Many file-related security vulnerabilities result from a program accessing an unintended file object because file names are only loosely bound to underlying file objects. File names provide no information regarding the nature of the file object itself. Furthermore, the binding of a file name to a file object is reasserted every time the file name is used in an operation.
Files can often be identified by other attributes in addition to the file name, for example, by comparing file creation time or modification times. Information about a file that has been created and closed can be stored and then used to validate the identity of the file when it is reopened.
Comparing multiple attributes of the file increases the likelihood that the reopened file is the same file that was previously operated on.
File identification is less of an issue if applications maintain their files in secure directories, where they can be accessed only by the owner of the file and (possibly) by a system administrator (see FIO00-J. Do not operate on files in shared directories).
Noncompliant Code Example
In this noncompliant code example, the file identified by the string filename is opened, processed, closed, and then reopened for reading.
//Identify a file by its path
String filename = // initialized
Path file1 = Paths.get(filename);
// Open the file for writing
BufferedWriter bw = new BufferedWriter(
new OutputStreamWriter(Files.newOutputStream(file1)));
// Write to file...
// Close the file
bw.close();
/*
* A race condition here allows for an attacker to switch
* out the file for another
*/
// Reopen the file for reading
Path file2 = Paths.get(filename);
BufferedReader br = new BufferedReader(
new InputStreamReader(Files.newInputStream(file2)));
String line;
while ((line = br.readLine()) != null) {
System.out.println(line);
}
// Close the file
br.close();
There is no guarantee that the file opened for reading is the same file that is opened for writing. An attacker can replace the original file (for example, with a symbolic link) between the first call to close() and the subsequent creation of the BufferedReader.
Noncompliant Code Example (isSameFile())
In this noncompliant code example, the programmer attempts to ensure that the file opened for reading is the same as the file previously opened for writing by calling the method isSameFile():
//Identify a file by its path
String filename = // initialized
Path file1 = Paths.get(filename);
// Open the file for writing
BufferedWriter bw = new BufferedWriter(
new OutputStreamWriter(Files.newOutputStream(file1)));
// Write to file...
// Close the file
bw.close();
// ...
// Reopen the file for reading
Path file2 = Paths.get(filename);
if (!Files.isSameFile(file1, file2)) {
System.out.println("File tampered with");
// Handle error
}
BufferedReader br = new BufferedReader(
new InputStreamReader(Files.newInputStream(file2)));
String line;
while ((line = br.readLine()) != null) {
System.out.println(line);
}
// Close the file
br.close();
Unfortunately, there is no guarantee that the method isSameFile() really checks that the files are the same file. The Java 7 API for isSameFile() says:
If both Path objects are equal then this method returns true without checking if the file exists.
That is, isSameFile() may simply check that the paths to the two files are the same. This does not exclude the possibility that the file at that path has been replaced by a different file between the two open operations.
Compliant Solution (Multiple Attributes)
This compliant solution checks the creation and last modified times of the files to ensure that the file opened for reading is the same file as the file that was written:
//Identify a file by its path
String filename = // initialized
Path file1 = Paths.get(filename);
BasicFileAttributes attr1 = Files.readAttributes(file1, BasicFileAttributes.class);
FileTime creation1 = attr1.creationTime();
FileTime modified1 = attr1.lastModifiedTime();
// Open the file for writing
BufferedWriter bw = new BufferedWriter(
new OutputStreamWriter(Files.newOutputStream(file1)));
// Write to file...
// Close the file
bw.close();
// ...
// Reopen the file for reading
Path file2 = Paths.get(filename);
BasicFileAttributes attr2 = Files.readAttributes(file2, BasicFileAttributes.class);
FileTime creation2 = attr2.creationTime();
FileTime modified2 = attr2.lastModifiedTime();
if ( (!creation1.equals(creation2)) || (!modified1.equals(modified2)) ) {
System.out.println("File tampered with");
// Handle error
}
BufferedReader br = new BufferedReader(
new InputStreamReader(Files.newInputStream(file2)));
String line;
while ((line = br.readLine()) != null) {
System.out.println(line);
}
// Close the file
br.close();
Although this solution is reasonably secure, a determined attacker could create a symbolic link with the same creation and last-modified times as the original file. Also, there is a time-of-check-time-of-use (TOCTOU) race condition between when the file's attributes are read and when the file is first opened. Likewise, there is another TOCTOU between the second attributes are read and the file is reopened.
Compliant Solution (POSIX fileKey Attribute)
In environments that support the fileKey attribute, a more reliable approach is to check that the fileKey attributes of the two files are the same. The fileKey attribute is an object which "uniquely identifies the file" [API 2011], as shown in this compliant solution:
//Identify a file by its path
String filename = // initialized
Path file1 = Paths.get(filename);
BasicFileAttributes attr1 = Files.readAttributes(file1, BasicFileAttributes.class);
Object key1 = attr1.fileKey();
// Open the file for writing
BufferedWriter bw = new BufferedWriter(
new OutputStreamWriter(Files.newOutputStream(file1)));
// Write to file...
// Close the file
bw.close();
// ...
// Reopen the file for reading
Path file2 = Paths.get(filename);
BasicFileAttributes attr2 = Files.readAttributes(file2, BasicFileAttributes.class);
Object key2 = attr2.fileKey();
if ( !key1.equals(key2) ) {
System.out.println("File tampered with");
// Handle error
}
BufferedReader br = new BufferedReader(
new InputStreamReader(Files.newInputStream(file2)));
String line;
while ((line = br.readLine()) != null) {
System.out.println(line);
}
// Close the file
br.close();
This approach will not work on all platforms. For example, on an Intel Core i5-2400 machine running Windows 7 Enterprise, all fileKey attributes are null.
This solution is not perfect. Like the previous compliant solution, it has a TOCTOU race window between when the file's attributes are read and when the file is first opened. Likewise, there is another TOCTOU between the second attributes are read and the file is reopened.
Applicability
Many file-related vulnerabilities are exploited to cause a program to access an unintended file. Proper file identification is necessary to prevent exploitation.
Bibliography