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Failure to filter sensitive information when propagating exceptions often results in information leaks that can assist an attacker's efforts to expand the attack surfacedevelop further exploits. 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 FileNotFoundException message of a FileNotFoundException reveals information about the file system layout, and the exception type reveals the absence of the requested file.

Wiki MarkupThis rule applies to server -side applications as well as to clients. Adversaries 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 Schönefeld discovered an exploit for the Opera v7.54 web browser, wherein an attacker could use the {{browser in which 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|AA. Bibliography#Schoenefeld 04]\[Schönefeld 2004].

All exceptions reveal information that can assist an attacker's efforts to carry out a denial of service (DoS) against the system. Consequently, programs must filter both exception messages and exception types that can propagate across trust boundaries. The following 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 When a Java program that 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 contain sensitive information about the program's internal structure. Consequently, command-line programs any program that may be run on a console accessible to an untrusted user must never abort because of due to 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 
 // 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 then wraps it in an unchecked a more general exception before re-throwing rethrowing 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 when 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.
Note that this example also violates FIO04-J. Release resources when they are no longer needed, as it fails to close the input stream in a finally block. Subsequent code examples also omit this finally block for brevity.

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 Although this exception is less likely to leak useful information than the previous noncompliant code examples to leak useful information, 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 with repeated by issuing queries that collectively cover the space of possible file names; queries that result in the sanitized message exclude the requested file, the remaining possibilities represent the actual . File names that cause the program to return the sanitized exception indicate nonexistent files, whereas file names that do not return exceptions reveal existing files.

Compliant Solution (

...

Security Policy)

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 when the file cannot be opened , or the file does not live in the proper directory. This serves to conceal any Any information about files outside c::\homepath is concealed.

The compliant solution also uses the File.getCanonicalFile() method to canonicalize the file , so that the to simplify subsequent path name comparison cannot be foiled by spurious instances of "\.", or symbolic links, or capitalizationcomparisons (see FIO16-J. Canonicalize path names before validating them for more information).

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 ERR14permitted by exception ERR08-J-EX2(see ERR08-J. Do not catch NullPointerException , RuntimeException, Exception, or Throwable, It also or any of its ancestors). It uses the MyExceptionReporter class described in rule ERR00-J. Do not suppress or ignore checked exceptions, which handles responsibility for filtering filters sensitive information from any resulting exceptions.

class ExceptionExample {
  public static void main(String[] args) {
    tryFileInputStream {
fis = null;
    FileInputStream fis = null;try {
      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 ERR07(see 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 scalability, 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.

Automated Detection

Related Vulnerabilities

CVE-2009-2897

Other Languages

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describes several cross-site scripting (XSS) vulnerabilities in several versions of SpringSource Hyperic HQ. These vulnerabilities allow remote attackers to inject arbitrary web script or HTML via invalid values for numerical parameters. They are demonstrated by an uncaught java.lang.NumberFormatException exception resulting from entering several invalid numeric parameters to the web interface.

CVE-2015-2080 describes a vulnerability in the Jetty web server, versions 9.2.3 to 9.2.8, where an illegal character passed in an HTML request causes the server to respond with an error message containing the text with the illegal character. But this error message can also contain sensitive information, such as cookies from previous web requests.

Related Guidelines

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Related Guidelines

Bibliography

Bibliography

Image Added Image Added Image AddedImage Removed      06. Exceptional Behavior (ERR)      ERR07-J. Prevent exceptions while logging data