ERR06-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 parameters that attempt to provoke exposure of internal structures and mechanisms of the application. Both the exception message text and the type of an exception can leak information information. For example, given an exception of type FileNotFoundException, the message reveals information regarding the file system layout and the exception type reveals the absence of the requested file.

This guideline applies to server side applications as well as to clients. Adversaries can glean sensitive information not only from vulnerable web servers but also from innocent users 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 errors 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 a few sensitive errors and exceptions:

Noncompliant Code Example (Leaks from Exception Message and Type)

Consider a program that must read a file supplied by the user, but the contents and layout of the filesystem might be considered sensitive. This noncompliant code example accepts a file name as an input argument, while doing nothing 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]);  
  }
}

An attacker can learn about the structure of the underlying file system by repeatedly passing constructed paths to fictitious files to the program. When a requested file is absent, the FileInputStream constructor throws a FileNotFoundException.

Noncompliant Code Example (Wrapping and Rethrowing Sensitive Exception)

This noncompliant code example logs the exception and wraps it in an unchecked 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 learn sensitive information about the filesystem 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 more cryptic to info disclosure than previous examples, it still reveals that the file being specified cannot be read. More specifically, the program reacts differently to nonexistant file paths than it does to valid ones, and an attacker can still infer sensitive information about the filesystem 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 via repeated queries that collectively cover the space of possible filenames; queries that result in the sanitized message exclude the requested file, the remaining possibilities represent the actual files.

Compliant Solution (Canonicalization)

This compliant solution operates under the policy that only files that live in c:\homepath may be opened by the user, and that the user must not learn anything about files outside this directory. So it 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 pathname 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 ERR14-J. Catch specific exceptions rather than the more general RuntimeException or Exception, It also uses the MyExceptionReporter class described in guideline ERR01-J. Use a class dedicated to reporting 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 guideline ERR03-J. Use a logging API to log critical security exceptions for additional information. The MyExceptionReporter class from guideline ERR01-J. Use a class dedicated to reporting exceptions demonstrates an acceptable approach for this logging and sanitization.

Risk Assessment

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

Related Vulnerabilities

CVE-2009-2897

Other Languages

This guideline appears in the C++ Secure Coding Standard as ERR12-CPP. Do not allow exceptions to transmit sensitive information.

Bibliography

[[Gong 2003]] 9.1 Security Exceptions
[[MITRE 2009]] CWE ID 209 "Error Message Information Leak", CWE ID 600 "Failure to Catch All Exceptions (Missing Catch Block)", CWE ID 497 "Information Leak of System Data"
[[SCG 2007]] Guideline 3-4 Purge sensitive information from exceptions

ERR05-J. Handle checked exceptions that can be thrown within a finally block      06. Exceptional Behavior (ERR)      ERR07-J. Prevent exceptions while logging data