SEC02-J. Guard doPrivileged blocks against untrusted invocation and leakage of sensitive data

The static method java.security.AccessController.doPrivileged affirms that the invoking method assumes responsibility for enforcing its own privileges and that the access permissions of its callers should be ignored. For example, an application may have permissions to operate on a sensitive file, however, a caller of the application may be allowed to operate with only the basic permissions. Invoking doPrivileged() in this context allows the application operating with basic permissions to use the sensitive file, for instance, when a user password change request requires an unprivileged application to use a more privileged application to set the new password.

Noncompliant Code Example

In this noncompliant code example, the doPrivileged method is called from the openPasswordFile() method. The openPasswordFile() method is privileged and returns a FileInputStream reference to its caller. This allows an untrusted caller to call openPasswordFile() directly and obtain a reference to the sensitive file because of the inherent privileges possessed by openPasswordFile's doPrivileged block.

public class Password {
  public static void changePassword(final String password_file) 
    throws FileNotFoundException {
    FileInputStream fin;
    fin = openPasswordFile(password_file);
  }

  public static FileInputStream openPasswordFile(String password_file) 
    throws FileNotFoundException {
    // Declare as final and assign before the body of the anonymous inner class
    // Array f[] is used to maintain language semantics while using final
    final FileInputStream f[] = {null};
    // Use own privilege to open the sensitive password file
    AccessController.doPrivileged(new PrivilegedAction() {
      public Object run() {
        try {
          f[0] = new FileInputStream(password_file);	// Perform privileged action
        } catch(FileNotFoundException cnf) {
          // cannot recover if password file is not found; log to file
        }
        return null;    // Still mandatory to return from run()
      }
    });
   return f[0];  // Returns a reference to privileged objects (inappropriate)
 }
}

In general, when any method containing the doPrivileged block exposes a field (such as a reference) beyond its own boundary, it becomes trivial for untrusted callers to exploit the program. Both compliant solutions (below) avoid exposing any reference to the privileged data.

Compliant Solution (Logging Exceptions)

The openPasswordFile() method controls access to the sensitive password file and returns its reference. For this reason, it should not be directly invokable. Instead, the changePassword() method must be used to forward any requests to openPasswordFile(). This is because changePassword() does not return a reference to the sensitive file to any caller and processes the file internally. Observe that caller supplied (tainted) inputs are not used because the name of the password file is hard-coded.

This compliant solution logs exceptions to a privileged log file. This choice is appropriate when data attached to the exception could potentially leak privileged information (e.g. the path to a privileged file).

class Password {
  private static void changePassword() {
    // Use own privilege to open the sensitive password file
    final String password_file = "password";
    final FileInputStream f[] = {null};
    AccessController.doPrivileged(new PrivilegedAction() {
      public Object run() {
        try {
          f[0] = openPasswordFile(password_file);  // call the privileged method here
        } catch(FileNotFoundException cnf) {
          // cannot recover if password file is not found; log to file
        }
        return null;
      }
    });
    // Perform other operations such as old password verification
  }

  private static FileInputStream openPasswordFile(String password_file) throws FileNotFoundException {
    FileInputStream f = new FileInputStream(password_file);
    // Perform read/write operations on password file
    return f;
  }
}

Compliant Solution (Throwing Wrapped Exceptions)

The previous compliant solution logs the exception instead of revealing sensitive information. (See guideline EXC06-J. Do not allow exceptions to transmit sensitive information for more information.) When none of the possible exceptions reveals sensitive information, we can use an equivalent mechanism that allows exceptions to be wrapped thus providing better diagnostic information for the caller. For example, an applet that lacks read-access to system files that contain fonts can accomplish the task from a privileged block without revealing any sensitive information. When non-sensitive exceptions provide more information, the client is better able to recognize the symptoms of a read failure.

public static void readFont() throws FileNotFoundException {
  // Use own privilege to open the font file
  final String font_file = "fontfile";
  try {
    final InputStream in =
    AccessController.doPrivileged(new PrivilegedExceptionAction<InputStream>() {

     public InputStream run() throws FileNotFoundException {
       return openFontFile(font_file); // call the privileged method here
     }
    });
    // Perform other operations
  } catch (PrivilegedActionException exc) {
      Exception cause = exc.getException();
      if (cause instanceof FileNotFoundException) {
        throw (FileNotFoundException)cause;
      } else { throw new Error("Unexpected exception type", cause); }
    }
}

In summary, if the code can throw a checked exception without leaking sensitive information, prefer the form of doPrivileged method that takes a PrivilegedExceptionAction instead of a PrivilegedAction.

Risk Assessment

Returning references to sensitive resources from within a doPrivileged block can break encapsulation and confinement. Any caller who can invoke the privileged code directly and obtain a reference to a sensitive resource or field can maliciously modify its elements.

Automated Detection

Identifying sensitive information requires assistance from the programmer; fully-automated identification of sensitive information is beyond the current state of the art.

If we had user-provided tagging of sensitive information, we could do some kind of escape analysis on the doPrivileged blocks and perhaps prove that nothing sensitive leaks out of them. We could even use something akin to thread coloring to identify the methods that either must (or must not) be called from doPrivileged blocks.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this guideline on the CERT website.

Bibliography

[[API 2006]] method doPrivileged()
[[Gong 2003]] Sections 6.4, AccessController and 9.5 Privileged Code
[[SCG 2007]] Guideline 6-1 Safely invoke java.security.AccessController.doPrivileged
[[MITRE 2009]] CWE ID 266 "Incorrect Privilege Assignment", CWE ID 272 "Least Privilege Violation"

SEC01-J. Minimize the accessibility of classes and their members      02. Platform Security (SEC)      SEC03-J. Do not allow tainted variables in doPrivileged blocks