ERR01-J. Do not allow exceptions to expose sensitive information

Failure to filter sensitive information when propagating exceptions often results in information leaks that can assist an attacker's efforts to expand the attack surface. An attacker may craft input arguments to expose internal structures and mechanisms of the application. Both the exception message text and the type of an exception can leak information. For example, the message of a FileNotFoundException reveals information about the file system layout and the exception type reveals the absence of the requested file.

This rule applies to server side applications as well as to clients. Attackers can glean sensitive information not only from vulnerable web servers but also from victims who use vulnerable web browsers. In 2004, Schoenefeld discovered an exploit for the Opera v7.54 web browser, wherein an attacker could use the sun.security.krb5.Credentials class in an applet as an oracle to "retrieve the name of the currently logged in user and parse his home directory from the information which is provided by the thrown java.security.AccessControlException" [[Schoenefeld 2004]].

All exceptions reveal information that can assist an attacker's efforts to carry out a denial of service against the system. Consequently, programs must filter both exception messages and exception types that can propagate across trust boundaries. The table shown below lists several problematic exceptions:

Printing the stack trace can also result in unintentionally leaking information about the structure and state of the process to an attacker. If a Java program is run within a console, and it terminates because of an uncaught exception, the exception's message and stack trace are displayed on the console; the stack trace may itself leak sensitive information about the program's internal structure. Consequently, command-line programs must never abort because of an uncaught exception.

Noncompliant Code Example (Leaks from Exception Message and Type)

In this noncompliant code example, the program that must read a file supplied by the user but the contents and layout of the file system are sensitive. The program accepts a file name as an input argument but fails to prevent any resulting exceptions from being presented to the user.

class ExceptionExample {
  public static void main(String[] args) throws FileNotFoundException {
    // Linux stores a user's home directory path in the environment variable 
    // $HOME, Windows in %APPDATA%
    FileInputStream fis = new FileInputStream(System.getenv("APPDATA") + args[0]);  
  }
}

When a requested file is absent, the FileInputStream constructor throws a FileNotFoundException allowing an attacker to reconstruct the underlying file system by repeatedly passing fictitious path names to the program.

Noncompliant Code Example (Wrapping and Rethrowing Sensitive Exception)

This noncompliant code example logs the exception and wraps it in a more general exception before re-throwing it.

try {
  FileInputStream fis = new FileInputStream(System.getenv("APPDATA") + args[0]);
} catch (FileNotFoundException e) {
  // Log the exception
  throw new IOException("Unable to retrieve file", e);
}

Even if the logged exception is not accessible to the user, the original exception is still informative and can be used by an attacker to discover sensitive information about the file system layout.

Noncompliant Code Example (Sanitized Exception)

This noncompliant code example logs the exception and throws a custom exception that does not wrap the FileNotFoundException.

class SecurityIOException extends IOException {/* ... */};

try {
  FileInputStream fis = new FileInputStream(System.getenv("APPDATA") + args[0]);
} catch (FileNotFoundException e) {
  // Log the exception
  throw new SecurityIOException();
}

While this exception is less likely to leak useful information than previous noncompliant code examples, it still reveals that the specified file cannot be read. More specifically, the program reacts differently to nonexistent file paths than it does to valid ones, and an attacker can still infer sensitive information about the file system from this program's behavior. Failure to restrict user input leaves the system vulnerable to a brute force attack in which the attacker discovers valid file names by issuing queries that collectively cover the space of possible file names. Filenames that cause the program to return the sanitized exception indicate nonexistent files, filenames that don't, reveal existing files.

Compliant Solution (Canonicalization)

This compliant solution implements the policy that only files that live in c:\homepath may be opened by the user, and that the user is not allowed to discover anything about files outside this directory. The solution issues a terse error message if the file cannot be opened, or the file does not live in the proper directory. This serves to conceal any information about files outside c::\homepath.

The compliant solution also uses the File.getCanonicalFile() method to canonicalize the file, so that the subsequent path name comparison cannot be foiled by spurious instances of "\.", or symbolic links, or capitalization.

class ExceptionExample {
  public static void main(String[] args) {

    File file = null;
    try {
      file = new File(System.getenv("APPDATA") + args[0]).getCanonicalFile();
      if (!file.getPath().startsWith("c:\\homepath")) {
        System.out.println("Invalid file");
        return;
      }
    } catch (IOException x) {
      System.out.println("Invalid file");
      return;
    }

    try {
      FileInputStream fis = new FileInputStream(file);
    } catch (FileNotFoundException x) {
      System.out.println("Invalid file");
      return;
    }
  }
}

Compliant Solution (Restricted Input)

This compliant solution operates under the policy that only c:\homepath\file1 and c:\homepath\file2 are permitted to be opened by the user.

It also catches Throwable, as warranted by EX0 of ERR08-J. Do not catch NullPointerException or any of its ancestors, It also uses the MyExceptionReporter class described in rule ERR00-J. Do not suppress or ignore checked exceptions, which handles responsibility for filtering sensitive information from any resulting exceptions.

class ExceptionExample {
  public static void main(String[] args) {
    try {
      FileInputStream fis = null;
      switch(Integer.valueOf(args[0])) {
        case 1: 
          fis = new FileInputStream("c:\\homepath\\file1"); 
          break;
        case 2: 
          fis = new FileInputStream("c:\\homepath\\file2");
          break;
        //...
        default:
          System.out.println("Invalid option"); 
          break;
      }      
    } catch (Throwable t) { 
      MyExceptionReporter.report(t); // Sanitize 
    } 
  }
}

Compliant solutions must ensure that security exceptions such as java.security.AccessControlException and java.lang.SecurityException continue to be logged and sanitized appropriately. See rule ERR02-J. Prevent exceptions while logging data for additional information. The MyExceptionReporter class from rule ERR00-J. Do not suppress or ignore checked exceptions demonstrates an acceptable approach for this logging and sanitization.

For scalablity, the switch statement should be replaced with some sort of mapping from integers to valid file names, or at least an enum type representing valid files.

Risk Assessment

Exceptions may inadvertently reveal sensitive information unless care is taken to limit the information disclosure.

Related Vulnerabilities

CVE-2009-2897

Related Guidelines

Bibliography

      06. Exceptional Behavior (ERR)