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 only be accessed by the owner of the file and (possibly) by a system administrator.
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 = "...";
Path file1 = Paths.get(filename);
// Open the file for writing
BufferedWriter bw = new BufferedWriter(
new OutputStreamWriter(Files.newOutputStream(file1)));
bw.write(...);
// 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 = "...";
Path file1 = Paths.get(filename);
// Open the file for writing
BufferedWriter bw = new BufferedWriter(
new OutputStreamWriter(Files.newOutputStream(file1)));
bw.write(...);
// 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);
if (!Files.isSameFile(file1, file2)) {
System.out.println("File tampered with");
// Deal with 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.
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 = "...";
Path file1 = Paths.get(filename);
// Open the file for writing
BufferedWriter bw = new BufferedWriter(
new OutputStreamWriter(Files.newOutputStream(file1)));
bw.write(...);
// 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);
BasicFileAttributes attr1 = Files.readAttributes(file1, BasicFileAttributes.class);
BasicFileAttributes attr2 = Files.readAttributes(file2, BasicFileAttributes.class);
FileTime creation1 = attr1.creationTime();
FileTime modified1 = attr1.lastModifiedTime();
FileTime creation2 = attr2.creationTime();
FileTime modified2 = attr2.lastModifiedTime();
if ( (!creation1.equals(creation2)) || (!modified1.equals(modified2)) ) {
System.out.println("File tampered with");
// Deal with 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.
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, as shown in this compliant solution.
//Identify a file by its path
String filename = "...";
Path file1 = Paths.get(filename);
// Open the file for writing
BufferedWriter bw = new BufferedWriter(
new OutputStreamWriter(Files.newOutputStream(file1)));
bw.write(...);
// 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);
BasicFileAttributes attr1 = Files.readAttributes(file1, BasicFileAttributes.class);
BasicFileAttributes attr2 = Files.readAttributes(file2, BasicFileAttributes.class);
Object key1 = attr1.fileKey();
Object key2 = attr2.fileKey();
if ( !key1.equals(key2) ) {
System.out.println("File tampered with");
// Deal with 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.
Risk Assessment
Many file-related vulnerabilities are exploited to cause a program to access an unintended file. Proper file identification is necessary to prevent exploitation.
Guideline |
Severity |
Likelihood |
Remediation Cost |
Priority |
Level |
|---|---|---|---|---|---|
FIO52-J |
medium |
probable |
medium |
P8 |
L2 |
Automated Detection
In general, it is not possible to automatically detect this problem.
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this guideline on the CERT website.
Other Languages
This guideline is based on FIO05-C. Identify files using multiple file attributes.
Bibliography
<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="4bd26287-b23f-4ef5-96f2-289ee32ba29a"><ac:plain-text-body><![CDATA[ |
[[API 2011 |
AA. References#API 11]] |
[Class File |
http://download.oracle.com/javase/7/docs/api/java/io/File.html], [Interface Path |
http://download.oracle.com/javase/7/docs/api/java/nio/file/Path.html], [Class Files |
http://download.oracle.com/javase/7/docs/api/java/nio/file/Files.html], [Interface BasicFileAttributes |
http://download.oracle.com/javase/7/docs/api/java/nio/file/attribute/BasicFileAttributes.html] |
]]></ac:plain-text-body></ac:structured-macro> |