONIMO-3489

Related Guidelines

Bibliography

FIO06-J. Close resources when they are no longer needed      12. Input Output (FIO)      FIO08-J. Do not log sensitive information outside a trust boundary